instrumentation tubing and their connections-nirbhay gupta

TECDOC-01 SEPTEMBER 2008

TECHNICAL DOCUMENT ON

INSTRUMENTATION TUBING AND THEIR

CONNECTIONS

Instrumentation Technical Document Series By: Nirbhay Gupta

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PREFACE

Instrumentation design and construction is a very interesting proposition. One is supposed to

know the electronics and electrical aspects as well as the mechanical aspects too.

Instrumentation tubing is one such field where an instrumentation engineer has to don the

robes of a mechanical engineer. In NPCIL, for a long time, it was felt that there is no single

document that can cater to the needs of budding as well as practicing engineers when they

want to search some information on instrumentation tubing and connections.

Instrumentation tubing covers both Impulse tubes (sensing lines) as well as pneumatic tubes.

Connections include tapping points, root valves and tube fittings. Usually one has to refer to

myriad technical documents, codes and standards to search for a specific aspect of tubing

design or construction. This technical note is an attempt to put all the information at one

place. The efforts have been put to expose the reader to all the aspects of tubing and make

him aware of all the developments in the world. A comprehensive list of all the reference

documents is given at the end and they have been liberally used while preparation of this note

was underway. Effort has been made to represent all the relevant information here however,

enterprising readers will benefit even more if they peruse the reference documents directly.

Attempt has been made to demonstrate analytically that if the design and installation

practices are followed as per this note then the sensing line will meet the intent of class -I

tubing. Readers may note that the word tube/tubing used here should be inferred as

instrumentation tubing only limited to maximum 1” size.

It may be noted that various tubing practices have not been discussed in this note. The

detailed installation practices for various process measurements will be discussed in respective

process measurement/field installation technical notes. However, salient issues common to all

installations have been discussed in detail.

Author is grateful to a large number of engineers with whom they had an opportunity to work

with during their long career in NPCIL and on the way a lot of design aspects were concluded.

Nirbhay Gupta

23rd September, 2008

Mumbai

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TABLE OF CONTENT Section TITLE Page

No.

1.0 INTRODUCTION 1

1.1 DIFFERENCE BETWEEN A PIPE AND A TUBE 1

1.2 MAJOR ADVANTAGES OF TUBING OVER PIPING SYSTEMS 2 1.3 TYPES OF TUBES 3

1.4 GUIDELINES FOR SELECTION OF INSTRUMENTATION TUBES 3

1.5 DIFFERENT SIZES OF TUBES 5

1.6 CRITERIA FOR SELECTING THE SIZE OF A TUBE 5

1.7 SELECTION AND DESIGN CRITERIA 6

2.0 DESIGN OF TUBING AND TUBING SYSTEMS 13

2.1 CLASS-I INSTRUMENTATION TUBING DESIGN 13 2.2 REQUIREMENTS OF MATERIAL FOR INSTRUMENT TUBING/PIPING AS PER NB-2000 13

2.3 DESIGN REQUIREMENTS OF INSTRUMENT PIPING/TUBING AS PER SUBSECTION NC (NC

3600)

13

2.4 PRESSURE DESIGN (INTERNAL PRESSURE) OF INSTRUMENT TUBING/ PIPING 14

2.5 ANALYSIS CRITERION OF TUBING/PIPING SYSTEM 15 2.6 ANALYSIS OF SS TUBES USED IN NPCIL 18

2.6.1 WALL THICKNESS AND PRESSURE RATING OF DIFFERENT SIZES OF INSTRUMENT TUBING 18

2.6.2 STRESS ANALYSIS OF TUBING SYSTEMS 19

2.6.2.1 ANALYSIS FOR SUSTAINED MECHANICAL LOADS 19

2.6.2.2 ANALYSIS FOR OCCASIONAL LOADS (LEVEL A&B SERVICE LIMITS) 19

2.6.2.3 ANALYSIS FOR STRESS DUE TO THERMAL EXPANSION AND OTHER SUSTAINED LOADS 20

2.7 CONSIDERATION FOR VARIOUS FORCES 23 2.8 TUBE BENDING CONSIDERATIONS 23

2.9 SPECIAL DESIGN ASPECTS TO MEET THE REQUIREMENTS OF CLASS-I TUBING AND

TUBING SYSTEMS

23

2.10 CONCLUSION 24

3.0 TECHNICAL REQUIREMENTS OF SS TUBES 25 4.0 PNEUMATIC TUBING 27

4.1 ADVANTAGES OF USING COPPER TUBES 27

4.2 DIFFERENT TYPES OF COPPER TUBES 28

4.3 RECOMMENDATIONS FOR SELECTION OF A TYPE OF COPPER TUBE 29

4.4 TECHNICAL REQUIREMENTS OF COPPER TUBE 34

4.5 APPLICABLE INTERNATIONAL STANDARDS FOR COPPER TUBES 35

5.0 ASTM TUBING SPECIFICATIONS OUTSIDE DIAMETER/WALL THICKNESS 36

6.0 EMBEDDED PENETRATIONS 38

7.0 METHODS OF CONNECTION OF INSTRUMENTATION TUBES 39

7.1 WELDED JOINTS 39

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7.2 FLARED, FLARELESS AND COMPRESSION JOINTS 39

7.3 THREADED JOINTS 40

8.0 GUIDELINES FOR TAKE OFF CONNECTIONS FOR SENSING LINES 41

8.1 LOCATION OF PRESSURE TAPS 41

8.2 CONSIDERATIONS FOR PRESSURE TAP DESIGN 42

8.3 RECOMMENDATIONS FOR PRESSURE TAP DESIGN 43

9.0 GUIDELINES FOR ROOT VALVES 44 10.0 INSTALLATION OF INSTRUMENTATION TUBING 45

10.1 BEST PRACTICES FOR IMPULSE TUBE INSTALLATION 45

10.2 SOME PRACTICAL GUIDELINES FOR TUBE LAYING AND BENDING 48

10.3 TUBE BENDING CHECK LIST 50

10.4 CHARACTERISTICS OF A WELL-MADE TUBING CIRCUIT 54

10.5 COMMON CAUSES OF IMPERFECT BENDS 55

10.6 ROUTING OF BENDS 57 10.7 GUIDELINES FOR COPPER TUBE INSTALLATION 60

10.8 GUIDELINES FOR COPPER TUBE BENDING 60

10.9 COPPER TUBE JOINTS 61

11.0 IMPULSE TUBE/SENSING LINE SUPPORT 62

12.0 IMPULSE TUBE INSTALLATION THROUGH EPS 64

13.0 TUBE FITTINGS 65

13.1 REQUIREMENTS OF A TUBE FITTING 65 13.2 CONSTRUCTION OF A TUBE FITTING 67

13.3 TYPES OF TUBE FITTINGS 68

13.4 FLARED FITTING 68

13.5 FLARELESS BITE TYPE TUBE FITTING 69

13.6 FLARELESS COMPRESSION TYPE TUBE FITTING 69

13.7 SINGLE FERRULE FLARELESS COMPRESSION TYPE TUBE FITTING 70

13.8 TWIN FERRULE FLARELESS COMPRESSION TYPE TUBE FITTING 71

13.8.1 FERRULE AND ITS PURPOSE 72 13.8.2 SWAGING 73

13.8.3 OPERATION OF A TWIN FERRULE TUBE FITTING 74

13.8.4 EFFECT OF TUBE THICKNESS ON SWAGING 78

13.8.5 SAFETY PRECAUTIONS FOR TUBE FITTING INSTALLATION 80

13.9 REPEATED ASSEMBLY AND DISASSEMBLY OF TUBE FITTING 82

13.10 SPECIFICATION FOR SS TUBE FITTINGS 83

13.11 SPECIFICATION FOR BRASS TUBE FITTINGS 85 14.0 THREADS USED FOR TUBE FITTINGS 87

14.1 EVOLUTION OF THREADS 87

14.2 TYPE OF THREADS 87

14.3 SIZES 88

14.4 TAPER/PARALLEL THREADED JOINTS 89

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14.5 DRY SEAL NPTF THREADS 93

15.0 WELDING METHODS 96

15.1 300 SERIES STAINLESS STEELS 96

15.2 C1018 FITTINGS 96

15.3 TIG WELDING 97

15.4 ORBITAL TUBE WELDING 98

15.4.1 ORBITAL WELDING EQUIPMENT 99 15.4.2 REASONS FOR USING ORBITAL WELDING EQUIPMENT 99

15.4.3 INDUSTRIAL APPLICATIONS FOR ORBITAL WELDING 100

15.4.4 GENERAL GUIDELINES FOR ORBITAL TUBE WELDING 101

15.4.5 THE PHYSICS OF THE GTAW PROCESS 102

15.4.6 MATERIAL WELDABILITY 102

15.4.7 WELD JOINT FIT-UP 103

15.4.8 SHIELD GAS (ES) 104 15.4.9 TUNGSTEN ELECTRODE 105

15.4.10 WELDING BASICS AND SET-UP 106

15.4.11 WELDING PARAMETER DEVELOPMENT 109

16.0 References and Suggested Reading 116

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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008

Nirbhay Gupta 1

1.0 Introduction Impulse sensing lines are the lines containing process fluid which run between the sensing instruments and process tapping points, and are usually made of tubing/piping, valves and tube fittings.

1.1 Difference between a pipe and a tube The fundamental difference between pipe and tube is the dimensional standard to which each is manufactured.

A tube is a hollow product of round or any other cross section having a continuous periphery. Round tube size may be specified with respect to any two, but not all three, of the following: Outside diameter, inside diameter, wall thickness; type K, L and M copper tube (See section6 for details) may also be specified by nominal size and type only. Dimensions and permissible variations (tolerances) are specified in the appropriate ASTM or ASME standard specifications.

Generally tubing is specified by giving O.D. and wall thickness whereas pipes are specified by giving nominal diameter & wall thickness (NB and Schedule).

A pipe is a tube with a round cross section conforming to the dimensional requirements for nominal pipe size as tabulated in ANSI B36.10, Table 2 and 4, and ANSI B36.19, Table 1. For special pipe having a diameter not listed in these tables, and also for round tube, the nominal diameter corresponds with the outside diameter.

Pipe versus Tubes Standard fluid line systems, whether for simple household use or for the more exacting requirements of industry, were for many years constructed from threaded pipe of assorted materials and were assembled with various standard pipe fitting shapes, unions and nipples. Such systems under high pressures were plagued with leakage problems besides being cumbersome, inefficient and costly to assemble and maintain. Therefore, the use of pipe in these systems has largely been replaced by tubing because of the many advantages it offers.

Figure 11 Tubing provides simplified, free flow system


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Old Method Each connection is threaded ‐ requires numerous fittings – system not flexible or easy to install and service connections not smooth inside ‐ pockets obstruct flow. Modern Method ‐ Bendable tubing needs fewer fittings ‐ no threading required ‐ system light and compact ‐ easy to install and service ‐ no internal pockets or obstructions to free flow.

1.2 Major Advantages of Tubing over Piping Systems 1. Bending Quality ‐ Tubing has strong but relatively thinner walls; is easy to bend.

Tube fabrication is simple.

2. Greater Strength ‐ Tubing is stronger as no threads are required for connection. No weakened sections from reduction of wall thickness by threading.

Figure 12: With no threading necessary, tubing does not require extra wall thickness

3. Less Turbulence ‐ Smooth bends result in streamlined flow passage and less pressure drop.

4. Economy of Space and Weight ‐ With its better bending qualities and a smaller outside diameter, tubing saves space and permits working in close quarters. Tube fittings are smaller and also weigh less.

5. Flexibility ‐ Tubing is less rigid, has less tendency to transmit vibration from one

connection to another.

6. Fewer Fittings ‐ Tubing bends substitute for elbows. Fewer fittings mean fewer joints, fewer leak paths.


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7. Tighter Joints ‐ Quality tube fittings, correctly assembled, give better assurance of leak‐free systems.

8. Better Appearance ‐ Tubing permits smoother contours with fewer fittings for a

professional look to tubing systems.

9. Cleaner Fabrication ‐ No sealing compounds on tube connections. Again no threading; minimum chance of scale, metal chips, foreign particles in system.

10. Easier Assembly and Disassembly ‐ Every tube connection serves as a union. Tube

connections can be reassembled repeatedly with easy wrench action.

11. Less Maintenance ‐ Advantages of tubing and tube fittings add up to dependable, trouble‐free installations.

1.3 Types of tubes Tubes can be categorized in different ways.

1. Categorization based on tube dimensional specifications: Tubes can be classified as a. Metric tubes, where dimensions are specified in mm units e.g. 10mm, 20 mm

etc. b. Fractional tubes, where dimensions are specified in inch units e.g. ½”, ¾”, 1”

etc. 2. Categorization based on material of tubes e.g. carbon steel tubes, PVC Tubes, Copper

tubes, SS tubes, Inconel tubes, etc. 3. Categorization based on method of tube drawing i.e. welded and drawn, seamless

etc.

1.4 Guidelines for selection of instrumentation tubes Proper Tubing Selection

1. Always Match Materials – S.S. Tubing should be used only with S.S. Fittings. The only exception to this rule is copper tubing with brass fittings. Mixing materials can cause galvanic corrosion.

Galvanic Corrosion (Electrochemical) All metals have a specific relative electrical potential. When dissimilar metals come in contact in the presence of moisture (electrolyte), a low intensity electric current flows from the metal having the higher potential to the metal having the lower potential. The result of this galvanic action is the corrosion of the metal with the higher potential (more anodic). (See Galvanic Series Chart)


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Figure13: Galvanic Series chart


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2. Select proper tubing hardness – Remember instrumentation tube Fittings are designed to work within specific hardness ranges. RB 90 maximum for S.S., RB 80 recommended. For proper swaging the hardness of the tube should be less than the hardness of the fitting.

3. Select proper tubing wall thickness – Proper wall thickness is necessary to accommodate accepted safety factors relative to desired working pressures.

4. Tubing surface finish – Always select tubing free of visible draw marks or surface scratches. If possible, cut off any undesirable sections. These “deep” scratches can cause leaks when attempting to seal low‐density gases such as argon, nitrogen, or helium. Proper surface finish ensures leak‐proof compression joint with fitting.

1.5 Different sizes of tubes Following tube sizes have been used in NPCIL NPPs

SS Tubes (metric): 6 mm, 10mm, 12mm, 20mm and 25mm.

SS tube (Fractional): ¼”, 3/8”, ½”, ¾” and 1”.

Copper tubes (metric): 6mm, 10mm, 12mm, 20mm and 25mm.

Copper tubes (Fractional): ¼”, 3/8”, ½”, ¾” and 1”.

1.6 Criteria for selecting the size of a tube The selection criteria for sizing the tube are as follows:

• The O.D. of the tubes/impulse tubes should be the same and not smaller than 6 mm even with clean liquids and non corrosive piping, owing to the chance of blockage after long service.

• If condensation is likely to occur or if gas bubbles are likely to be liberated, the O.D. should not be smaller than 10 mm.

• When long runs cannot be avoided, the internal diameter of impulse tubing/piping may be selected as per the following table‐1‐1:


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TABLE – 11 Pressure signal transmission distance (meter)

Inside Dia. in mm of impulse tubing/piping for different process fluids

Water/steam Dry air/gas

Wet air or gas Oil of low to med. viscosity

Very dirty liquid or gas

0 ‐ 16 7 to 9 13 13 2516 ‐ 45 10 13 19 2545 ‐ 90 13 13 25 38As very long runs of impulse tubing/piping are not expected in our systems and also process fluid is expected to be clean, 10 mm OD tubing having I.D. of 7.6 mm has been found to be adequate, for pressure/ ΔP measurement except for some cases for level measurement in tanks/vessels using ΔP principle.

• Based on hold up, installation and material cost, radiation streaming considerations, higher size (>10 mm OD) tubing is not recommended for pressure/∆P measurement in primary/nuclear system in general.

1.7 Selection and Design criteria Following requirements should be met for impulse tubing for sensing the pressure/differential pressure signal for all types of process systems including for safety and safety related systems.

The most important consideration in the selection of suitable tubing for any application is the compatibility of the tubing material with the media to be contained. Table 1‐2 lists common materials and their associated general application. Table 1‐2 also lists the maximum and minimum operating temperature for the various tubing materials. Properly designed tubing/piping based on service conditions, should only be used for sensing lines.

The practice of mixing materials should be strongly discouraged. The only exception is brass fittings with copper tubing. Dissimilar materials in contact may be susceptible to galvanic corrosion. Further, different materials have different levels of hardness, and can adversely affect the fittings ability to seal on the tubing.

The use of a particular type of tube for a specific usage depends on the application and the process condition. The following table briefly describes the application guidelines for a specific tube material.


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Table12

1. For operating temperatures above 800 °F (425 °C), consideration should be given to media. 300 Series

Stainless Steels are susceptible to carbide precipitation which may lead to intergranular corrosion at elevated temperatures.

2. All temperature ratings based on temperatures as per ASME/ANSI B313 Chemical Plant and Petroleum Refinery Piping Code, 1999 Edition.

Gas Service Special care must be taken when selecting tubing for gas service. In order to achieve a gas‐tight seal, ferrules in instrument fittings must seal any surface imperfections. This is accomplished by the ferrules penetrating the surface of the tubing. Penetration can only be


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achieved if the tubing provides radial resistance and if the tubing material is softer than the ferrules. Thick walled tubing helps to provide resistance. Tables‐1‐3 to 1‐10 below indicate the minimum acceptable wall thickness for various materials in gas service. The ratings in white indicate combinations of diameter and wall thickness which are suitable for gas service. Acceptable tubing hardness for general application is listed in Table 1‐12. These values are the maximum allowed by the ASTM. For gas service, better results can be obtained by using tubing well below this maximum hardness. For example, a desirable hardness of 80 RB is suitable for stainless steel. The maximum allowed by ASTM is 90 RB.

System Pressure The system operating pressure is another important factor in determining the type, and more importantly, the size of tubing to be used. In general, high pressure installations require strong materials such as steel or stainless steel. Heavy walled softer tubing such as copper may be used if chemical compatibility exists with the media. However, the higher strength of steel or stainless steel permits the use of thinner tubes without reducing the ultimate rating of the system. In any event, tube fitting assemblies should never be pressurized beyond the recommended working pressure. The following tables (1‐3 to 1‐10) list by material the maximum suggested working pressure (in psi) of various tubing sizes. Acceptable tubing diameters and wall thicknesses are those for which a rating is listed. Combinations which do not have a pressure rating are not recommended for use with instrument fittings.

Table13: Fractional 316 or 304 STAINLESS STEEL (Seamless)


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Table14: Fractional 316 or 304 STAINLESS STEEL (Welded & Drawn)

Table15: Seamless Stainless Steel metric tubing


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Table17: Carbon Steel Metric tubing

Table16: Fractional Carbon Steel (Seamless)

Tube OD in.

Tube Wall Thickness, in.

0.028 0.035 0.049 0.065 0.083 0.095

0.109 0.120 0.134 0.148 0.165 0.180 0.220

Working Pressure, psig Note: For gas service, select a tube wall thickness outside of the shaded area.

1/8 8000 10 200 3/16 5100 6 600 9600 1/4 3700 4 800 7000 9600 5/16 3 700 5500 7500 3/8 3 100 4500 6200 1/2 2 300 3200 4500 5900 5/8 1 800 2600 3500 4600 5300

3/4 2100 2900 3700 4300

5100

7/8 1800 2400 3200 3700

4300

1 1500 2100 2700 3200

3700 4100

1 1/4

1600 2100 2500

2900 3200 3600 4000 4600 5000

1 1/2

1800 2000

2400 2600 2900 3300 3700 4100 5100

2 1500 1700 1900 2100 2400 2700 3000 3700


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Table18: ALUMINIUM (SEAMLESS) Table19: COPPER (SEAMLESS)

Table110: MONEL 400 (SEAMLESS)

Note:

• All working pressures have been calculated using the maximum allowable stress levels in accordance with ASME/ANSI B31.3, Chemical Plant and Petroleum Refinery Piping or ASME/ANSI B31.1 Power Piping.

• All calculations are based on maximum outside diameter and minimum wall thickness. • All working pressures are at ambient (72°F) temperature. • Ratings in gray are not suitable for gas services.


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Systems Temperature Operating temperature is another factor in determining the proper tubing material. Copper and aluminum tubing are suitable for low temperature media. Stainless steel and carbon steel tubing are suitable for higher temperature media. Special alloys such as Alloy 600 are recommended for extremely high temperature (see Table 1‐2). Table 1‐11 lists de‐rating factors which should be applied to the working pressures listed in Table 1‐3 to 1‐10 for elevated temperature (see Table 1‐2). Simply locate the correct factor in Table 1‐11 and multiply this by the appropriate value in Tables 1‐3 to 1‐10 for the elevated temperature working pressure.

Table-1-11 Temperature Derating Factors

Temperature Monel °F (°C) Copper Aluminum 316

SS 304 SS

Steel 400

100 (38) 1.00 1.00 1.00 1.00 1.00 1.00 200 (93) .80 1.00 1.00 1.00 .96 .88 300 (149) .78 .81 1.00 1.00 .90 .82 400 (204) .50 .40 .97 .94 .86 .79 500 (260) .90 .88 .82 .79 600 (316) .85 .82 .77 .79 700 (371) .82 .80 .73 .79 800 (427) .80 .76 .59 .76 900 (486) .78 .73

1000 (538) .77 .69 1100 (593) .62 .49 1200 (649) .37 .30

EXAMPLE: 1/2 inch x .049 wall seamless stainless steel tubing has a working pressure of 3700 psi @ room temperature. If the system were to operate @ 800°F (425°C), a factor of 80% (or .80) would apply (see Table 111 above) and the “at temperature” system pressure would be 3700 psi x .80 = 2960 psi


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Table‐1‐12 Material Type ASTM Tubing Spec. Condition Max.

Recommended Hardness

Stainless Steel

304, 316, 316L

ASTM‐A‐269, A‐249, A‐213, A632

Fully Annealed 90 RB

Copper K or L ASTM‐B75 B68, B88* (K or L)

Soft Annealed Temper 0 60 Max. Rockwell 15T

Carbon Steel

1010 SAE‐J524b, J525b Fully Annealed 72 RB

ASTM‐A‐179Aluminum Alloy 6061 ASTM B‐210 T6 Temper 56 RBMonel™ 400 ASTM B‐165 Fully Annealed 75 RBAlloy C‐276

C‐276 ASTM‐B‐622, B‐626 Fully Annealed 90 RB

Alloy 600 600 ASTM B‐167 Fully Annealed 90 RBCarpenter

20™ 20CB‐3 ASTM B‐468 Fully Annealed 90 RB

Titanium Commercially Pure Grade 2

ASTM B‐338 Fully Annealed 99 RB 200 Brinell Typical


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2.0 DESIGN OF TUBING AND TUBING SYSTEMS

2.1 CLASSI INSTRUMENTATION TUBING DESIGN In ASME Section III‐Division‐I sub‐section NB (Class I components), the design criterion/design requirements for instrument tubing has not been covered separately. Thus design guidelines given for small size of piping is being followed for Class I instrument tubing also. Also as the outside diameter of instrument tubing is being limited to 1” (25 mm); so any design concession permitted for lower size piping (<1”) will also be applicable to instrument tubing. As per NB 3630 (Piping design and analysis criteria) the piping of 1” NB or less, which have been classified as class I in design specification, may be designed and analyzed as per subsection NC. Thus for instrument tubing, the material & testing requirements shall be as per subsection NB whereas the design and analysis will be as per subsection NC.

2.2 REQUIREMENTS OF MATERIAL FOR INSTRUMENT TUBING/PIPING AS PER NB2000

a. Pressure retaining material should confirm to the requirements of one of the

specifications for material given in NB‐2121. b. Impact testing for austenitic stainless steel is not required. Also impact testing is not

required for a pipe/tube with a nominal pipe size less than 6”, irrespective of wall thickness.

c. Seamless pipes, tubes and fittings need not be examined by the rule of NB‐2510 (examination of pressure retaining material).

d. Wrought seamless and welded (without filler metal) pipes and tubes shall be examined and may be repaired in accordance with the requirements of class‐I seamless and welded (without filler metal) piping and tubing of SA‐655 (specification for special requirements for pipe and tubing for nuclear and other applications).

2.3 DESIGN REQUIREMENTS OF INSTRUMENT PIPING/TUBING AS PER SUBSECTION NC (NC 3600) i. MAXIMUM ALLOWABLE STRESS

For design/calculating minimum wall thickness of instrument tubing/piping, the maximum allowable stress for the material at design temperature shall be used as given in ANSI/ASME B36.19.


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ii. PRESSURE AND TEMPERATURE RATINGS The pressure ratings at the corresponding temperature given in ANSI/ASME B36.19 shall not be exceeded and piping/tubing product shall not be used at temperature in excess of those given in ANSI/ASME B36.19 for all the materials of which the tubing is made.

iii. ALLOWANCES Increased wall thickness of tubing shall be taken for providing allowances for corrosion or erosion, mechanical strength & bending etc.

iv. DYNAMIC EFFECTS Impact forces caused by either external or internal loads shall be considered in the piping/tubing design. Also the effect of earthquake and non‐seismic vibration shall be considered in the tubing design.

2.4 PRESSURE DESIGN (INTERNAL PRESSURE) OF INSTRUMENT TUBING/ PIPNG (Ref. NC3640)

a) Minimum Wall Thickness of straight tube/pipe: The minimum wall thickness of straight tube/pipe shall not be less than that determined by eq. (I) as follows:

tm=P* Do2 S+PY

+A …….. (1)

tm = minimum required wall thickness, mm P = Internal design pressure, kPag DO = Outside diameter of tube/pipe, mm S = Maximum allowable stress in the material due to internal pressure and joint

efficiency at design temperature, kPa A = Additional thickness, to provide for material removed in threading, corrosion

and erosion allowances and allowance for structural strength needed during erection.

Y = a coefficient having a value of 0.4. For pipe/tube with a ratio less than 6, the value of ‘Y’ for ferritic and austenitic steels designed for temperature of 480 oC and below should be taken as per eq. (2) below

………. (2)

Where d = Inside diameter of tube/pipe.


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b) Wherever bending of tubing/piping is likely to be involved in installations, the minimum wall thickness after bending shall not be less than the minimum wall thickness calculated as per eq. (1) for straight tube/pipe. To meet this requirement, actual wall thickness of tubing/piping is to be increased as per following Table –2‐1 (This is based on NC 3000):

TABLE – 21 Bend Radius Minimum thickness recommended Prior to

bending 6 tube/pipe diameter or greater

5 tube/pipe diameter 4 tube/pipe diameter 3 tube/pipe diameter

1.06 tm 1.08 tm 1.16 tm1 1.25 tm

1 As per ANSI/ASME B31.1 this value is 1.14. However NC3000 is more conservative. tm = minimum wall thickness required as per eq. (1) above.

c) Also, unless otherwise justified by the design calculation the ovality of tubing/piping after bending should not exceed 8% as determined by following eq. (3).

% 100 ……….. (3)

Where

Do = Nominal outside diameter of tube/pipe Dmax = the maximum outside diameter after bending or forming Dmin = the minimum outside diameter after bending or forming

2.5 ANALYSIS CRITERION OF TUBING/PIPING SYSTEM Analysis requirements for tubing/piping systems as per NC‐3650 are given below. “The design of complete piping system shall be analyzed between anchors for the effects of thermal expansion, weight and other sustained and occasional loads.”

The detail requirements/analysis criteria are given in following sub‐sections.

a. CONSIDERATION OF DESIGN CONDITIONS (STRESS DUE TO SUSTAINED LOADS)(Refer NC 3652)

The effects of pressure, weight and other sustained mechanical loads must meet the requirements of following eq. (4).

1.5 …………………… (4)


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Ssl = Stress due to sustained loads, kPa P = Internal design pressure, mm Do = Outside diameter of tube/pipe, mm B1, B2 = Primary stress indices for the pipe/tube (As per Figure below) NC 3673.2 (b)1

MA = Resultant moment loading on cross section due to weight and other sustained loads, kN‐m. NC 3653.3

Z = Sectional modulus of pipe/tube, mm3 Sh = Basic material allowable stress at design temperature consistent with loading

under consideration. tn = Nominal wall thickness, mm

b. CONSIDERATION OF LEVEL A AND B SERVICE LIMITS (REF. NC3653) i. STRESS DUE TO SUSTAINED PLUS OCCASIONAL LOADS

The effect of pressure, weight, other sustained loads and occasional loads including earthquake, for which level B service limits are designated, must meat the requirements of following eq. (5).

1 2 1.8 ………. (5)

But not greater than 1.5 Sy

Where Mb = resultant moment loading on cross section due to non reversing dynamic loads e.g.

occasional loads such as thrust from relief and safety valves loads from pressure and flow transients and earthquake.

Sy = material yield strength at temperature consistent with the loading under consideration, kPa.

Sol = stress due to occasional loads, kPa. Pmax = Peak pressure, kPa

ii. SUSTAINED PLUS THERMAL EXPANSION STRESSES The effects of pressure, weight, other sustained loads and thermal expansion for which level A and B service limits are designated, shall meet the requirements of following eq. (6).

0.75 ………(6)

0.75 i shall not be less than 1.0

Where

Ste = Sustained plus thermal expansion stresses. MC = range of resultant moments due to thermal expansion SA = Allowable stress range for expansion stresses. i = Stress intensification factor (refer NC‐3673.2)


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= ratio of bending moment producing fatigue in a given number of cycles in a straight pipe/tube with girth butt weld to that producing failure in the same number of cycles in the fitting or joint under consideration.

Other terms are same as of eq. (4) Allowable stress range for expansion stresses (SA) can be calculated using following equation

SA = f (1.25 SC + 0.25 Sh) ……. (7) SC = Basic material allowable stress at minimum (cold) temperature. Sh = Basic material allowable stress at maximum (hot) temperature. f = stress range reduction factor for cyclic conditions for total number N of full

temperature cycles over total number of years during which system is expected to be in service from table‐2‐1A below NC 3611.2 (e)‐1

TABLE: 21A

Number of equivalent full temperature cycles (N)

Stress range reduction factor (f)

7000 and less 1.07000 to 14000 0.914000 to 22000 0.822000 to 45000 0.745000 to 100000 0.6100000 and over 0.5

Stress intensification factor ‘i’ can be calculated using following equation (8)

2 …… (8) Where C2 and K2 are stress indices for class‐1 piping products or joints from NB 3681 (a)‐1. For straight pipe/tube the value of C2 and k2 are 1. For curved pipe/tube or welded elbows ‘I’ can be computed as per equation (9) below (refer NB 3681)

. …… (9)

where …… (10)

tn = nominal wall thickness of tube/pipe R = bend radius r = mean radius of tube/pipe

iii. CONSIDERATION OF LEVEL C SERVICE LIMITS


Nirbhay Gupta 20

In section II in calculating the resultant moment MB, moment due to SEE conditions is proposed to be used which is more conservative, thus separate analysis for level C service limits is not required.

iv. TESTING REQUIREMENTS AS PER SUBSECTION – NB

Requirements of material testing as per subsection NB is briefly mentioned above. In addition to examination/testing requirements as per SA‐655, tubing should be hydrostatically tested at not less than 1.25 times the design pressure with minimum holding time of 10 min.

2.6 ANALYSIS OF SS TUBES USED IN NPCIL

2.6.1 WALL THICKNESS AND PRESSURE RATING OF DIFFERENT SIZES OF INSTRUMENT TUBING The maximum design pressure and temperature are taken as 195 kg/cm2 and 310oC respectively. Though the above pressure and temperature may not exist simultaneously in any system, still to be on conservative side, all the sizes of tubing will be designed for above ratings. Using eq. (1) in the analysis criteria above, the minimum wall thickness of straight tubing can be calculated.

Thus following equation can be used

tm=P* Do2 S+PY

+A

We can make following assumptions

• There will be no threading on the tubes • Corrosion, erosion is negligible (hence allowance for corrosion and erosion may

be neglected) • Bend radius is not less than 3Do. The actual wall thickness is to be increased as

per Table‐2‐1 above.

Following data may be used

P = design pressure (= 195 kg/cm2) S = maximum allowable stress of S.S. 304L material at 310oC temp. (= 986 kg/cm2) Y = 0.4

By putting the above variables, the minimum wall thickness for different sizes (Do) of straight tubing is tabulated in following Table‐2‐2.


Nirbhay Gupta 21

TABLE – 22 Tube O.D. (Do) mm

Required minimum wall thickness of straight instrument tubing (tm) in mm (for design pressure of 195 kg/cm2 and design temp. of 310oC)

Minimum recommended wall thickness (mm) prior to bending (1.25 tm )

Specified wall thickness as per (PB‐

M‐17) in mm

6 0.55 0.69 1.210 0.92 1.15 1.212 1.1 1.38 1.516 1.47 1.83 1.8(See note below)20 1.83 2.29 2.525 2.29 2.86 3.0

Note: It can be seen from Tables – 22 & 23 that specified wall thickness of all sizes of tubing as per PBM17 is more than required wall thickness as per ASME Section III except for 16 mm size. As maximum pressure and temperature may not be simultaneous so 1.8 mm wall thickness instead for 1.83 mm of 16 mm size will be adequate from pressure rating considerations.

“For example, the maximum pressure & temperature in PHT system will be 125 kg/cm2 and 310oC respectively. For this application, the required minimum wall thickness for 16mm OD tube, including the bending allowance, should be 1.3 mm, which is less than specified wall thickness of 1.8 mm. Similarly, in some applications like F/M supply circuit, the maximum pressure and temperature may be 195 kg/cm2 and 40oC respectively. For this service also, the minimum required wall thickness including the bending allowance for 16mm OD tube should be 1.62mm which is less than specified wall thickness of 1.8 mm”.

2.6.2 STRESS ANALYSIS OF TUBING SYSTEMS (TUBING CONFORMING TO PBM17)

2.6.2.1 ANALYSIS FOR SUSTAINED MECHANICAL LOADS When the tubing is installed in the field, the effects of pressure, weights and other sustained mechanical loads must meet the requirements of eq. (4) i.e. Stress due to Sustained load = Ssl < 1.5 Sh The above equation may be verified for different sizes of tubing having wall thickness as given in Table‐2‐2 and other constants to be calculated/taken as below: B1 = 0.5 (as per NB – 3680) 2 .

/ and

2

Where tn = nominal wall thickness of tube


Nirbhay Gupta 22

R = Bend radius r = (Do – t)/2 = mean radius of tubing

32

–

Thus for different sizes of tubing systems Ssl value is tabulated in Table‐2‐4

2.6.2.2 ANALYSIS FOR OCCASIONAL LOADS (LEVEL A&B SERVICE LIMITS) As per requirement of ASME – Section III installed tubing system should satisfy the equation (5) of Section 4.2.1 as given below:

1 2 1.8

Based on the seismic analysis carried out for different tubing layouts, the recommended conservative value of Mb is 200 kg mm for all sizes of tubing systems for SSE level of earthquake. Thus for different sizes of tubing systems Sol value is tabulated in Table‐2‐4.

This can be seen that Sol is less than 1.8 Sh for all the sizes of tubing thus satisfying the above equation.

2.6.2.3 ANALYSIS FOR STRESS DUE TO THERMAL EXPANSION AND OTHER SUSTAINED LOADS As per requirement of ASME Section III installed tubing system should satisfy the following equation

4 0.75

The maximum value of stress (iMc/Z) due to thermal loading (temperature variation from 25oC to 310oC) for different tubing systems comes out to be 1600 kg/cm2 provided that tubing system is supported as per recommended practices. Based on the above data and other parameters/constants, Ste has been calculated & tabulated in TABLE‐2‐3 for different sizes of tubing.

This may be seen from the table that Ste value for different sizes of tubing is less than the value of Sh + SA (viz. 2615 kg/cm2).

TABLE 23 Tube Size Nominal Wall Calculated value to be STE Sh + SA


Nirbhay Gupta 23

(OD) mm Thickness (tn) (mm) (See note2)

used for analysis (kg/cm2)

6 1.2 0.42 1.33 2164.94 10 1.2 0.67 1.33 2169.92 12 1.5 0.65 1.33 2205.82 261516 1.8 0.71 1.33 2112.3 20 2.5 0.65 1.33 2043.24 25 3 0.67 1.33 2045.4

Note: 1. The values of MA, Z, P, Sh used for calculation of STE are same as given in Table24. 2. The value of used is based on requirement such that 0.75 should not be less than 1.0 3. SA = f (1.25 Sc + 0.25 Sh) where f = 1 & Sc = 1106 (kg/cm2)


Nirbhay Gupta 24

TABLE24

Tube size OD (Do) mm

Wall Thickness (t) (mm)

Calculated B2

2.

B2 value to be used for analysis (

as B2 cannot be less than 1)

Z (mm

3 )

32

–

WL (wt. of 1m of tube filled with D

2O) kg/m

WF (wt. of fitting) (kg)

Ma(kg/m

m)

}

Ssl (kg/cm

2 )

B1PDo

2tn

B2M

aZ

1.5Sh (kg/cm

2 )

Whether Ssl < 1.5 Sh

Mb (Kg mm) 1

2 2

1.8 S h

Whether Sol < 1.8 Sh

1 2 3 4 5 6 7 8 9

6 1.2

0.61

1.0

18.46

0.156

0.192

67.5

475.34

Yes

1558.76

Yes

10

1.2

0.97

1.0

65.42

0.315

0.32

119.37

449.07

Yes

754.78

Yes

12

1.5

0.94

1.0

115.97

0.466

0.384

154.25

383.7

Yes

200

556.17

1774.8

Yes

16

1.8

1.03

1.03

257.06

0.775

0.512

224.87

383.4

1479

Yes

463.54

Yes

20

2.5

0.93

1.0

536.89

1.293

0.64

321.62

310.5

Yes

347.75

Yes

23

3 0.94

1.0

1022.21

1.971

0.8

446.37

310.27

Yes

329.83

Yes


Nirbhay Gupta 23

2.7 Consideration for various forces The design of tubing/piping systems for sensing lines should take account of all

the forces and moments resulting from thermal expansion and contraction and

from the effects of expansion joints if any.

2.8 Tube Bending Considerations Bend radius in instrument tubing/piping should be subject to following

limitations;

i) Minimum wall thickness at any point in the completed bends should not be less than required minimum wall thickness for the design pressure.

ii) The ovality of instrument tubing/piping after bending should not exceed

8% as calculated below:

Ovality (%) = 100 (D max – D min)/Do

Where –

Do = Nominal O.D. of tube/pipe

Dmin = The min. outside diameter of tube/pipe after bending

Dmax = The max. outside diameter after bending

The above requirements are met if bend radius is more than 3D o.

2.9 Special design aspects to meet the requirements of class-I tubing

and tubing systems In addition to the general requirements of impulse connections as mentioned above, the following requirements should also be met for impulse connections for pressure/differential pressure measurement in safety and safety related systems.

For safety and safety-related systems the safety classification of instrument

sensing lines including the first accessible isolating valves should at least remain

the same as that of process systems, and from the valves up to instruments they

should meet at least the requirements of ANSI-B-31.1.

SS tubes should meet the design intent of ASME Section III sub-section NB/NC.


Nirbhay Gupta 24

For seismic classification the instrument sensing lines should be of SSE Category for safety and safety-related instrumentation systems.

A single instrument sensing line should not be used to perform both a safety-related function and a non safety-related function unless the following can be shown:

a. The failure of the common sensing line would not simultaneously 1. cause an action in a non-safety-related system that results in a plant

condition requiring protective action and 2. also prevent proper action of a protection system channel designed

to protect against the condition. Tubing system should be such that the failure of non safety impulse line/tubing should not affect the reading of safety system.

2.10 CONCLUSION 1) MATERIAL SELECTION

a. Based on the requirements of corrosion resistance, tensile strength,

hardness and weldability, austenitic stainless steel grade SS-304L material

as per ASTM A-213/SA655 has been selected and specified for instrument

tubing. Also the instrument SS tubing should be seamless, cold finished and

full annealed. From welding consideration the tubing should have delta

ferrite of 5 to 10%.

b. Based on the requirements of different applications the tubing in different

sizes have been specified i.e. OD of 6mm, 10mm, 12mm, 16mm, 20mm and

25mm.

2) NON-DESTRUCTIVE INSPECTION

All finished tubing should be inspected by ultrasonic or eddy current methods or any combination of these methods in accordance with the requirements of NB-2550.

3) Based on the analysis of tubing systems carried out above for our installations the

stress values for different loading (service limits) are well within the required

limits.

4) Thus, if SS 304L instrument tubing are supplied as per specification above and

installation of tubing systems is done as per recommended practices(see

section-10) then instrument impulse tubing systems will be meeting the intent

of ASME Section III-Sub-Section NB-Class I components.


Nirbhay Gupta 25

3.0 Technical Requirements of SS tubes

Following design requirements should be specified while ordering SS tubes.

1. TYPE : SEAMLESS, AS PER ASTM-A213

2. MATERIAL : SS 304L

3. SIZE & THICKNESS : As per the Table below

Out side

dia. [mm]

Wall thickness [min]

and tolerance [mm]

Tolerances on O.D.

[mm]

Length of tube

Pieces

6

10

12

16

20

25

1.2 + 15%; -0%

1.2 + 15%; -0%

1.5 + 10%; -0%

1.8 + 10%, -0%

2.5 + 10%, -0%

3.0 + 10%; -0%

-00 + 0.10

-00 + 0.10

-00 +0. 10

-00 +0. 12

-00+ 0. 12

-00 + 0.12

6 meters

4. FLUID : Water/Steam/Lube oil

5. MAX. PRESSURE: 200 kg/cm2(g)

6. MAXIMUM TEMPERATURE : 320ºC

7. OVALITY VARIATION: < 8.0% OF O.D.

8. HARDNESS : > ROCKWELL B-65 & < ROCKWELL B-80

9. SURFACE FINISH: BETTER THAN 8.2 microns FOR O.D. & I.D.

10. MECHANICAL PROPERTIES :

a. TENSILE STRENGTH : ≥ 4920 kg/cm2 (g)

b. YIELD POINT : ≥1760 kg/cm2 (g)

c. ELONGATION % IN 50MM GAUGE LENGTH : ≥ 35 %

11. TYPE TESTS

a. HARDNESS TEST : On one test piece of each size and each batch as per

relevant ASTM standard


Nirbhay Gupta 26

b. EXPANSION TEST : On one sample piece of each size and each batch as per


c. TENSILE TEST : On one sample piece of each size and each batch as per


d. FLATTENING AND DOUBLING OVER TEST : On one sample piece of

each size and each batch as per relevant ASTM standard

e. CHEMICAL ANALYSIS : One sample of each batch as per relevant ASTM

standard

12. ROUTINE TESTS

a. DIMENSIONAL TEST : Required to be done on 10 % of the lot

b. HYDROSTATIC TEST : At pressure of 300 kg/cm2 (g) for 10 min. required to

be done on each sizes of each batch

13. LENGTH OF EACH TUBE: 6 meters

Relevant standards for SS tubes Following standards should be followed while specifying or testing SS tubes.

Sr.

No.

Code/Standard Description

1. ASTM-A-213 Seamless Ferritic & Austenitic Alloy Steel Boiler , Super

heater & Heat Exchanger Tubes

2. ASTM-A-450 General requirement for carbon, Ferritic & Austenitic Alloy

steel Tubes.

3. ASTM A 262 Standard Practices for Detecting susceptibility to inter-

granular attack in stainless steel .

4. ASTM A 370 Standard test method and definitions for mechanical testing

of steel products.

5. ASME SEC. III NB 2550

Examination & Repair Of seam less and welded (without

filler metal) tubular products and fittings

6. PB-M-17 Specifications for Seamless Austenitic SS tubes


Nirbhay Gupta 27

4.0 Pneumatic Tubing

Copper tubes are primarily used for pneumatic connections. Earlier pneumatic

instruments were more popular and used (controllers, transmitters, indicators etc.). Thus

pneumatic tubing was used widely. However now-a-days most of the instruments that

are used are electronic instruments, thus the use of pneumatic tubing is limited. Still, at

present this is used to connect the pneumatic actuator and its accessories viz. positioners,

I/P converters, solenoid valves etc. which are quite important from plant operation point

of view. Pneumatic instruments are still prevalent in hazardous areas. Even though the

pneumatic instruments are passé, they still provide a very reliable alternative to

electronic instruments.

4.1 Advantages of using copper tubes Strong, corrosion resistant, copper tube is the leading choice for pneumatic piping. There are seven primary reasons for this:

1. Copper is economical. Easy handling, forming and joining permits savings

in installation time, material and overall costs. Long-term performance and

reliability mean fewer callbacks, and that makes copper the ideal cost-

effective tubing material.

2. Copper is lightweight. Copper tube does not require the heavy thickness of

ferrous or threaded pipe of the same internal diameter. This means copper

costs less to transport, handles more easily and, when installed, takes less

space.

3. Copper is formable. Because copper tube can be bent and formed, it is

frequently possible to eliminate elbows and joints. Smooth bends permit the

tube to follow contours and corners of almost any angle. With soft temper

tube, particularly when used for renovation or modernization projects,

much less wall and ceiling space is needed.

4. Copper is easy to join. Copper tube can be joined with capillary fittings.

These fittings save material and make smooth, neat, strong and leak-proof

joints. No extra thickness or weight is necessary to compensate for material

removed by threading.

5. Copper is safe. Copper tube will not burn or support combustion and de-

compose to toxic gases. Therefore, it will not carry fire through floors, walls

and ceilings. Volatile organic compounds are not required for installation.


Nirbhay Gupta 28

6. Copper is dependable. Copper tube is manufactured to well-defined

composition standards and marked with permanent identification so you

know exactly what it is and who made it.

7. Copper resists corrosion. Excellent resistance to corrosion and scaling

assures long, trouble-free service, which means satisfied customers.

4.2 Different types of copper tubes

Table 4-1 below identifies the six standard types of copper tube and their most

common applications2. The table also shows the ASTM Standard appropriate to the

use of each type along with a listing of its commercially available lengths, sizes and

tempers.

Types K, L, M, DWV and Medical Gas tube are designated by ASTM standard sizes,

with the actual outside diameter always 1/8-inch larger than the standard size

designation. Each type represents a series of sizes with different wall thicknesses.

Type K tube has thicker walls than Type L tube, and Type L walls are thicker than

Type M, for any given diameter. All inside diameters depend on tube size and wall

thickness.

Copper tube for air-conditioning and refrigeration field service (ACR) is designated

by actual outside diameter.

“Temper” describes the strength and hardness of the tube. In the piping trades,

drawn temper tube is often referred to as “hard” tube and annealed as “soft” tube.

Tube in the hard temper condition is usually joined by soldering or brazing, using

capillary fittings or

by welding. Tube in the soft temper can be joined by the same techniques and is also

commonly joined by the use of flare-type and compression fittings.

It is also possible to expand the end of one tube so that it can be joined to another by

soldering or brazing without a capillary fitting—a procedure that can be efficient

and economical in many installations.

Tube in both the hard and soft tempers can also be joined by a variety of

“mechanical” joints that can be assembled without the use of the heat source

required for soldering and brazing.


Nirbhay Gupta 29

Table-4- 1

1. There are many other copper and copper alloy tubes and pipes available for specialized applications. 2. Individual manufacturers may have commercially available lengths in addition to those shown in this table. 3. Tube made to other ASTM standards is also intended for plumbing applications, although ASTM B 88 is by far the most widely used. ASTM Standard Classification B 698 lists six plumbing tube standards including B 88. 4. Available as special order only.

4.3 Recommendations for selection of a type of copper tube It is up to the designer to select the type of copper tube for use in a particular

application. Strength, formability and other mechanical factors often determine the

choice. Plumbing and mechanical codes govern what types may be used. When a


Nirbhay Gupta 30

choice can be made, it is helpful to know which type of copper tube has and can

serve successfully and economically in the following applications:

a. Underground Water Service: Use Type M hard for straight lengths joined with fittings, and Type L soft where coils are more convenient.

b. Water Distribution Systems: Use Type M for above and below ground. c. Chilled Water Main: Use Type M for all sizes. d. Drainage and Vent System: Use Type DWV for above- and below-ground

waste, soil and vent lines, roof and building drains and sewers. e. Heating: For radiant panel and hydronic heating and for snow melting

systems, use Type L soft temper where coils are formed in place or prefabricated, Type M where straight lengths are used. For water heating and low-pressure steam, use Type M for all sizes. For condensate return lines, Type L is successfully used.

f. Solar Heating: See ‘Heating’ section above. For information on solar

installation and on solar collectors, write CDA. g. Fuel Oil, L.P. and Natural Gas Services: Use Type L or Type ACR tube with

flared joints in accessible locations and brazed joints made using AWS A5.8 BAg series brazing filler metals in concealed locations.

h. Nonflammable Medical Gas Systems: Use Medical Gas tube Types K or L,

suitably cleaned for oxygen service per NFPA Standard No. 99, Health Care Facilities.

i. Air-Conditioning and Refrigeration Systems: Copper is the preferred

material for use with most refrigerants. Use Types L, ACR or as specified. j. Ground Source Heat Pump Systems: Use Types L or ACR where the ground

coils are formed in place or prefabricated, or as specified.

k. Fire Sprinkler Systems: Use Type M hard. Where bending is required, Types K or L is recommended. Types K, L and M are all accepted by NFPA.

l. Low Temperature Applications –

Use copper tube of Type determined by rated internal working pressures at room temperature as shown in Tables below. Copper tube retains excellent ductility at low temperatures to –452°F and yield strength and tensile strength increase as temperature is reduced to this point. This plus its


Nirbhay Gupta 31

excellent thermal conductivity makes an unusual combination of properties for heat exchangers, piping, and other components in cryogenic plants and other low temperature applications.

m. Compressed Air—Use copper tube of Types K, L or M determined by the

rated internal working pressures as shown in tables 4-2 to 4-9 below. Brazed joints are recommended.

Table-4-2: Rated Internal Working Pressures for Copper Tube: TYPE DWV*

Table-4-3: Rated Internal Working Pressures for Copper Tube: TYPE K*


Nirbhay Gupta 32

Table-4-4: Rated Internal Working Pressures for Copper Tube: TYPE L*

Table-4-5: Rated Internal Working Pressures for Copper Tube: TYPE M*


Nirbhay Gupta 33

Table-4-6: Rated Internal Working Pressures for Copper Tube: TYPE ACR*

NOTE: * Based on ‘S’, the maximum allowable stress in tension (psi) for the indicated temperatures (°F). ** When brazing or welding is used to join drawn tube, the corresponding annealed rating must be used. ***Types M and DWV are not normally available in the annealed temper. Shaded values are provided for guidance when drawn temper tube is brazed or welded.


Nirbhay Gupta 34

4.4 Technical Requirements of Copper tube Following parameters are to be specified while preparing the specifications for

copper tubes for pneumatic piping.

1. Type : Annealed Copper, Seamless copper tubes as per ASTM-B-68M.

2. Length: 15 m< L < 80 meters

3. Size (mm):

Out Side Dia. in mm

Tolerance On OD In mm Wall Thickness[In mm] And Tolerance

6 +0.1

-0

1.2 + 0.16

-0

10 +0.1

-0

1.2 + 0.16

-0

12 +0.1

-0

1.2 + 0.16

-0

20 +0.12

-0

1.5 + 0.20

-0

25 +012

-0

1.5 + 0.20

-0

Note: Though from pressure rating consideration the thickness requirement may

be less but while deciding the thickness due consideration is to be given to the

strength.

4. Fluid: Air /oil / water

5. Max. Pressure: 8.5 kg/cm2(g)

6. Max. Temperature: up to 100oC

7. Hardness: Rockwell F50

8. Ovality Variation : < 0.7% Of O.D.

9. Surface Finish : Better than 8.2 Microns For O.D & I.D

10. Mechanical Properties:

a. Tensile Strength: 2200 kg/Cm2 (g)

b. Yield Point: 650 kg/cm2 (g)

c. Elongation (%) in 50 mm Gauge Length: 40%


Nirbhay Gupta 35

11. Tests

11.1 Type Tests a. Hardness Test: On one test piece of each size and each batch as per

ASTM-E-18 b. Expansion Test: On one sample piece of each size and each batch as per

ASTM-B-153 c. Tensile Test: On one sample piece Of Each Size And Each Batch As Per

ASTM-E-8M d. Flattening And Doubling Over Test: On One sample piece Of Each

Size and Each Batch As Per BS-2871 & ASTM-E-255 e. Chemical Analysis: one sample of each batch as per ASTM-E-53 & ASTM-

B-55M

11.2 Routine Tests a. Dimensional Test: Required to be done on 10% of the lot

b. Hydrostatic Test: At pressure of 50 kg/cm2(g); for 10 min. Required to

be done on each size each batch

c. Pneumatic Test: At a pressure 8.5 kg/cm2 (g); for 10 min. Required to

be done on each size each batch.

4.5 Applicable international standards for copper tubes Besides NPCIL specifications following international codes and standards may be

referred while specifying copper tubes.

ASTM-B-68M: Standard specification for seamless copper tube, bright Annealed [metric] ASTM-E-8M: Standard test Method for tension testing of metallic materials [metric] ASTM-E-18: Standard test method for Rockwell hardness and Rockwell superficial hardness of metallic materials ASTM-E-53: Method for chemical analysis of copper ASTM-B-153: Standard test method for expansion [pin test] of copper and copper alloy pipe and tubing ASTM-E-243: Standard practice for electro-magnetic [eddy current] examination of copper and copper alloy tubes. ASTM-B-251M: Standard specification for general requirement for wrought seamless copper and alloy tubes [metric] ASTM-E-255: Practice for sampling copper and copper alloy for determination of chemical composition. BS-2871: Copper and copper alloys tubes


Nirbhay Gupta 36

5.0 ASTM Tubing Specifications outside diameter/wall thickness

It is important to understand that both of the above can affect the ferrule(s) ability to seal on the tubing. It is recommended to order tubing manufactured to the plus (+) side of the outside diameter tolerance. Wall thickness variations can affect pressure ratings and flow characteristics. The following tables should explain the allowable variations.

ASTM Dimensional Specifications for Tubing Table-A: Permissible Variations in Outside Diameter

Table-B: Permissible Variations in Wall Thickness

Table-C: Permissible Variations in Wall Thickness for ASTM B68 and ASTM B75 cold

drawn copper tubes

Table-A Permissible Variations in Outside Diameter


Nirbhay Gupta 37

Table-B: Permissible Variations in Wall Thickness

Table-C: Permissible Variations in Wall Thickness for ASTM B68 and ASTM

B75 cold drawn copper tubes


Nirbhay Gupta 38

6.0 EMBEDDED PENETRATIONS

Whenever the tubes have to pass through some floor slab or a wall an embedded

penetration is used. This serves two purposes viz.

a. Supporting the tube

b. Making a leak tight passage. This prevents streaming of radioactivity from an

active area to a non-active area

Guidelines for EP Design

Following general guidelines should be followed:

(a) There should be enough space between two penetrations so that tube fittings (bore-through connector) can be installed easily using spanner or a suitable tool.

(b) Lead shielding balls are filled in the floor penetration EPs.

(c) In floor penetration EPs bore through connector is installed only on the top opening of the EP. Bottom opening is left as it is. This allows thermal expansion of the tube through the EP and no additional stress is caused on the EP.

(d) In the top plate of a floor EP a hole is made and a suitable plug is installed in

it. This hole is used to fill the Steel/lead shielding balls in the EP.

(e) In the bottom plate of a floor EP a hole is made and a suitable plug is installed in it. This hole is used to take off the Steel/lead shielding balls from the EP.

Note: For further details on the EP installation refer section 12. 0 of this

technical note.


Nirbhay Gupta 39

7.0 METHODS OF CONNECTION OF INSTRUMENTATION TUBES

Different types of connections/joints can be used in instrument sensing lines as discussed

below:

7.1 WELDED JOINTS

a) Butt-welded joints should be made wherever possible.

b) Socket-welded joints are permitted but limited to tube/pipe size of 50 mm

and less. Socket welded joints should conform to requirements of ANSI-B-

16.11. While performing socket-welding, approximately 1.6 mm should be

provided between the end of pipe/tube and bottom of socket, before

welding.

c) Socket welds should not be used where the existence of crevices could

accelerate corrosion.

7.2 FLARED, FLARELESS & COMPRESSION JOINTS

Flareless and compression type tube fittings may be used in instrument sensing

lines for the tube sizes not exceeding 25mm (1”) O.D. provided following

conditions are met.

a) Fittings and their joints should be of compatible material with the tubing or

pipe material with which they are used and should conform to the range of

wall thickness and method of assembly recommended by the

manufacturers.

b) Tube fittings should be used at pressure-temperature ratings, not exceeding

the recommendations of the manufacturer.

c) Wherever compression type flareless tube fittings are used, they should be

made of design in which the gripping members or sleeve shall grip or bite

into the outer surface of the tube hold the tube against pressure (to prevent

blow out) but, without seriously deforming the inside diameter. The

gripping member or sleeve should form a pressure seal against a fitting

body.


Nirbhay Gupta 40

d) Tube fittings should be installed in accordance with manufacturer’s

recommended procedures.

e) The fitting parts of the original flareless tube fittings should be designed and

manufactured by same company. Replacement parts from different

manufacturers should be qualified to meet and seal properly at the design

service conditions, or the complete fittings should be replaced when needed.

f) The fittings selected should not degrade the inherent strength of the tubing

specified.

g) Service conditions such as vibration and thermal cycling should also be

considered in the application.

h) Metal tubing material should be softer than fitting material. For example,

stainless steel tubing should not be used with brass fittings.

i) When tubing and fittings are made of the same material, tubing must be

fully annealed and ferrule of fittings should have more hardness than tubing

material.

7.3 THREADED JOINTS

Threaded joints in which the threads provide the only seal may be avoided as far

as possible. Threaded joints should not be used between the process taps and

accessible isolating valves. However, threaded joints between accessible isolating

valve and instrument may be used within the limitations specified below:

a) Thread size should not exceed ¾” (N.B.)

b) Pipe threads should be taper pipe threads in accordance with applicable

standard. Suitable sealant should be used on pipe threads depending upon

the type of threads.

c) Threaded joints should not be used when severe erosion, crevice corrosion,

shock or vibrations are expected to occur.

d) In case of steam or hot water if service temperature is more than 100oC,

threaded connections should be used only if process pressure is less than

100 kg/cm2.

e) Threaded connections should not be used at temperature above 495oC.


Nirbhay Gupta 41

8.0 GUIDELINES FOR TAKE OFF CONNECTIONS FOR SENSING LINES

Following guidelines pertaining to the take off connections (pressure taps) should be

followed.

8.1. LOCATION OF PRESSURE TAPS a. In general the location of pressure taps in a pipe line having flowing fluid,

should be at a point where the flow is uniform. b. Location of pressure taps should be at least 5 diameters downstream from

a symmetrical pipe fitting (as a reducer) and at least 10 diameters from any unsymmetrical fitting (as a tee, elbow or valve). It should also be at least 2 diameters upstream from any fitting or irregularity.

c. LOCATION OF PRESSURE TAPS IN HORIZONTAL PIPES/VESEL:

I. GAS AS PROCESS FLUID: The location of pressure taps in the vertical meridian upwards is preferred mainly for wet gas. For following draining to occur, taps location angle should be less than 450 off the vertical meridian plane.

II. LIQUID AS PROCESS FLUID: The pressure taps should be located in a

meridian plane with which the horizontal meridian is forming an angle not greater than 45O above or below according to the position of the measuring/sensing device. If the liquid is clean, it is advisable to avoid the risk of gas in impulse

line by using tap location below the pipe horizontal meridian plane. If

on the other hand the liquid has significant solid content, then a

position above the horizontal centre line is recommended. In neither

case should the taps be more than 450 from the horizontal.

In cases where there is considerable volume of gas in liquid line and

needs special considerations a horizontal tap position should be used in

conjunction with gas vent connection and gas collecting chambers in the

impulse lines.

d. LOCATION OF PRESSURE TAPS IN VERTICAL PIPES/VESSELS In case of vertical pipes/vessel, there are generally no problems as far as the radial position of pressure taps is concerned.


Nirbhay Gupta 42

8.2. CONSIDERATIONS FOR PRESSURE TAP DESIGN A. CONSIDERATIONS:

Following aspects should be kept in mind while designing the pressure taps for

take off connections.

I. MATERIAL CONSIDERATIONS: Take of connections at the source together with attaching bosses or adapters should be made of material at least equivalent to material of process pipes/vessels to which they are attached. They should be designed to withstand full line pressure, temperature and stresses.

II. DYNAMIC RESPONSE CONSIDERATIONS: From measurement/accuracy considerations specially when the

pipes/vessels contain compressible flowing fluids, the higher size of pressure

taps are preferred for minimizing the possibility of plugging and for

minimizing the error due to friction, inertia and lag in the connections while

measuring dynamic pressures. When dynamics effects are not important

smaller tapping size and impulse tubing may be used.

III. TURBULENCE ERRORS: It may be noted that for measurement of static pressure in above case if

higher size of opening is used as static pressure tap, the laminar flow past the

tap is affected so that the velocity of tap opening is not effectively zero. Thus

the pressure at tap involves some velocity pressure and is not true static

pressure. Also large tap will produce eddies in the stream and as these

eddies form and break away downstream, the pressure at tap fluctuates even

when the undisturbed pressure is constant.

The turbulence error is also a function of edge condition of pressure tap hole.

With the burrs up stream, flow is diverted away from the hole and the

pressure sensed at the hole is less than the true static pressure. Conversely,

if the burr is on downstream edge of the hole, the pressure at the hole will

have a positive velocity effect and the pressure at the hole will be greater

than the true static pressure.

IV. VELOCITY ERRORS: In general the measurement errors are proportional to velocity pressure.

Thus when stream velocity is high, it is necessary to take great care in tap

construction with minimum possible tap diameter and special attention to

the sharpness and squareness of the edges of the hole. When the stream


Nirbhay Gupta 43

velocity is low, the size and workmanship of the tap hole are not so

important.

During fast transients, measurement errors may be more as the nozzle drop

(take off connection losses) is proportional to the square of the velocity.

However during steady state this loss can be neglected. Thus, in the system

where fast transients are expected this requirement is more crucial.

Thus based on above considerations the size of pressure tap holes should be

decided depending upon the process fluid condition, reliability of measurement,

accuracy, response time and requirement of physical strength.

8.3. RECOMMENDATIONS FOR PRESSURE TAP DESIGN

Briefly the design and sizing recommendations for pressure taps for different

applications are given below.

a. In venturi tubes/orifice plates as the low pressure tap holes are to be located in high velocity region, it is recommended to use small size of pressure tap holes uniformly for HP & LP connections as recommended in ASME/PTC-9.5 (Flow measurement).

b. In all other applications where turbulence error does not come in picture, it is recommended to use 20 mm NB pressure tap holes from the consideration of reliable measurement/minimizing possibility of plugging/choking fast response and mechanical/physical strength.

c. Workmanship of pressure taps holes plays an important role in accurate/reliable measurement of pressure signals. Proper care should be taken while machining the pressure tap holes. There should be no burrs, wire edges or other irregularities on the inside of pipe/vessel at pressure tap connections or along the edge of the hole through the pipe/vessel wall.

d. In no case shall any fittings project beyond the inner surface of the pipe/vessel wall.

e. Also there should be no change in the pressure tap hole diameter for at least a distance of 2.5 d as measured from the inner surface of the pipe/vessels ( d = inside dia. of pressure tap hole).

f. Redundant taps should be located a distance apart such that the failure of one tap does not adversely affect the reading through the other taps.


Nirbhay Gupta 44

9.0 GUIDELINES FOR ROOT VALVES

Isolating root valves are provided at take off connections to isolate the entire

measurement system from the main pipe line/vessel when necessary. Following

recommendations should be followed.

a. Root valves should not affect the pressure signal during normal operation. b. Root valves should be capable of withstanding the maximum working

pressure and temperature of the piping/vessel system to which the take off adapters or nipples are attached.

c. Isolating/root valves should be located immediately following the tapping point.

d. It is preferred to use gate valves for root valves in order to: i) Avoid trapping gas bubbles in the valve structure, in case of liquid flow ii) Avoid trapping liquid in the valve structure, in case of gas flow.

e. Root valve bore/should not be less than the inside diameter of impulse tubing/piping.

f. It is recommended that the root valves be ¾ inch unless special requirements necessitate a different size.

g. The root valves may or may not perform the function of the accessible isolation valve, dependent on its location.

h. Root valves should be of the same material as that of the pipe they are connected to.


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10.0 INSTALLATION OF INSTRUMENTATION TUBING

10.1 Best practices for impulse tubing installation

Separation between redundant instrument sensing lines should be provided

by free air space or barriers or both such that no single failure can cause the

failure of more than one redundant sensing line.

The minimum separation between redundant sensing lines should be at least

450 mm. As an alternate a suitable steel or concrete barrier can be used.

Instrument sensing lines should be run along the walls, columns or ceilings

wherever practicable, avoiding open or exposed area to decrease the

likelihood of persons supporting themselves on the lines or of damage to the

sensing lines by pipe whip, jet forces or falling objects.

Routing of instrument sensing lines should ensure that the lines are not

subjected to vibrations, abnormal heat or stress.

Where redundant instrument sensing lines penetrate a wall or floor the

required separation (Minimum 450 mm) should be maintained. Care should

be taken to ensure that the tubing/piping does not rest on or against any

abrasive surface.

They should be kept as short as possible. This is good for two reasons;

a. The speed of response is reduced for long runs

b. Resonance frequency is increased for longer tube runs. This is

detrimental from vibratory and seismic considerations.

The distance of transmission for instrumentation tubing should be limited to

16 meters only. Beyond this limit electrical or pneumatic transmission should

be used.

They should not cause any obstructions that would prohibit personnel or

traffic access.

They should not interfere with the accessibility for maintenance of other

items of equipment/instrument.

They should avoid hot environments or potential fire risk area.

Sensing lines should be located with sufficient clearance to permit sagging.


Nirbhay Gupta 46

The two impulse lines especially in case of ΔP /flow measurements should be

kept close together to avoid a false pressure difference arising from a

difference in temperature.

There should be provision for thermal expansion and contraction preferably

by tube/pipe bends, elbows, offsets or changes in direction of sensing lines.

The number of joints should be kept to a minimum.

Sensing lines should be adequately supported along its entire length.

Supports, brackets, clips, or hangers shall not be fastened to the instrument

sensing lines for the purpose of supporting cable trays or any other

equipment.

Sensing lines should continuously slope downwards towards the sensing

instruments in case of liquid and upwards in case of gases. The slope should

not be less than 1:12. The slope should be increased if the liquid in impulse

lines is more viscous than water.

Bends rather than fittings should be used to change the direction of a run of

piping or tubing. A bending tool should be used when bending the tubing in

cold condition. Fittings are permitted where the use of bends is not practical.

Sharp bends should be avoided.

While installing the sensing lines the bend radius of tubing should not be

less than 3 Do.

Tubes at different temperatures should not be run together for level

measurement. This may affect the density of fluid in reference or

measurement legs.

The instrument sensing tubing or piping runs pertaining to a nuclear safety-

related instrument channel should be identified and coded so as to identify its

channel.

Each instrument-sensing line and associated valves in this channel should

have an identification tag showing the channel and unique line or valve

identification number.

If multiple sensing lines are installed in a single tray, the tray should be

identified with the appropriate sensing line numbers, colors, etc.

Each instrument sensing line, as a minimum, should be tagged at its process

line root-valve connection, at the instrument, and at any point in between

where the sensing line passes through a wall or a floor (on both sides of such

penetration). Each valve also should be tagged.

Where tubing penetrates a radiation, fire, water, or air seal, care should be

taken to ensure that the seal is not degraded by the sensing line's seismic or

thermal movements. In addition, the mechanical properties of the seal shall be


Nirbhay Gupta 47

reviewed to ensure that the seal does not anchor the sensing line when a

guide is required.

All sensing lines including trays, supports, instrumentation, valves, and other

in-line devices should be installed to avoid contact interferences caused by

relative motion between the sensing line and other adjacent equipment or

devices. Sources of relative motion that should be considered are thermal

expansion, seismic motions, vibrations, and design-basis accidents or events.

The Code classification of the sensing line will determine the requirements for

relative motion that shall be considered.

Routing of the nuclear safety-related sensing lines shall ensure that the

function of these lines is not affected by thermal motions due to “hot blow

down” of the sensing lines. One of the following methods should be used to

ensure that the sensing-line function is not affected:

1. Demonstrate by documented analysis or calculations that the majority

of the sensing line routing is at ambient temperature, and “hot blow

down” is not a design loading.

Or

2. Design the sensing line routing using the process design temperature

as the temperature value used in the design analysis.

Routing of the nuclear safety-related sensing lines shall ensure that the

function of these lines is not affected by the movement of the main process

(piping, ductwork, equipment, etc.) to which the sensing line is connected.

One of the following methods should be used to ensure that the sensing line

function is not affected:

1. Demonstrate by documented analysis or calculations that the process

movements are negligible.

Or

2. Demonstrate by documented analysis or calculations that sufficient

flexibility has been provided to accommodate the process movements.

Flexible hose may be used in sensing lines to accommodate the process

thermal, seismic, and vibrational movements if its ratings equal or exceed the

design requirements, including service life. Installation considerations should

include maintaining slope and no low points.

Instrument sensing lines and accessories inside the Containment Building

shall withstand the pressure profile during containment leak-rate testing.


Nirbhay Gupta 48

Tubing Handling

Imperfection on the tube OD can be potential source of problems in a tubing

system. Handling of the tube shall be done very carefully to avoid scratches

and protect the finish of the tubes.

Dragging the tube across any surface that could scratch the surface can

cause seal corrosion and sealing problems. On offshore facilities

scratches on tube may lead to corrosion of SS tubing from salt water

pitting.

It is a good practice to visually inspect tubing to ensure it is free from

scratches and other damage.

When cutting the tubing hacksaw must not be used, the correct tool is a

tube cutter with a sharp blade.

Correct deburring tool shall be used for deburring both inside and

outside edge of tube ends.

It is good practice to clean the tubing with dry instrument air. If the

surface requires higher degree of cleanliness then a cleaning agent

should be used.

10.2 Some Practical Guidelines for Tube Laying and Bending 1. Measure Exactly - Bend Accurately These are the two most important rules which must be observed when fabricating a tube line. (See Figure-10-1 below)


Nirbhay Gupta 49

EXACT MEASUREMENT is required to insure that you obtain the desired distance between bends. If you do not measure exactly, the tube line will not fit. (See Figure-10-2 below)

ACCURATE BENDING is necessary to achieve the exact angles required for the tube line. If you do not bend accurately, the tube line will not fit. (See Figure 10-3 below)

2. Tube Centerline Basis for Measurement: The centerline of the tube is the basis for all tube line measurement (See Figure-10-4 below). Always measure from the centerline except from the first bend which


Nirbhay Gupta 50

is measured from the end of the tube. On most benders, the edge of the radius block is at the centerline of the tube.

3. Control Accuracy Remember only you can control the accuracy of your work. Use good, careful workmanship at all times.

10.3 Tube Bending Check list Follow this list to insure good results on each bend.

a. Measure and mark exactly. Insert tube in bender. b. Always try to bend in the same direction! If you backbend, be sure to

compensate for gain or pickup. Remember, gain always occurs to the right side of the tube radius block.

c. Clamp tubing securely in bender. d. Check to make certain length mark is tangent to desired angle on radius

block or in line with the desired degree on the link member. e. Bend accurately to the desired angle plus spring back allowance. f. Open bender, remove tube. g. Double check bend angle with triangle. h. Check measurement length with tape or ruler. i. The bending radius selected must be at least three times the outside

diameter of the tube.

Keep Track of Changes of Plane

Benders bend in only one direction. Changes in plane are accomplished by rotating the tubing in the bender. To insure that the tubing is correctly placed for the desired change in plane, a reference mark on the tube is very helpful.

Bend Direction Mark

One method for keeping track of changes in plane is to

use a longitudinal or lengthwise bend direction mark.


Nirbhay Gupta 51

(See Figure 10-5 above)Put the mark on the side opposite the direction in which you wish

to bend. When you put the tube in the bender, center the mark face up in the groove of

the radius block. (See Figure 10-

6)This will insure that you bend

in the correct direction. It also

gives you a reference mark in

case you must leave your work

unfinished.

Marking the Tube Whenever you make a mark on tubing, use a sharp pencil. Use a ferrule as a guide to make measurement marks all the way around the tube so that the mark is always visible. (See Figure 10-7) Don’t use grease pencils or crayons as these make too wide a line which can easily affect accuracy.

Measure and Mark

Never use a sharp tool to scratch marks onto tubing. Scratches create points where corrosion or stress concentration can ruin or dangerously weaken the tube.

Rules for Positioning Tubing in Bender

A line which is tangent to the desired angle mark on the radius block and which passes through the measurement mark at the centerline of the tube, is used to control the distance between bend centerlines. (See Figure below)


Nirbhay Gupta 52

Tube Positioning Rules 90° angles - tangent flush with length mark (refer to dotted line xy tangent to radius block @ 90° fig. 10-8 (above). Angles less than 90° - tangent intersects length mark at centerline. Angles more than 90° - position for a 90° bend and continue on to desired angle, i.e. 135°, 145°. (i.e. Length mark @ 90° on link member) Horseshoe or u-bends – measure first leg, position for 90°, bend around to 180°.

Compensate for springback:

a. Test a piece of the material before you start fabricating a line to see how much it springs back on a 90° bend. b. Overbend by the amount of springback. For example, if the material springs back 3° on a 90° bend, bend to 93° to secure a finished 90° bend, or to 46-1/2° to obtain finished 45° bend. This works especially well with large heavy-wall tubing. c. Remember, it is always better to underbend slightly. You can always bend a little more if needed, but it’s almost impossible to remove or straighten a bend, especially with large, heavy-wall tubing. REMEMBER - A TUBE BENDER BENDS - IT CAN NOT UNBEND.

Tube Stretch or Pickup

When bent, tubing seems to stretch or pick up length. This is because it takes a curved shortcut across the inside of the angle. A good “rule of thumb” for most standard tubing materials and radius blocks is that the tubing will stretch approximately one tube diameter for each 90° bend. Always try to bend in the same direction - away from the original starting end. If you reverse the direction of bending (bending towards instead of away from the original starting end) you will “trap” the stretch. Thus,


Nirbhay Gupta 53

if you unknowingly make a reverse bend of 90°, you will trap the gain, in table 10-1 (approximately one tube O.D.) and increase your length between bends by that amount. If bend direction for either 45° or 90° bend must be reversed, subtract the “gain” amount listed in table 10-1. While our rule of thumb is approximately correct, the amount of stretch is related to the diameter of the radius block used. This chart (Figure 10-11) gives the accurate increase in length that occurs with the most commonly used sizes of radius blocks. As long as you measure and bend with the tube inserted from the left, and measure centerline, “pickup” will not affect your actual center-to- center measurement.

NOTE: 1. Some radius blocks may differ. Consult individual

radius block manufacturers for details on other radius diameters.

2. For metric tubes the size and radius can be computed in the similar way.

Pre-Measuring You may pre-measure a series of bends. Measure the first bend from the end of the tube, the correct length. Compensate for each bend after the first by subtracting the amount of gain from your chart for each 90° of bend to allow for stretch (Figure 4-11). Always custom measure for the last bend.

“Rule of Thumb” Method Compensate each measurement after the first by subtracting the gain listed in table 10-1. Best Way to Measure For maximum

Table-10-1

Radius of

Tube size (in

inches) Size Bender Gain Gain

(in inches) 90° 45°

1/8 2 3/8 .16 .02 3/16 3 7/16 .19 .02 1/4 4 9/16 .24 .02

5/16 5 11/16 .30 .03 3/8 6 15/16 .40 .04 1/2 8 1-1/2 .64 .06 5/8 10 1-7/8 .80 .08 3/4 12 2-1/4 .97 .10 7/8 14 2-5/8 1.13 .11

1 16 3 1.29 .13 1-1/4 20 3-3/4 1.61 .16

1-1/2 24 4-1/2 1.93 .19 2 32 8 3.43 .34


Nirbhay Gupta 54

accuracy, measure and bend exactly for each individual bend in the tubing line. We recommend the practice of Measure and Bend then again Measure and Bend, etc.

10.4 Characteristics of a Well-Made Tubing Circuit

In a well made tubing circuit or line, bends are accurate, measurement exact. The run is

plumb, square and level. Tube ends rest firmly in the fittings and entry into the fittings is

straight. Straight tube entry is very important to insure that fittings are not under stress

and can be assembled without leaks. (See Figure 4-13below)

Remember too, that length magnifies bend angles errors. If the leg following the bend is

fairly long, an error of 1° may result in the tube line missing the desired point completely.


Nirbhay Gupta 55

Recommended Free Tubing

Lengths

It is important to consider the length of tubing from the end in the fitting body to the beginning of the bend. Table: 10-2 lists the recommended lengths “L” and “D” for various sizes of tubes.

Table: 10-2

TU

BE

O.D

. in

ches

1/1

6

1/8

3/1

6

1/4

5/1

6

3/8

1/2

5/8

3/4

7/8

1

1-1

/4

1-1

/2

2

“L”

Fre

e L

engt

h

of

Stra

igh

t T

ub

ing

(i

nch

es)

.50

.70

.75

.80

.88

.94

1.1

9

1.2

5

1.2

5

1.3

1

1.5

0

1.9

4

2.4

1

3.2

5

“D”

Tu

be

Inse

rtio

n

Dep

th

(in

ches

)

.38

.52

.56

.61

.66

.69

.94

.98

.98

1.0

5

1.2

2

1.6

1

1.9

6

2.6

5

10.5 Common Causes of Imperfect Bends Figure A shows an ideal bend. Bends with little or no flattening are produced when correct equipment and methods are employed; when proper consideration is given to co-relationship of the radius of the bend, material wall thickness and hardness of the tube. Figure B shows a flattened bend, caused by trying to bend too short a radius, or bending smaller diameter tube in larger radius block. Figure C shows a kinked and flattened bend, caused by the tube slipping in the bender, or by using non-annealed tubing. Tubes must be firmly clamped by clamp block to prevent slippage during bending process. Figure D shows a wrinkled bend, sometimes produced when thin wall tube is bent. Breakage will sometimes occur when mandrel is too far forward in tube, or when too short a radius is attempts with hard tube.


Nirbhay Gupta 56

Offset Bends To form a tube offset, it is obviously necessary to make two bends. With the tube benders, it is easy to make double 45° bends. To make an offset bend simply follow the “Offset Bend Allowance” steps below to determine the proper distance between the two 45°

bends. Here’s the procedure. STEP 1 First, determine the total amount of offset required (dimension “F” in the diagram). STEP 2 Next, determine the angle of offset - 30° or 45°. The latter (45°) is recommended because benders are calibrated for 45° bending.

STEP 3 Figure the length of the tube required to meet your offset requirements (“L” dimension) in the diagram. For 30° bends multiply desired offset “F”x 2= 30° offset dimension “L”. For 45° bends multiply desired offset “F”x 1414=45° offset dimension “L”. STEP 4 Determine where you want the offset bend of the tube to start; and make a reference mark (A). Now measure off the “L” dimension (determined in Step 3), starting from the reference mark and make a second mark (B). You are now ready to make the bends. STEP 5 Align mark (A) with reference mark 45° on bender shoe handle (measurement end to the left) and proceed with first bend. Then align (B) with 45° mark and make second bend in proper direction (measurement end to the left). Follow previous detailed instructions for making 45° bends in one plane.


Nirbhay Gupta 57

10.6 Routing of Bends Routing of lines is probably the most difficult yet most significant of these system design considerations. Proper routing involves getting a connecting line from one point to another through the most logical path. The most logical path should: Avoid excessive strain on joints - A strained joint will eventually leak.

Figure-10-17


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Figure-10-18

Allow for expansion and contraction - Use a “U” bend in long lines to allow for expansion and contraction.

Allow for motion under load - Even some apparently rigid systems do move under load.


Nirbhay Gupta 59

Get around obstructions without using excessive amount of 90° bends. Pressure drop due to one 90° bend is greater than that due to two 45° bends.

Keep tube lines away from components that require regular maintenance.

Have a neat appearance and allow for easy trouble shooting, maintenance and repair.


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10.7 Guidelines for Copper Tube Installation Following additional guidelines for the installation of copper tubes should be followed.

Generally long runs of copper tubes are not used because of slow response.

Therefore, extension of tube length is not required. However, if it is needed union

is used (instead of brazing and welding).

Separation is being maintained between the pneumatic tubing used for redundant

valves/instruments.

Because of response time considerations 6 mm tubes are for short distances

whereas 10 mm tubes are used for air supply connections.

Pneumatic tubing for redundant instruments should be taken from different

supply headers.

Considering the strength and hardness of copper tubing, brass tube fitting

becomes the preferred choice. The tube fitting that is used is Brass compression

type single ferrule tube fitting.

The installation procedure of brass tube fitting is more or less similar to that of SS

tube fitting. However galling problem of straight threads is not there to that extent

as that of SS tube fittings.

Moisture free instrument air is recommended from the consideration of corrosion

of tubing.

Tubing should not be laid at a place where human or machine movements are

expected. As these tubes are soft they may get damaged.

10.8 Guidelines for Copper tube bending Because of its exceptional formability, copper can be formed as desired at the job site. Copper tube, properly bent, will not collapse on the outside of the bend and will not buckle on the inside of the bend. Tests demonstrate that the bursting strength of a bent copper tube can actually be greater than it was before bending. Because copper is readily formed, expansion loops and other bends necessary in an assembly are quickly and simply made if the proper method and equipment are used. Simple hand tools employing mandrels, dies, forms and fillers, or power-


Nirbhay Gupta 61

operated bending machines can be used. Both annealed tube and hard drawn tube can be bent with the appropriate hand benders. The proper size of bender for each size tube must be used. For a guide to typical bend radii, see Table10-3. The procedure for bending copper tube with a lever-type hand bender is illustrated in Figure below

10.9 Copper tube Joints Soldered joints, with capillary fittings, are used in plumbing for water lines and for sanitary drainage. Brazed joints, with capillary fittings, are used where greater joint strength is required or where service temperatures are as high as 350°F. Brazing is preferred, and often required, for joints in refrigeration piping. Mechanical joints are used frequently for underground tubing, for joints where the use of heat is impractical and for joints that may have to be disconnected from time to time. Copper tube may also be joined by butt-welding without the use of fittings. Care must be taken to use proper welding procedures.


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11.0 Impulse tube/Sensing line support

Following principals should be followed while designing the instrument sensing line

supports.

Hanger, support and clamps design should include provision for seismic, pipe whip and thermal expansion of process taps and instrument sensing lines to which the hangers, supports or clamps may be subjected during normal operation, seismic or other credible events.

Material for hanger, clamps, pads and spacers in contact with sensing lines should be compatible to avoid corrosion.

From the consideration of seismic qualification the following supporting criterion should be followed:

b) Supports should be placed at a distance of about 150 mm from each end of tube fittings as well as bends in the tubing layout.

c) Instrument isolating valves or other instrumentation valves coming in the sensing lines should be supported with suitable clamps.

d) In the intermediate tubing runs supports should be located in such a way that the maximum unsupported span is not more than 1m.

From the consideration of thermal loading, tube fittings and bends coming in the

sensing lines should not be supported. This criterion will be applicable for sensing

lines/tubing where temperature cycling is expected.

Tube Clamping Once you’ve taken the time to make good bends and installed them, it’s not enough to just let them lay suspended in mid-air. When tubing is left unsupported, shock and vibration will cause the tubing to shake, and in turn, cause the fitting to loosen and leak or even

allow tube to fall through fatigue. Tube support and clamping is a necessary requirement in the fluid power industry. Tubing can be clamped individually, in sets, and can also be stacked. The most important part of any clamping system is having enough clamps to attain the final result. That being, a well supported, vibration and noise free system. Also, most manufacturers specify SAE and JIC approved components on their


Nirbhay Gupta 63

equipment. The best way to meet these specs concerning clamps is to utilize a clamp that employs both an upper and lower unit made of metal and a rubber split bushing which surrounds the tube or pipe and fits on the inside of the clamping units. Parker Hannifin offers a tube clamp support system by the name of “ParKlamp”. ParKlamp can clamp and support tube from 1/4” to 2” and pipe or hose from 1/4” to 1-1/2”. It comes standard in steel and uses a rubber grommet around the tube for vibration dampening.

Alongside Table-11-1 is showing maximum permissible spacing between clamps. It is recommended to clamp as close to each bend of the tube as possible; and one must clamp each side. This eliminates thrust in all directions. The tube should be

clamped at 1 m distance preferably. (See class-1 Tubing design)

Table-11-1

FOOT SPACING

EQUIVALENT SPACING IN

TUBE TUBE BETWEEN METERS

O.D.” (mm) SUPPORTS (Approx.)

1/4” - 1/2” 6 - 13 mm 3 ft. .9 m

3/8” - 7/8” 14 - 22 mm 4 ft. 1.2 m

1” 23 - 30 mm 5 ft. 1.5 m

1-1/4” & up 31 & up mm 7 ft. 2.1


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12.0 Impulse tube installation through EPs

In case where tubing/piping are

penetrating shielding wall, care should

be taken to avoid personnel exposure

to radiation streaming from radioactive

sources to surrounding areas through

instrument sensing lines penetrations

in the shielding wall. To take care of

the above, sensing lines penetrating the

shield wall should be passed through

embedded parts and surrounded by a

pipe sleeve in the shielding wall. The

open space between the pipe

Figure-12-1

sleeve and the sensing lines should be filled with the suitable radiation absorbing

material. Sensing lines (tubing/piping) should pass the E.P. through seal plate and

suitable sealing arrangement should be

provided on the seal plate using suitable

fittings/bore through tube fittings etc.

Figure-12-2


Nirbhay Gupta 65

13.0 TUBE FITTINGS

Tube fittings are used to join or connect a tube end to another member, whether that

other member be another tube end such as through T-fittings and elbow fittings, for

example, or a device that needs to be in fluid communication with the tube end, such as

for example, a valve.

Any tube fitting must accomplish two important functions within the pressure,

temperature and vibration criteria that the tube fitting is designed to meet. First, the tube

fitting must grip the tube end so as to prevent loss of seal or tube blow out. Secondly, the

tube fitting must maintain a primary seal against leakage.

The requirement that a tube fitting accomplish these two functions has been the driving

factor in tube fitting design for decades. A multitude of factors influence the design of a

tube fitting to meet a desired grip and seal performance criteria, but basic to any tube

fitting design will be:

1.0 The characteristics of the tubing that the fitting must work with, including

the material, outside diameter and wall thickness; and

2.0 The tube grip and seal performance level required of the tube fitting for its

intended applications.

13.1 Requirements of a tube fitting

Tube fittings that are intended for use with stainless steel tubing, for example, are

particularly challenging to design in order to achieve the desired tube grip and

seal functions. This arises from the nature of stainless steel which, in terms of

typical commercially available tubing material, is a very hard material, usually on

the order of up to 200 Vickers. Stainless steel tubing is also used for high pressure

applications in which the tubing wall thickness is substantial (referred to in this

paper as "heavy walled" tubing). Heavy wall tubing is difficult to grip because it is

not only hard but it is also not particularly ductile. Low ductility makes it more

difficult to deform the tubing plastically so as to achieve a desired tube grip.

A tube fitting has to meet the following requirements:

Offer reliable installation over a range of field conditions, since improper

make-up and tightening remain the leading causes for leakage.


Nirbhay Gupta 66

Cope with the wide variation in tubing characteristics, including differences

in wall thickness, hardness, ovality, and burst pressures.

Deliver a predictable, consistent “feel” to installers, who sometimes judge

installation quality by effort (torque) rather than the recommended

installation practice. Fittings that require high installation torque or that

vary widely in the “feel” and effort required to achieve complete pull-up

may cause installers to improperly tighten components and severely

degrade tube fitting performance.

Fittings should be of a compatible material with the tubing or pipe material

on which they are used to avoid electrolysis and to provide acceptable weld

joints.

Tube fittings should be used at pressure-temperature ratings not exceeding

the recommendation of the tube fitting manufacturer and to meet the

environmental and process system requirements.

Tube fittings should be installed in accordance with manufacturer's

recommendations.

In the absence of any existing standards, the designer should determine

that the type of fitting selected is qualified for design conditions (including

vibration, pressure, and thermal shock and applicable environmental

conditions) or should demonstrate this by testing the fitting's ability to

perform its intended function. The fittings selected should not degrade the

inherent strength of the tubing specified.

Screwed joints in which pipe threads provide the only seal may be used, as

long as they are in compliance with the appropriate code and system

temperature and pressure requirements.

Thread sealant should be suitable for the required service conditions

considering the process media, radiation environment, and compatibility

with the materials of construction.

It should withstand the temperature and pressure cycling as per

appropriate standard (PB-E-146).

Pull Out Capability: Tube fitting should provide sufficiently robust grip on

the tube such that when a tensile load (e.g. during hydro test or during

operating conditions) is applied on it the tube does not pull out of the grip.

Generally the acceptable pull out tension load is more than four times the

hydrostatic test pressure load.

It is recommended that compression type tube fitting should not be used

for welded tubes. As in such type of tubes the hardness differs at the point

of welding. This difference makes the gripping action of the ferrule


Nirbhay Gupta 67

unreliable. (This may be noted that NAPS onwards welded tubes have

replaced by Seamless annealed tubing in all NPCIL plants/projects)

13.2 Construction of a tube fitting

Tube fittings for stainless steel tubing typically include an assembly of

a tube gripping device, often in the form of a ferrule or ferrules, or a

gripping ring-like structure, and

a pull-up mechanism for causing the tube gripping device to be installed on

a tube end so as to grip the tube end and provide a seal against leakage.

The term "pull-up" simply refers to the operation of tightening the tube fitting

assembly so as to complete the assembly of the fitting onto the tube end with the

desired tube grip and seal.

Usually a stainless steel tube fitting is first assembled in a "finger tight" condition

and then a wrench or other suitable tool is used to tighten or "pull up" the fitting

to its final initial and complete assembled condition. In some cases, especially for

larger tube sizes, a swaging tool is used to pre-install a ferrule onto the tubing. The

pull up mechanism most commonly used is a threaded connection of a female

threaded nut component and a male threaded body component, with the tube

gripping device being acted upon by these two components as they are threaded

and tightened together. The body includes a tube end receiving bore with an

angled camming surface at the outer portion of that bore. The most commonly

used camming surfaces are frusto-conical such that the term "camming angle"

refers to the cone angle of the camming surface relative to the tube end

longitudinal axis or outer surface. The tube end is axially inserted into the body

bore and extends past the frusto-conical camming surface. The gripping device is

slipped onto the tube end and the nut is partially threaded onto the body to the

finger tight position such that the tube gripping device captured axially between

the camming surface and the nut. The nut typically includes an inward shoulder

that drives the tube gripping device into engagement with the angled camming

surface on the body as the nut and body components are threadably tightened

together. The angled camming surface imparts a radial compression to the tube

gripping device, forcing the tube gripping device into a gripping engagement with

the tube end.

The tube gripping device typically is to form a seal against the outer surface of the

tubing and also against the angled camming surface.


Nirbhay Gupta 68

13.3 Types of tube fittings

Generally following types of fittings are available:

(a) Flareless Compression Type (Single Ferrule) (b) Flareless Compression Type (Double Ferrule) (c) Bite Type (d) Flared Fitting

A flareless tube fitting generally refers to a type of tube fitting in which the tube

end remains substantially tubular, in contrast to a flared tube fitting in which the

tube end is outwardly flared over a fitting component. Flared tube ends are

commonly encountered in use with plastic tubing and plastic tube fittings.

The present note is not directed to plastic tubing or tube fittings because such

fittings have significantly different challenges and material properties that affect

the ability of the fitting to both grip the tube and provide an adequate seal.

Operating pressures and temperatures are also typically substantially lower in the

plastics tubing systems. In other words, with respect to tube grip and seal,

whatever works in a plastic tube fitting provides little or no guidance for a non-

plastic tube fitting.

Among the above, the recommended fitting is flareless compression type twin

ferrule tube fitting. Because of its ease of installation and higher reliability this

type of fitting is most commonly used.

13.4 Flared Fitting

This is made up of a nut, sleeve and body with a flare or coned end. In some instances, the sleeve is used as a self-flaring option, usually on thinner wall or softer tubing materials. Compared to the original compression fitting, the flare fitting can handle higher pressures and wider system parameters. It is also available in a larger variety of materials and has a larger seal area, which provides remake capabilities in maintenance applications. However, special flaring tools are required to prepare the tubing for installation. Additionally, flaring of the tubing can cause stress risers at the base of the flare or cause axial cracks on thin or brittle tubing. Uneven tube cuts will create an uneven sealing surface.


Nirbhay Gupta 69

13.5 Flareless Bite type tube fitting

A Flareless bite type fitting consists of a body, a

special case hardened ferrule (a one-piece precision

machined ferrule) and a nut, put together in a

standard way. On assembly, the ferrule "bites" into

the outer surface of the tube with sufficient strength

to hold the tube against pressure, without

significant distortion of the inside tube diameter.

Hence, the name "bite type fitting". As used herein,

the term "bite" refers to the plastic deformation of

the ferrule into the outer surface of the tube end so

as to plastically deform and indent the tubing with

an almost cutting- like action to create a generally radial shoulder or wall at the

front end of the ferrule. This "bite" thus serves as a strong structural feature to

prevent tube blow out at high pressure, particularly for larger diameter tubing

such as 1/2"and higher.

As compared to ordinary compression joints, the ferrule holds the pipe in its place

to give a proper seal when the nut is screwed on to the body. When it is fully

tightened, the case hardened ferrule is pushed slightly in the middle where it acts

as a spring. This maintains a continuous friction between the body and nut and

which help prevent the nut from loosening under stress and repeated vibration.

Bite-type fittings are typically single ferrule in design. This requires the nose of the

ferrule to perform two functions: to bite into the tube to hold it, and to provide a

sealing element for the coupling body, an action that can easily compromise one or

both functions. A two-ferrule separation of functions (the first to seal, the second

to hold the tube) would solve this problem, as the separation would permit each of

the elements to be designed specifically for the task.

13.6 FLARELESS COMPRESSION TYPE TUBE FITTING

Over the years there have been numerous tube fitting designs that do not rely on

a "bite" type action, but rather merely radially compress the tube gripping device

against the tubing outer surface, some with the effect of indenting into the tubing

without creating a bite.

The most common commercially available stainless steel tube fittings especially

for high pressure applications have historically been of two radically distinct


Nirbhay Gupta 70

designs of the tube gripping device--single ferrule tube fittings and two ferrule

tube fittings.

13.7 SINGLE FERRULE FLARELESS COMPRESSION TYPE TUBE

FITTING

A single ferrule tube fitting, as the name implies, uses a single ferrule to accomplish both the tube grip and seal functions. For single ferrule tube fittings, the tube gripping action is usually associated with the single ferrule being designed to bow in a radially outward direction from the tube wall in the central region or mid-portion of the single ferrule body between the front and back ends thereof. The front end of the ferrule is driven against the angled camming surface of the body by the nut pushing against the back end of the ferrule. The bowing action helps direct the front end of the single ferrule into the tube end. The bowing action is also used to cause the back end of the ferrule to likewise engage and grip the tube end. This is accomplished usually by providing an angled drive surface on the nut shoulder that engages the back end of the single ferrule so as to radially compress the back end of the ferrule into a gripping action on the tube end. In some single ferrule designs, the back end of the ferrule apparently is intended to bite into the tube end. This back end tube grip is sometimes used with the single ferrule in order to attempt to improve the tube fitting's performance under vibration because the back end grip attempts to isolate down-tube vibration from affecting the front end tube bite. The use of a back end tube grip actually works against the effort to grip the tube end at the front end of the single ferrule. Ideally, the single ferrule should be completely in three dimensional compression between the nut and the camming surface of the body. Providing a back end grip actually places a counter acting tension to the single ferrule that works against the front end compression being used to provide the tube grip. Additionally, the outward bowing action tends to work against the effort to grip the tube at the front end of the single ferrule because, in order to enable the outward bowing action, the single ferrule requires a lessened mass that is adjacent the tube gripping "bite". The outward bowing action radially displaces ferrule mass from central of the ferrule body to away from the tube end. Consequently, an outwardly bowed single ferrule fitting could be more susceptible to ferrule collapse, loss of seal and possibly tube blow out at higher pressures.


Nirbhay Gupta 71

In order to achieve an adequate tube grip on stainless steel tubing, single ferrule stainless steel tube fittings have historically used a rather shallow camming angle of between 10o and 20o. This range of angles is referred to herein as "shallow" only as a term of convenience in that the angle is rather small. The shallow camming angle has been used in single ferrule fittings to obtain a mechanical advantage because the shallow angle provides an axially elongated camming surface against which to slide and radially compress the single ferrule front end to bite into the tube end outer surface. Hard stainless steel tubing material necessitated this elongated sliding camming action in order to be able to get the single ferrule to create an adequate bite for tube grip. Over the years, the single ferrule has been ‘through hardened’ or ‘case hardened’ so as to be significantly harder than the stainless steel tubing, however, the shallow camming angle is still used today in such single ferrule fittings to obtain a mechanical advantage from the ferrule sliding along the camming surface to produce the "bite" so as to assure an adequate tube grip. An example of a commercially available single ferrule tube fitting that uses a case hardened ferrule and a shallow camming angle of about twenty degrees is the CPI fitting line available from Parker-Hannifin Corporation. Another example is the EO fitting line available from Ermeto GmbH that uses a through hardened single ferrule and a twelve degree camming angle. In some single ferrule designs, a non-conical camming surface has been tried whereby an attempt is made to simply press the ferrule against the outer surface of the tube end, thereby not creating a bite. The result in such cases however is a low grip or low-pressure-only fitting that are not well suited to stainless steel fittings.

13.8 TWIN FERRULE FLARELESS COMP RESSION TYPE TUBE

FITTING

It is becoming increasingly recognized that the two primary functions of a tube fitting viz. tube gripping and sealing are at odds with each other when designing a tube fitting that can meet a desired tube grip and seal performance criteria. This is because the design criteria needed to assure that the tube fitting achieves an adequate tube grip usually works against the ability of the single ferrule to also provide an effective seal. Consequently, although single ferrule fittings can achieve adequate tube grip in some cases, this tube grip performance comes at the expense of having a less effective seal. The shallow camming angle and elongated camming surface and axial movement needed to achieve an adequate tube grip with a single ferrule fitting, however, compromises the ability of the single ferrule to achieve the seal function, especially in extreme environments and for sealing gas. This is because the front end of the single ferrule attempts to make the seal against the axially elongated camming surface. The radially outward bowing action causes a larger portion of the outer surface of the front end of the single


Nirbhay Gupta 72

ferrule to come into contact with the camming surface against which it is being driven. The result necessarily is a larger seal surface area between the outer surface of the single ferrule and the camming surface. This enlarged seal area causes an unwanted distribution of the sealing force between the single ferrule and the camming surface, and also creates a larger area for surface imperfections to allow leaks to occur. This is particularly a metal to metal seal issue (as contrasted to non-metal to non-metal seals: for example, in a plastic fitting it is usually desirable to provide an enlarged seal contact area because the more highly ductile plastic material can better form a seal between the two surfaces.) One result of this situation is that some single ferrule tube fittings have been designed with additional components and techniques to achieve an adequate seal. Less than optimum seal performance is particularly noted in single ferrule fittings that attempt to seal against gas, and especially high pressure gas. Single ferrule tube fittings thus are usually more suited to lower pressure liquid applications such as hydraulics, however, even in such lower pressure applications single ferrule seal performance remains less than desired. The double ferrule fitting has the ability to lock onto the tube with a 'double bite' feature. Each ring bites in to the tube giving two separate sealing areas. This style of fitting does so without transmitting torque or twisting the tube ensuring that the tube does not become 'stressed'. Therefore, the mechanical properties of the tube are maintained. A further sealing point occurs at the bottom of the tube abutment. The abutment has an angle which the tube is forced into when the rings bite and drive the tube forward.

13. 8.1 Ferrule and its purpose

The ferrule, perhaps the most-critical component in

compression fittings, appears rather simple. Yet it is

highly engineered and, to function properly,

requires considerable design, metallurgy, and

production expertise. Not all products on the

market meet these stringent requirements.

For instance, the ferrule must precisely deform

elastically and plastically during fitting assembly to

properly grip and seal the tubing. Its front edge

must be harder than the tubing to grip and seal through surface scratches and

defects, but if the entire ferrule is too hard, it may not deform properly.

Therefore, only the gripping edge of the ferrule is hardened while the rest has

different, tightly controlled mechanical properties. Also, the hardening process

must not compromise stainless steel's corrosion resistance. And finally,


Nirbhay Gupta 73

production processes must consistently turn out defect-free ferrules that hold

tight tolerances and maintain metallurgical specifications.

Ferrules provide a reliable, leak-proof connection in instrumentation and process

tubing systems. These tube fittings consist of four precision-machined

components: body, front ferrules, back ferrules, and nut. Ferrules make up for the

variation in the tubing material, hardness and thickness of the tube wall in order

to provide leak-proof connections in a large number of applications. They also

reduce the number of potential leak paths in the connection, boosting safety,

reliability and integrity. They also simplify assembly and maintenance.

Ferrules can generally handle pressures up to 15,000 PSI / 1,034bar. They

eliminate the time-consuming 'coning and threading' that usually needs to be

performed when applying traditional high-pressure flared fittings, allowing

fittings to be installed in seconds by simply tightening a nut.

Back and front ferrules are designed to provide leak resistant, secure and tight

connections for operations at high pressures. These fittings provide a tight

pressure seal and have a long thread area for improved resistance to pressure

and load on ferrules. Long support area of back ferrules improves resistance to

vibration and line loads.

13. 8.2 Swaging

Swaging is a metal-forming technique in which the dimensions of an item

are altered using a die or dies, into which the item is forced. Swaging is a

forging process, usually performed cold, however it can be done hot . T he

most common use of swaging is to attach fitt ings to pipes or cables ( also

called wire ropes); the parts loo sely fit together, and a mechanical or

hydraulic tool compress es and deforms the fitt ing, creating a permanent

joint . Pipe flaring machines are another example.

http://en.wikipedia.org/wiki/Metal

http://en.wikipedia.org/wiki/Die_%28manufacturing%29

http://en.wikipedia.org/wiki/Forging

http://en.wikipedia.org/wiki/Cold_work

http://en.wikipedia.org/wiki/Pipe

http://en.wikipedia.org/wiki/Cable

http://en.wikipedia.org/wiki/Wire_rope

http://en.wikipedia.org/wiki/Hydraulic


Nirbhay Gupta 74

Swaging is a process that is used to reduce or increase the diameter of tubes. A swaged piece is created by placing the tube inside a die that applies compressive force by hammering radially.

Swaging can be further expanded by placing a mandrel inside the tube and applying radial compressive forces on the outer diameter. Thus, through the swage process, the inner tube diameter can be a different shape, for example a hexagon, and the outer is still circular. Flared piece of pipes are sometimes known as "swage nipples," "pipe swages," "swedge nipples," or "reducing nipples".

13.8.3 Operation of a twin ferrule tube fitting

Function of Front ferrule

In the two ferrule fitting, the tube grip and seal functions also are separately achieved by the use of two ferrules. The forward or front ferrule provides an excellent seal even against gas, and the back or rear ferrule provides an excellent tube grip. The front ferrule achieves an excellent seal by camming against a shallow camming surface angle such as twenty degrees. This is because the front ferrule does not need to slide excessively on the camming surface in order to achieve a tube grip function. Likewise, the front ferrule is not case hardened because the primary purpose of the front ferrule is to seal and is not to bite into the tube end. Thus the relatively "softer" front ferrule achieves an excellent seal, particularly against gas, even though the body conical camming surface presents a camming angle of about twenty degrees.

Function of a Back Ferrule

The back ferrule achieves the tube grip function in the two ferrule tube fitting. The back ferrule is case hardened to be substantially harder than the tube end. Tube fittings depend on a balance of factors to ensure proper installation and performance. In a two-ferrule tube fitting design, the back ferrule moves the front ferrule forward to spring load the fitting assembly, burnish and seal with the fitting body, and create the primary tubing seal. The front end of the back ferrule cams against a frusto-conical camming surface formed in the back end of the front ferrule. The ostensible angle of this camming surface is forty-five degrees, but due to the sliding movement of the front ferrule, the effective camming angle is actually a shallow angle of about fifteen to twenty degrees. Although the effective camming angle for the back ferrule is shallow, the back ferrule is not required to provide a primary seal (although it can form secondary or backup seals). The back ferrule also does not exhibit the undesired bowing action but rather grips


Nirbhay Gupta 75

the tube end as a function of a radially inward hinging action. As used herein, the term "hinging" refers to a controlled deformation of the ferrule such that a central region or mid-portion of the ferrule body undergoes an inwardly radial compression, as distinctly contrasted to a bowing or radially outward displacement. Thus, the effective shallow camming angle not only does not compromise the fitting seal capability, it actually substantially enhances the overall performance of the tube fitting especially for stainless steel tubing. By using separate ferrules for each to achieve primarily only one of the

key tube fitt ing functions, the two ferrule tube fitt ing achieves

tremendous tube grip and seal functions.

The back ferrule also swages the tube to provide the grip needed to keep the fitting and tubing firmly in place. To swage and grip the tube properly, the back ferrule’s leading edge must be sufficiently harder than the tube. Two methods of producing this differential hardness may be employed—

1. Complete surface hardening of the back ferrule: The use of complete surface hardening on a conventional back ferrule can

have several drawbacks.

First, it typically increases installation torque because a surface-hardened, conventional back ferrule is unable to flex or “hinge” downward to improve swaging action on the tube. Instead, it must be wedged into position using installer torque, and as a result, more torque typically is required. Second, because it is not engineered to hinge and absorb installer torque on remakes, a conventional surface-hardened back ferrule can tend to overdrive the front ferrule when remade. This condition can potentially damage the tubing and fitting body and compromise the front ferrule action required for consistent gas-tight remakes.

2. Selectively hardened back ferrule: Use of a selectively hardened back ferrule, Swagelok reduced installation torque while providing the swaging and gripping action needed to perform in combination with a wide variation of commercial grade tubing. In manufacturing back ferrules selectively hardening the nose of the back ferrule is done, yet the center section and rear flange are left softer. During make-up, this softer center section acts as a hinge point when force is applied to the flange. This hinging mechanism helps limit the amount of torque required by the installer, yet delivers the right amount of swaging action through the nose of the back ferrule.

The improved engineered hinging action of the back ferrule (Figure 4, next page) provides several benefits:


Nirbhay Gupta 76

It advances and seals the front ferrule predictably and accurately.

It flexes to maintain installation torque at a predictable and manageable level, even on hard materials.

It smoothly and efficiently delivers more swaging energy earlier in the pull-up process. As a result, it reduces the potential for improper installation and leakage in cases where the fitting is less than properly tightened.

Its proprietary metallurgy and hinging action can absorb excess torque inputs to help prevent overdriving of the front ferrule, thus ensuring more predictable gas-tight sealing during remakes.

An important aspect of the cho ice of materials is that the ferrule preferably

should be cas e or through hardened to a ratio of at least about 3.3 and

preferably 4 or more times harder than the hardest tubing material that the

fitt ing will be used with. Therefore, the ferrule need not be made of the

same material as the tubing itself . For example, the ferrule may be s elected

from the stainless stee l materials or other suitable materials that can be

case hardened, such as magnesium, t itanium and aluminum, to name some

additional examples.

Fi gure 13-4: 316 SS Advanced S wage lok Tube Fi t ti ng Pri or to Make -up

The elements of the fitting are depicted in cross-section prior to make-up: the fitting nut (top), the advanced geometry back ferrule (left), the front ferrule (center), and the fitting body (right). The tube wall section is shown below the ferrules and body.


Nirbhay Gupta 77

Fi gure 13-5: 316 SS Advanced S wage lok Tube Fi t ti ng After Make -up

During make-up, the front ferrule (center) is driven into the body of the fitting (right) and the tube (bottom) to create primary seals (tube and body), while the back ferrule (left) hinges inward to create a strong grip on the tube. The back ferrule geometry allows for an improved engineering hinging action that translates axial (forward) motion into radial swaging action on the tube, yet operates with a low input force (torque) requirement. The improved radial colleting action of the back ferrule (the area to the left of the swage point) isolates and protects the swaged area of the tube, preventing the exposed vibration stress riser that is typical of bite-type fittings.

A distinct advantage of the contoured back ferrule is that pull up forces between the nut drive surface and the contoured face of the Back ferrule are more uniformly distributed across the surface of the back ferrule, thus reducing and substantially eliminating force concentrations. This further reduction of force concentrations on the drive nut reduces pull up torque and reduces galling, thus facilitating re-make of the fitting.


Nirbhay Gupta 78

13.8.4 Effect of Tube thickness on Swaging

The strength of the fitting is such that the tube contained will burst before the fitting shows any sign of a leak or movement. This is subject to certain constraints on the wall thickness of the tube. Tube thickness is decided by following factors

a. Pressure rating b. Corrosion/Threading allowance c. Swaging considerations

For swaging over thickness may lead to unreliable joint and in very thin tube it may lead to distortion of tube leading to leakage. Thus considering all the above factors, optimal thickness should be selected when use of compression type of tube fittings is envisaged. A heavy wall tube resists ferrule action more than a thin wall tube, allowing the ferrules to coin out minor surface imperfections. If the wall is too heavy the rings will not bite. A thin wall tube offers less resistance to ferrule action during installation, reducing the chance of coining out surface defects, such as scratches. When the tube wall is too thin, the tube will collapse rather than allow the rings to bite fully. Within the applicable suggested allowable working pressure table, select a tube wall thickness whose working pressure is outside of the shaded areas. Reference to the manufacturers' product information should be made in all instances. The tube should generally have a hardness of no more than 80 on the Rockwell 'B' scale.


Nirbhay Gupta 79

Advanced Swagelok Fitting

The adv a nced Swa gelok two -f errul e tu be f itti ng of f ers predict abl e, l eak f ree perf orm ance up to t he b urs t

pres s ure of ANSI 316 and 304 s tai nles s s teel tu bin g. A s um m ary of its benef its includ e:

Wi der Target for Pr oper Ins tal l ati on: The e ngi ne ered hi ngi ng actio n of t he back f errul e d eliv ers en ergy

to not only s eal t he f ront f errul e, bu t als o to de liv er grea ter s wagin g actio n thro ugho ut th e pu ll -up

proces s . As a r es ult, this f ittin g red uces th e pot ent ial f or im prop er ins tall atio n an d s y s tem leakage , ev e n in

cas es where the f itti ng was les s than prop erly tighte ne d.

Enhanced Gas Seal : The b ack f errul e h ing e d eliv ers s teady f orce to s eal t he f ront f err ule c ons is ten tly on a

wide ran ge of tu bin g. B eca us e th e adv a nce d back f err ule c an hin ge a nd a bs orb m ore e nergy th an a

conv ent iona l hard en ed b ack f errul e, th is des ign r edu ces th e pot enti al f or ov erdriv ing t he f ront f errul e,

there by ens urin g relia ble op erati on an d gas s eal f or repeat ed rem akes .

Vi brati on Fati gue R esi s tance: T he e ngi ne ered back f errul e h ing ing act ion de liv ers a m or e co ns is tent

radial col let ing ac tion to giv e im prov e d s upport to th e tub e beh ind t he poi nt of grip. This coll etin g prote cts

the s waged ar ea of the tub e m ore ef f ectiv ely f rom s y s tem v ibration and f atig ue .

Greate r Margi n of Performan ce on C omme rci al Tubi ng: Textbook calc ula tions , s uc h as Lam e’ s f orm ula

f or determ ini ng m inim um ru ptur e pres s ure of a tu be , us e th e m inim um al lowabl e ult im ate t ens il e s treng th,

m inim um allowabl e wall t hick nes s , an d m axim um al lowabl e out er diam eter f or tub e burs t c alcu latio ns —as

they s hould. Howev er , thes e c alcu latio ns of f er a cons erv ativ e es tim ate of the tube’ s pres s ure -cont ain ing

ability . In r eali ty , s tain les s s tee l tu bin g m an uf actur ers do not a lway s run t heir proc es s es f or the m in im um

requir ed m at erial s tre ngth v al ues ci ted by ASTM and other s ta ndar ds f or determ ini ng t he ru ptur e pres s ure

of a tube. Th e res ul t is s tronger, h arder t ubi ng with b urs t pres s ures of ten s ign if ican tl y high er tha n what

occurs und er leas t c as e cond itio ns . The adv a nce d Swagelok t ube f itti ng is robus t e noug h to grip a nd e xc eed

the b urs t pres s ure of th es e s trong er, av ailab le t ubi ng m at erials . I n ad ditio n, t he unif orm s urf ace -har de ned

des ign of the ba ck f er rule of f ers high corros ion res is tanc e.

C ompati bi l i ty wi th Ori gi nal Desi gn Swagel ok Tube Fi tti ngs: Th e adv an ced Swag elok f itti ng p ulls up

us ing th e s am e one-an d-one-q uart er-turn proc ed ure as the origi nal d es ign Swage lok tube f itti ng. In

addit ion, t he adv anc ed Swag elok f ittin g us es th e s am e ins tall atio n ins p ectio n ga uges as bef ore . Howev er,

what ev ery ins tall er wil l no tic e is a m ore cons is te nt f e el, f rom a m ore c ons is ten t ra nge of torq ue on ev ery

pull-u p to an ev e n m ore cons is tent , leak f ree co nn ectio n.

Appl i cabi li ty to New Al l oys: The adv an ced Swag elok f ittin g dem ons trat es it is practic al to dev el op an

eas y -to ins tall, hig h-perf orm anc e tu be f itti ng t hat c an b e bu ilt us i ng a dv ance d all oy s , s uch as s uper d upl ex

s teel, d es pit e their i ncre as ed s trengt h an d adv anc ed m ech anic al prop erti es .


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13. 8.5 Safety precautions for tube fitt ing installation

Following safety precautions should be taken while installing the tube fitting1

Do not bleed the system by loosening the fitting nut or fitting plug.

Do not make up and tighten fittings when the system is pressurized.

Make sure that the tubing rests firmly on the shoulder of the tube fitting body

before tightening the nut.

Use the gap inspection gauge to ensure sufficient pull-up upon initial

installation.

Never allow problems to go unreported.

Always use proper thread sealants on tapered pipe threads. In NPCIL a Nickel

compound based sealant is used (Never Siege compound) to avoid galling

Do not mix materials or fitting components from various manufacturers—

tubing, ferrules, nuts, and fitting bodies.

Never turn the fitting body. Instead, hold the fitting body and turn the nut.

Avoid unnecessary disassembly of unused fittings.

LUBRICATION

Stainless-steel parts that rub together under high pressure have a strong

tendency to cold weld and seize. And to form high-integrity, leak-free tubing

connections, ferrules must only slide forward during assembly and not rotate

with the nut. To prevent seizing and ensure only linear ferrule movement,

surface conditions and lubrication at the nut/ferrule and nut/body interfaces

should be precisely controlled..

All mating surfaces must be smooth and free of defects, which exacerbate

seizing. A bonded molybdenum-disulfide coating is the recommended

lubricant for many compression fittings.

Solid molybdenum disulfide readily adheres to surfaces, is noted for its

lubrication and anti-seizing properties, and the solid does not squeeze out like

liquid or soft, waxy lubricants under extreme pressure. The result is low

assembly torque and consistent performance, even with repeated remakes.

Additional tubing considerations:

Always use an insert with extremely soft or pliable plastic tubing.

Wall thickness should always be checked against the fitting

manufacturer’s suggested minimum and maximum wall thickness

limitations.


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Surface finish is very important to proper sealing. Tubing with any

kind of depression, scratch, raised portion, or other surface defect will

be difficult to seal, particularly in gas service.

Tubing that is oval and will not easily fit through fitting nuts, ferrules,

and bodies should never be forced into the fitting.

When installing fittings near tube bends, there must be a sufficient

straight length of tubing to allow the tube to be Bottomed in the fitting

(see figure-13-7). The following table indicates the minimum straight

length required.

Figure 13-7: Tube fitting at a bend

Special precautions for Gas Service

Gases (air, hydrogen, helium, nitrogen, etc.) have very small molecules that can escape through even the most minute leak path. Some surface defects on the tubing can provide such a leak path. As tube outside diameter (OD) increases, so does th e likelihood of a scratch or other surface defect interfering with proper sealing. The most successful connection for gas service will occur if all installation instructions are carefully followed and the heavier wall thicknesses of tubing on the tables-1-3 to 1-10 are selected.


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13.9 Repeated assembly and Disassembly of tube fitting

Figure-13-8: Tube fitting in assembled condition

Repeated assembly and disassembly of the tube fitting causes the reduction in the

distance between the two ferrules. As the distance between the two ferrules reduces over

a period of time the back ferrule’s spring action diminishes and a time comes when both

the ferrules touch each other and the leak tightness provided by this assembly is no

longer assured. Therefore it is necessary to keep this gap under check and whenever this

gap is found to be very little the new tube fitting and swaging should be used. The

figure:13-8 shows this type of action.


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13.10 SPECIFICATIONS FOR SS TUBE FITTINGS

1 Type : Flareless compression, twin ferrule 2 Material / type : SS-316 as per ASTMA-A-314 - forged

3 Fluid : water / steam/lube oil 4 Max. Pressure : 200 kg/cm2 (g) 5 Maximum

Temperature : 320 °C

6 Overall Dimensions

: As per ANSI-B-1.20.1

7 Hardness : > Rockwell B-90

8.0 Tests

8.1 Type Test

8.1.1 Chemical Composition test

: Incoming material as per ASTM-A-314

8.1.2 Ferrule Hardness Test

: On each size of ferrule as per ASTM-A-3145

8.1.3 Test on Raw Material

: Raw material for body, nut and ferrules should be tested for physical properties (i.e. tensile strength, % elongation, hardness)

8.1.4 Seismic Test : Required. Vibration should be in the frequency range from 1-33 Hz, Peak acceleration at frequencies 1, 1.26, 1.59, 2 and 2.5 Hz should preferably be between 1g and 3.5g or capability of shake table. For and beyond 3.75 Hz Peak acceleration should be 3.5 g. Test duration should be 30 seconds at each frequency. Frequency should be increased in step of 1/3 octave in the specified frequency range 1-33 Hz. Before and after the test the fitting assembly should be tested and should qualify all the other tests.

8.2 Routine Tests

8.2.1 Overall Dimension Thread

: Required to be done on 1% items of each type

8.2.2 Check for End Connections

:

8.2.3 Hydrostatic test : At pressure of 300 kg/cm2 (g); duration = 10 min. Required to be done on 10% items of each type

8.2.4 Pneumatic leak : At a pressure of 4 kg/cm2 (g); duration = 10 min.


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test (required to be done on 10% items of each type.) 8.2.5 Reassembly Test : Required to be done 25 times on 1% items (min. 2 nos.),

from each lot.

9.0 Applicable codes & standards:

ASME Boiler and Pressure vessel code

As per section III, Division I, Sub section NB for Class I components.

ANSI-B-1.1 Unified inch screw threads (UN AND UNR thread form) equivalent for tube end

ANSI-B.1.20.1 Pipe threads, general purpose

ANSI-B-1.20.2 ASTM-A-213 Standard specification for seamless ferritic & austenitic

alloy steel boiler super heater and Heat exchanger tubes ASTM-A-262 Standard practice for detecting susceptibility to inter

granular attack in austenitic stainless Steels. ASTM-A-276 Standard specification for stainless steels bars &shapes. ASTM-A-314 Standard specification for stainless steels billets and

bars for forging. ASTM-A-473 Standard specification for stainless steels forgings SAE – J -514 Hydraulic tube fittings ANSI-B-31.1

ASTM-A-269

ISOR-206


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13.11 SPECIFICATIONS FOR BRASS TUBE FITTINGS

1 Type : Flareless compression, twin ferrule 2 Material / type : Brass as Per ASTM-B-124 / Forged

3 Fluid : Air 4 Max. Pressure : 40 kg/cm2 (g) @ ambient temperature 5 Maximum

Temperature : 320 °C

6 Overall Dimensions

: As per ANSI-B-1.20.1

7 Hardness : > Rockwell B-90

8.0 Tests

8.1 Type Test

8.1.1 Chemical Composition test

: Incoming Material as per ASTM-E-54/478

8.1.2 Ferrule Hardness Test

: On each size of Ferrule as per ASTM-B-124

8.1.3 Test on Raw Material

: Raw material for body, nut and ferrules should be tested for physical properties (i.e. tensile strength, % elongation, hardness)

8.1.4 Seismic Test : Required. Vibration should be in the frequency range from 1-33 Hz, Peak acceleration at frequencies 1, 1.26, 1.59, 2 and 2.5 Hz should preferably be between 1g and 3.5g or capability of shake table. For and beyond 3.75 Hz Peak acceleration should be 3.5 g. Test duration should be 30 seconds at each frequency. Frequency should be increased in step of 1/3 octave in the specified frequency range 1-33 Hz. Before and after the test the fitting assembly should be tested and should qualify all the other tests.

8.2 Routine Tests

8.2.1 Overall Dimension Thread


8.2.2 Check for End Connections


8.2.3 Hydrostatic test : At a pressure of 60 kg/cm2 (g); duration = 10 min., required to be done on 10% items of each type

8.2.4 Pneumatic leak : At a pressure of 4 kg/cm2 (g); duration = 10 min.


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test (required to be done on 10% items of each type.) 8.2.5 Reassembly Test : Required to be done on 1% items (min. 2 nos., from each

lot, six times after sixth, pneumatic test)

9.0 Applicable codes & standards:

ASTM-B-124 Copper and copper alloy forging rod bar and shapes

ANSI-B-1.1 Unified inch screw threads (UN AND UNR thread form) equivalent for tube end

ANSI-B.1.20.1 Pipe threads, general purpose ANSI-B-1.20.2 SAE J 514 Hydraulic Tube fittings

ASTM-B-16 Free cutting brass rod, bar and shapes for use in screw machine.

IS 4218 Indian standard for Metric Straight Pipe Threads

Notes:

1. Pipe threads should be NPT threads as per ANSI-B-1.20.1

2. Needs the above seismic test is required on 1% of each type of fitting. This test

should be performed on limited samples taken from the lot.


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14.0 THREADS USED FOR TUBE FITTINGS

Different types of screw threads have evolved for fastening, and hydraulic systems. Of

special concern are plastic-to-metal, taper/parallel threaded joints in hydraulic circuits. A

discussion and recommendations are provided to create an awareness of diff erent types of

threads and how they are used.

14.1. Evolution of threads In the nineteenth century, many different types of screw threads were required for hydraulic and pneumatic circuits as well as fastening components. As a result, manufacturers started to devise their own fastening systems. This resulted in compatibility problems. The English mechanical engineer and inventor, Sir Joseph Whitworth devised a uniform threading system in 1841 to address the incompatibility problem. The Whitworth thread form is based on a 55 degree thread angle with rounded roots and crests.

In America, William Sellers set the standard for nuts, bolts, and screws which became the National Pipe Tapered (NPT) Thread in 1864. His 60 degree thread angle, in common use by early American clockmakers, enabled the American Industrial

Revolution. These thread forms later became the American National Standard.

The Whitworth thread form was selected as a connecting thread for pipes, which was made self sealing by cutting at least one of the threads on a taper. This became known as the British Standard Pipe thread (BSP Taper or BSP Parallel thread). The Whitworth thread is now used internationally as a standard thread for jointing low

carbon steel pipes.

The best known and most widely used connection where the pipe thread provides both

the mechanical joint and the hydraulic seal is the American National Pipe Tapered

Thread, or NPT Thread. NPT has a tapered male and female thread which seals with

Teflon tape or jointing compound.

14.2. Type of Threads Pipe threads used in hydraulic circuits can be divided into two types:

I. Jointing threads – are pipe threads for joints made pressure tight by sealing on the

threads and are taper external and parallel or taper internal threads. The sealing

effect is improved by using a jointing compound.


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II. Fastening threads – are pipe threads where pressure tight joints are not made on the

threads. Both threads are parallel and sealing is affected by compression of a soft

material onto the external thread, or a flat gasket.

14.3. Sizes

Pipe thread sizes are based on an inside diameter (ID) or flow size. For example, “1/2 –

14 NPT” identifies a pipe thread with a nominal inside diameter of 1/2 inch and 14

threads to the inch, made according to the NPT standard. If “LH” is added, the p ipe has

a left hand thread. The most common global pipe thread forms are:

NPT American Standard Pipe Taper Thread

NPSC American Standard Straight Coupling Pipe Thread NPTR American Standard Taper Railing Pipe Thread NPSM American Standard Straight Mechanical Pipe Thread

NPSL American Standard Straight Locknut Pipe Thread NPTF American Standard Pipe Thread Tapered (Dryseal)

BSPP British Standard Pipe Thread Parallel BSPT British Standard Pipe Thread Tapered

Plastic injection molded thread forms are manufactured to ANSI B2.1 and SAE J476

standards. The word “tapered” in several of the above names points to the big

difference between many pipe threads and those on bolts and screws. Many pipe

threads must make not only a mechanical joint but also a leak proof hydraulic seal.

This is accomplished by the tapered thread form of the male matching the thread form

of the female tapered thread and the use of pipe sealant to fill any voids between the

two threads which could cause a spiral leak. The bottoms of the threads aren't on a

cylinder, but a cone; they taper. The taper is 1⁄16 inch in an inch, which is the same as

3/4 inch in a foot.

Because of the taper, a pipe thread can only screw into a fitting a certain distance

before it jams. The standard specifies this distance as the length of hand tight

engagement, the distance the pipe thread can be screwed in by hand. It also specifies

another distance – the effective thread, this is the length of the thread which makes the

seal on a conventional machined pipe thread. For workers, instead of these distances, it

is more convenient to know how many turns to make by hand and how many with a

wrench. A simple rule of thumb for installing tapered pipe threads, both metal and

plastic, is finger tight plus one to two turns with a wrench. Torque installation values

can be determined as per application, but due to the variations involved in pipe joints

such as dissimilar materials of male and female threads, type of sealants used, and


Nirbhay Gupta 89

internal variations in product wall thickness, a standard torque specificatio n cannot be

generically applied.

This table shows the distances and number of turns called for in the standard. A

tolerance of plus or minus one turn is allowed, and in pract ice threads are often

routinely cut shorter than the standard specifies. All dimensions are in inches.

Table-XVI-American Standard Taper Pipe External Thread Nominal

size Actual

OD Threads per inch

Length of engagement (tightened by hand)

Length of effective thread

1/8 0.407 27 0.124 ≈ 3.3 turns 0.260

1⁄4 0.546 18 0.172 ≈ 3.1 turns 0.401

3/8 0.681 18 0.184 ≈ 3.3 turns 0.408

1/2 0.850 14 0.248 ≈ 3.4 turns 0.534

3/4 1.060 14 0.267 ≈ 3.7 turns 0.546

1 1.327 11.5 0.313 ≈ 3.6 turns 0.682

14.4. Taper/Parallel Threaded Joints Despite the standards created to maintain uniform fittings, tapered pipe threads are

inexact and during the course of use and repair the threads can become damaged and

susceptible to leakage. The area where the crest and the root of the thread meet can

form a spiral leak path no amount

of tightening will eliminate.

A pressure tight joint is achieved

by the compression in the threads

resulting from tightening. This

compression and sealing occurs in

the first few turns of the internal

thread. As wrenching takes place,

material from both the male and

female threads deform into each

other. This ensures full thread

contact which minimizes spiral

leakages. Variations between

injection molded plastic and

machined metal thread forms can

occur due to different

manufacturing processes.


Nirbhay Gupta 90

Pipe threads were originally designed as machined thread forms. With the use of

thermoplastics and plastic injection molding in the manufacture of plastic pipe thread

forms, mold shrinkage and plastic sink make it difficult to insure leak free joints. For

this reason, the use of a Teflon based sealant is recommended on all plastic pipe

threads. The most common form of sealant is Teflon tape wrapped 2 to 3 turns around

the male thread before assembly. Liquid Teflon based sealants are also used

successfully to ensure a pressure tight seal. It is always important to use care when

applying sealants to avoid introducing the sealant material into the system flow path.

The following sections show examples of how different threads are used and issues

that can arise in attempting to create a leak free connection.

When a BSP tapered male thread is tightened into a straight female thread (BSPP) the

seal can only be made at the base of the female port with 1 or 2 threads. See figure -14-

1. Sealing is compromised by the lack of thread form control in BSP specifications.

Variation in crests and roots may cause a mismatch in the thread and create a spiral

leak. Thread sealant is required to seal this combination.

Using both tapered male and female

BSPT threads would offer a better

chance of sealing since you are now

matching the taper of the male and

female thread. See figure-14-2. This

offers more threads a chance of sealing

against spiral leakage. Crest and root

control is still missing, but with thread

sealant, a pressure tight joint would be

easier to accomplish.

A number of variations of the NPT thread have been introduced to overcome the problem of spiral leakage and are known as Dryseal threads (See SAE standard J476). The best known is the NPTF (F for Fuel). With this

thread design, there are controls on the crests and roots of both the male and the female threads to ensure the crest crushes or displaces material into the root of the mating thread. The interference fit between the crest of one thread and the root of the other along with the thread flanks matching, seals against spiral leakage. Figure 14-3 shows an NPTF male tightened into an NPTF female hand tight. You can see the crest of both the male and female thread flanks meet.


Nirbhay Gupta 91

Figure 14-4 shows the NPTF male and female threads tightened approximately 1 turn past hand tight, and you can see the flanks meet and the crests are displaced into the roots. Although these threads are considered Dryseal, a Teflon tape or liquid is still recommended to aid in the assembly process. The Teflon works as a lubricant to avoid galling of the material when tightening the two threads together and also fills any voids that may cause leakage. However, in Nuclear power plants Teflon is not used since its properties deteriorate very fast under radiation conditions. A variation of the Dryseal thread is the NPSF (National Pipe Straight Fuel). It is used for internal threads and a NPTF external thread can be screwed into it to provide a satisfactory mechanical connection and a hydraulic seal. The combination of a parallel and tapered is not regarded as ideal but is widely used. High quality plastic quick disconnect couplings typically use NPT threads.

Another tapered thread is the British Standard Pipe

taper, or BSP, covered by British Standard 21. BSP

thread is commonly used for low pressure plumbing,

but is not recommended for medium and high

pressure hydraulic systems. This form uses the

Whitworth thread with an angle of 55°and a 1 in 16

taper. It is not interchangeable with the American NPT

thread, though at the 1/2" and 3/4" size, they both

have 14 threads per inch.

Problems arise when threading a NPT male thread

form into a BSP female straight thread form. The

1/16”, 1/8”, 1/4”, and 3/8” sizes have a dissimilar

pitch, which causes a misalignment of the threads. The flank angles of the threads are also

different between NPT and BSP. NPT has a 60° thread where the BSP has a 55° thread.

Figure 14-5 shows a male NPT tightened into a BSPP. Because of the smaller size of the

BSPP and the pitch difference, the NPT tightens with only a few turns.


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Figure 14-6 shows an NPT tightened into a BSPT. The NPT thread to engage further, but

pitch difference eventually causes a binding of the threads. Pitch and thread angle

differences will allow spiral leakage.

The 1/2” and 3/4” sizes in the NPT and BSP are all 14 threads per inch, and the NPT will engage the BSP fairly well. Although these threads are the same pitch and engage well there are still issues with the tread form. The thread angles and the crest and root tolerances being different will allow spiral leakage as shown in figure 14-6. These threads might be used effectively together if

an appropriate thread sealant is incorporated. Many issues arise when plastic quick disconnect couplings, with their corresponding injection plumbed into metal piped hydraulic systems. Leaks and plastic thread form failures may occur if care is not taken. When investigating a metal to plastic pipe joint failure, two factors viz. chemical attack and over tightening, need to be considered. Chemical attack can occur when improper thread sealants are used. Thread sealing is an attempt to block the spiral leak path which occurs when the crests and roots of the thread forms do not match. Anaerobic thread sealants should be avoided when sealing plastic thread forms. These sealants contain chemicals which may attack plastics. Use of a Teflon-based pipe thread sealant is a better choice for plastic threads.

Over tightening of any plastic pipe thread will have adverse affects on the function of the

joint. The major difference between plastics and metals is the behavior of polymers.

Injection-molded plastic parts continue to deform if they are held under a constant load e.g.

creep. Creep is the continued extension or deformation of a plastic part under continuous

load. Typically the plastic material in an injection-molded plastic pipe thread form will

creep from being over tightened into a female tapered port. The deformation of the part’s

internal features can lead to part failure.


Nirbhay Gupta 93

14.5. Dry Seal NPTF Threads Dryseal pipe threads are based on the USA (American) pipe thread; however, they

differ from the USA (American) pipe thread in that they are designed to seal

pressure tight joints without the necessity of using sealing compounds. To

accomplish this some modification of thread form and greater accuracy in

manufacture is required.

The roots of both external and internal threads are truncated slightly more than the

crest, i.e. roots have wider flats than the crests, so that metal to metal contact occurs

as the crests and the roots coincident with or prior to flank contact, see figure -14-7.

Thus as the threads are assembled wrenching, the roots of the threads crush the

sharper +crests of the mating threads .This sealing action at both the major and

minor diameters tends to prevent spiral leakage and pressure tight without the

necessity of using sealing compounds, provided that the mating threads are in

accordance with standard specification and tolerance and are damaged by galling in

the assembly. The control of crest and root truncation is simplified by

the use of properly designed threading tools. Also it is desirable that both for the

length. However, where not functionally objectionable, the use of a compatible

lubricant or sealant may be used to minimize the possibility of galling. This is

desirable in assembling dryseal pipe threads in refrigeration and other systems to

affect a pressure tight seal.

In order to obtain a pressure tight seal using dryseal pipe threads without a sealer, it

is necessary to hold crest and truncation of both internal and external threads

within the limits specified. Unless this is done by use of threading tools with the

crest and root truncation controlled so assure reproduction on the product of

threads, it is necessary to use a system of measuring or a system of gauging and

measuring to determine conformance.

There are two classes of Dryseal pipe threads viz. Class-I and Class-II Dryseal pipe

threads. The classes differ only in inspection requirements. For class-I threads,

inspection of roots and crest is not required while for class-Ii threads these

inspections are required.

External Dryseal threads are tapered only while internal Dryseal threads may be

either straight or tapered. Also, the thread lengths may be either standard or short

depending on the requirement of the application. Short threads are obtained by

shortening the length of the standard thread by one pitch.


Nirbhay Gupta 94

The minimum material condition as shown at the left is established by having the mating

crests and roots of equal truncation so as to assure metal to metal contact at these points

coincident with flank contact. The condition is established at the sharpest root and the

flattest crest and gives no clearance. Tolerances at the crests and the roots are established

in the direction of interference only, therefore the maximum material condition shown at

the right is established by having the extreme combination of sharpest crests and flattest

roots, which provide the maximum interference.

When threaded joints are made wrench tight, it is intended that the flanks and crests and

roots shall be in contact.


Nirbhay Gupta 95

Figure14-8-: Basic dimensions of NPTF threads


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15.0 Welding Methods

15.1 300 Series Stainless Steels May be welded by the TIG, MIG, or stick arc-weld process. TIG welding is

recommended as being best for welding Weld fitting systems because it allows

better operator control of heat penetration and filler material deposition. Stick arc

welding is not recommended in many cases because of the likelihood of excessive

burn-through and improper root penetration. In all cases where stick welding is

used, it is recommended that backing gas be used. MIG welding gives the same

characteristics as stick electrode welding with faster deposition of the filler

material.

As this process runs “hotter” than the stick process, the use of a backing gas is

mandatory. It should be noted that in welding the relatively small fitting sizes, filler

deposition rate economies are not a factor and therefore the MIG method is not

commonly applied.

15.2 C1018 Steel Fittings

May be welded by the TIG, MIG, stick and oxyacetylene methods. As scale formation

remains a problem, the use of a backing gas is still recommended.

Carbide Precipitation

When un-stabilized stainless steels are heated to 800° - 1500° F during welding, the

chromium in the steel combines with the carbon to form chrome carbides which

tend to form along the grain boundaries of the metal (carbide precipitation). This

lowers the dissolved chromium content in these areas and thus lowers their

corrosion resistance, making them vulnerable to intergranular corrosion. Carbide

precipitation is reduced by holding the carbon content of the material to a very low

value. This limits the amount of carbon available to combine with the chromium.

The “L” series (extra low carbon) stainless steels are often used for this purpose, but

their use reduces system design stress by approximately 15%. Weld fittings are

made from a select 316 series with carbon content in the low range of 0.04 to 0.07

percent. This results in a welded fitting with good corrosion resistance and a high

strength factor. All weld fittings in stainless steel are supplied in the solution-

treated condition, capable of passing ASTM-A-262 Tests for Detecting Susceptibility

to Intergranular Corrosion.


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15.3 TIG WELDING The "TIG" in TIG welding stands for Tungsten Inert Gas. But before it was called TIG"

it was given the name "Heliarc" because helium was the gas that was used when the

process was invented. But then someone discovered that argon worked better and

so it was called TIG because inert gas could refer to either helium or argon. But wait,

then someone else discovered that small additions of hydrogen worked well for

some metals. The word "Inert" no longer held true so it was decided that a new

name was required. So nowadays, the technical term for what used to be called ‘TIG’

and ‘Heliarc’ is Gas Tungsten Arc Welding or "GTAW". People still call it TIG and

even Heliarc. In fact more people call it TIG welding than Gas Tungsten Arc Welding.

TIG welding is akin to gas welding as far as welding technique in that the torch is

held in one hand and the filler rod is manipulated with the other hand. It is

considered more difficult than other arc welding processes because it requires the

use of both hands. Often times a foot pedal amperage control is also used which

adds another layer of difficulty.

A TIG torch can be either water cooled or air cooled and is designed to provide

shielding gas as well as welding current through a tungsten electrode. A ceramic

nozzle directs the shielding gas to the weld puddle and internal copper parts like the

collet and collet body hold the electrode in place. The tungsten electrode is

sharpened for applications where the arc needs to be pinpointed and for very low

amperage. The heat the melts the metal and makes the weld puddle comes from the

arc that is created between the tungsten electrode and the work piece. The arc is

shielded by argon, helium, or a mixture of the two. Sometimes for certain alloys,

hydrogen is added in small percentages to improve the way the puddle flows. The

arc is very smooth and quiet and clean when DC current is used. When the TIG

welding machine is set on Alternating current, it is slightly more noisy but still clean

and smooth.

What Metals can be welded using the TIG process?

Almost any metal can be welded with TIG. Carbon and low alloys steels like 1010

carbon steel and 4130 chromoly steel, Stainless steels like 304, 321, and 17-7ph,

Nickel alloys like inconel 718 and Hastelloy X, Aluminum alloys like 6061, 5052,

Magnesium alloys like az31b, Titanium alloys like commercially pure, and 6al4v,

Cobalt alloys like Stellite 6b and l605, copper alloys like Nibral bronze and pure

copper, All can be welding using the TIG welding process.


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15.4 ORBITAL TUBE WELDING Orbital welding was first used in the 1960's when the aerospace industry

recognized the need for a superior joining technique for aerospace hydraulic lines. A

mechanism was developed in which the arc from a tungsten electrode was rotated

around the tubing weld joint. The arc welding current was regulated with a control

system thus automating the entire process. The result was a more precision and

reliable method than the manual welding method it replaced.

Orbital welding became practical for many

industries in the early 1980's when combination

power supply/control systems were developed that

operated from 240 VAC and were physically small

enough to be carried from place to place on a

construction site for multiple in-place welds.

Modern day orbital welding systems offer computer

control where welding parameters for a variety of

applications can be stored in memory and called up

when needed for a specific application. The skills of

a certified welder are thus built into the welding

system, producing enormous numbers of identical welds and leaving significantly

less room for error or defects.


Nirbhay Gupta 99

15.4.1 Orbital Welding Equipment

In the orbital welding process, tubes/pipes are clamped in place and an orbital

weldhead rotates an electrode and electric arc around the weld joint to make the

required weld. An orbital welding system consists of a power supply and an orbital

weldhead.

Power Supply: The power supply/control system supplies and controls the welding

parameters according to the specific weld program created or recalled from

memory. The power supply provides the control parameters, the arc welding

current, the power to drive the motor in the weld head and switches the shield

gas(es) on/off as necessary.

Weld Head: Orbital weld heads are normally of the enclosed type and provide an

inert atmosphere chamber that surrounds the weld joint. Standard enclosed orbital

weld heads are practical in welding tube sizes from 1/16 inch (1.6mm) to 6 inches

(152mm) with wall thickness' of up to .154 inches (3.9mm) Larger diameters and

wall thickness' can be accommodated with open style weld heads.

15.4.2 Reasons for Using Orbital Welding Equipment

There are many reasons for using orbital welding equipment. The ability to make

high quality, consistent welds repeatedly at a speed close to the maximum weld

speed offer many benefits to the user:

Productivity. An orbital welding system will drastically outperform manual

welders, many times paying for the cost of the orbital equipment in a single

job.

Quality. The quality of a weld created by an orbital welding system with the

correct weld program will be superior to that of manual welding. In

applications such as semiconductor or pharmaceutical tube welding, orbital

welding is the only means to reach the weld quality requirements.

Consistency. Once a weld program has been established an orbital welding

system can repeatedly perform the same weld hundreds of times, eliminating

the normal variability, inconsistencies, errors and defects of manual welding.

Skill level. Certified welders are increasingly hard to find. With orbital

welding equipment you don't need a certified welding operator. All it takes is

a skilled mechanic with some weld training.


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Orbital welding may be used in applications where a tube or pipe to be

welded cannot be rotated or where rotation of the part is not practical.

Orbital welding may be used in applications where access space restrictions

limit the physical size of the welding device. Weld heads may be used in rows

of boiler tubing where it would be difficult for a manual welder to use a

welding torch or view the weld joint.

Many other reasons exist for the use of orbital equipment over manual

welding. Examples are applications where inspection of the internal weld is

not practical for each weld created. By making a sample weld coupon that

passes certification, the logic holds that if the sample weld is acceptable, that

successive welds created by an automatic machine with the same input

parameters should also be sound.

15.4.3 Industrial Applications for Orbital Welding

Aerospace: As noted earlier, the aerospace industry was the first industry to

recognize the requirement for orbital welding. The high pressure systems of a single

plane can have over 1,500 welded joints, all automatically created with orbital

equipment.

Boiler Tube: Boiler tube installation and repairs offer a perfect application for

orbital welding. Compact orbital weld heads can be clamped in place between rows

of heat exchanger tubing where a manual welder would experience severe difficulty

making repeatable welds.

Food, Dairy and Beverage Industries: The food, dairy and beverage industries

require consistent full penetration welds on all weld joints. Most of these

tubing/piping systems have schedules for cleaning and sterilization in place. For

maximum piping system efficiency the tubing must be as smooth as possible. Any

pit, crevice, crack or incomplete weld joint can form a place for the fluid inside the

tubing to be trapped and form a bacteria harbor.

Nuclear Piping/Tubing: The nuclear industry with its severe operating

environment and associated specifications for high quality welds has long been an

advocate of orbital welding.

Offshore Applications: Sub-sea hydraulic lines use materials whose properties can

be altered during the thermal changes that are normal with a weld cycle. Hydraulic


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joints welded with orbital equipment offer superior corrosion resistance and

mechanical properties.

Pharmaceutical Industry: Pharmaceutical process lines and piping systems deliver

high quality water to their processes. This requires high quality welds to ensur e a

source of water from the tubes that is uncontaminated by bacteria, rust or other

contaminant. Orbital welding ensures full penetration welds with no overheating

occurring that could undermine the corrosion resistance of the final weld zone.

Semiconductor Industry: The semiconductor industry requires piping/tubing

systems with extremely smooth internal surface finish in order to prevent

contaminant buildup on the tubing walls or weld joints. Once large enough, a build

up of particulate, moisture or contaminant could release and ruin the batch process.

Tube/Pipe Fittings, Valves and Regulators: Hydraulic lines, and liquid and gas

delivery systems all require tubing with connector fittings. Orbital systems provide

a means to ensure high productivity of welding and improved weld quality.

Sometimes the tubing may be welded in place to a valve or regulator body. Here the

orbital weldhead provides the ability to produce high quality welds in applications

with restricted access to the weld joint.

15.4.4 General Guidelines for Orbital Tube Welding

For orbital welding in many precision or high purity applications, the base material

to be welded, the tube diameter(s), weld joint and part fit -up requirements, shield

gas type and purity, arc length, and tungsten electrode material, tip geometry and

surface condition may already be written into a specification covering the specific

application.

Each orbital welding equipment supplier differs slightly in recommended welding

practices and procedures. Where possible, follow the recommendations of your

orbital equipment supplier for equipment set-up and use, especially in areas that

pertain to warranty issues.

This section is intended as a guideline for those applications where no specification

exists and the engineer responsible for the welding must create the welding set-up,

and derive the welding parameters in order to arrive at the optimum welding

solution.


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15.4.5 The Physics of the GTAW Process

The orbital welding process uses the Gas Tungsten Arc Welding process (GTAW) as

the source of the electric arc that melts the base material and forms the weld. In the

GTAW process (also referred to as the Tungsten Inert Gas process - TIG) an electric

arc is established between a Tungsten electrode and the part to be welded. To start

the arc, an RF or high voltage signal (usually 3.5 to 7 KV) is used to break down

(ionize) the insulation properties of the shield gas and make it electrically

conductive in order to pass through a tiny amount of current. A capacitor dumps

current into this electrical path, which reduces the arc voltage to a level where the

power supply can then supply current for the arc. The power supply responds to the

demand and provides weld current to keep the arc established. The metal to be

welded is melted by the intense heat of the arc and fuses together. No additional

filler material is used in this process of welding.

15.4.6 Material Weldability

The material selected varies according to the application and environment the

tubing must survive. The mechanical, thermal, stability, and corrosion resistance

requirements of the application will dictate the material chosen. For co mplex

applications a significant amount of testing will be necessary to ensure the long

term suitability of the chosen material from a functionality and cost viewpoint.

In general, the most commonly used 300 series stainless steels have a high degree of

weldability with the exception of 303/303SE which contain additives for ease of

machining. 400 series stainless steels are often weldable but may require post weld

heat treatment.

Accommodation must be made for the potential differences of different material

heats. The chemical composition of each heat batch number will have minor

differences in the concentration of alloying and trace elements. These trace

elements can vary the conductivity and melting characteristics slightly for each heat.

When a change in heat number is made a test coupon should be made for the new

heat. Minor changes in amperage may be required to return the weld to its original

profile.


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It is important that certain elements of the material be held to close tolerances.

Minor deviations in elements such as sulfur can vary the fluid flow in the weld pool

thus completely changing the weld profile and also causing arc wander.

15.4.7 Weld Joint Fit-Up

Weld joint fit-up is dependent on the weld specification requirements on tube

straightness, weld concavity, reinforcement and drop through. If no specification

exists the laws of physics will require that the molten material flow and compensate

for tube mismatch and any gap in the weld joint.

Tubing is produced according to tolerances that are rigid or loose according to the

application for which the tube was purchased. It is important that the wall

thickness is repeatable at the weld joint from part to part . Differences in

tube diameter or out -of-roundness will cause weld joint mismatch and arc

gap variations from one welding set up to another.

Tube and pipe end prep facing equipment is recommended in order to help ensure

end squareness and end flatness. Both the ID and OD should be burr free with no

chamfer.


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When two tubes are butted together for welding, two of the main considerations are

mismatch and gaps. In general, the following rules apply:

Any gap should be less than 5% of the wall thickness. It is possible to weld

with gaps of up to 10% (or greater) of wall thickness, but the resultant

quality of weld will suffer greatly and repeatability will also become a

significant challenge.

Wall thickness variations at the weld zone should not be more than ± 5% of

nominal wall thickness. Again, the laws of physics will allow welding with

mismatch of up to 25% of wall thickness if this is the only challenge but

again, the resultant quality of weld will suffer greatly and repeatability will

also become a significant issue.

Alignment mismatch (high-low) should be avoided by using engineering

stands and clamps to align the two tubes to be welded. This system also

removes the mechanical requirement of aligning the tubes from the orbital

weldhead.

15.4.8 Shield Gas (es)

An inert gas is required on the tube OD and ID during welding to prevent the

molten material from combining with the oxygen in the ambient atmosphere. The

objective of the welder should be to create a weld which has zero tint at the weld

zone ID.

Argon is the most commonly used shield gas (for the OD of the tube)and the purge

gas (for the ID of the tube). Helium is often used for welding on copper material.

Mixed gases such as 98% Argon/2% Hydrogen, 95% Argon/5% Hydrogen, 90%

Argon/10% Hydrogen or 75% Helium/25% Argon my be used when the wall

thickness to be welded is heavy (.1" or above). Using mixtures of 95% Argon/5%

Hydrogen is incompatible with carbon steels and some exotic alloys, often causing

hydrogen embrittlement in the resultant weld. As a general rule use 100% argon

gas, for simplicity and reduction of shield gas cost.

Gas purity is dictated by the application. For high purity situations where the

concern for micro-contamination is paramount, such as semiconductor and

pharmaceutical applications, the shield and purge gases must minimize the heat

tint that could otherwise be undesirable. In these applications, ultra high purity gas

or gas with a local purifier are employed. For non-critical applications, commercial

grade argon gas may be used.


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15.4.9 Tungsten Electrode

The tungsten welding electrode, the source of the welding arc, is one of the most

important elements of the welding system that is most commonly ignored by

welding systems users. While no one would refute the importance of the ignition

device on an automobile airbag, the rip cord for a parachute, or quality tires for

automobiles, the importance of tungsten electrode for quality welding is often

overlooked. Users continue to manually grind and wonder why they produce

inconsistent results. Whether in manual or automatic welding, this is the area

where manufacturing organizations can improve the consistency of their welding

output with minor effort.

The objective for the choice of tungsten parameters is to balance the benefits of a

clean arc start and reduced arc wander with good weld penetration and a

satisfactory electrode life.

Electrode Materials: For quite some time, tungsten manufacturers have added an

oxide to pure tungsten to improve the arc starting characteristics and the longevity

of pure tungsten electrodes. In the orbital welding industry, the most commonly

used electrode materials are 2% thoriated tungsten and 2% ceriated tungsten.

Safety: The safety issues of tungsten electrode material are now being looked at

more closely. Many users of the TIG welding process do not realize that the welding

electrode they use contains Thorium, a radioactive element added to the tungsten.

While the radioactivity is of a low level, it brings an issue of danger especially with

the radioactive dust generated when grinding the electrodes to a point for welding.

Alternative, non-radioactive tungsten materials are now available, such as 2%

ceriated electrodes, which often offer superior arc welding. While these materials

are commercially available they have been largely ignored until recently.

Recommended Electrode Materials: Cerium, as a base material, has a lower work

function than thorium, thus it offers superior emission characteristics. Thus, not

only do ceriated electrodes offer an advance in electrode safety, they also improve

the arc starting ability of the orbital equipment. However, as mentioned earlier, it is

always best to follow the advice of your orbital equipment manufacturer. 2%

ceriated and 2% thoriated electrodes are the most commonly recommended

materials for orbital welding equipment.

Electrode Tip Geometry: Given the ever increasing weld quality requirements of the

final weld, more and more companies are looking for ways to ensure that their


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weld quality is up to par. Consistency and repeatability are key to welding

applications. The shape and quality of the tungsten electrode tip is finally being

recognized as a vital process variable. Once a weld procedure has been established,

it is important that consistent electrode material, tip geometry and surface

condition be used.

15.4.10 Welding Basics and Set-Up

Figure-15-5: Weld Electrode tip diameter

To produce high consistent welds the Tungsten electrode must provide the

following:

1. High quality electrode material

2. The electrode tip dimensions shown must be held to close tolerances

3. The surface finish (ground or polished) of the electrode grind must be

consistent.

Welders should follow an equipment supplier's suggested procedures and

dimensions first, because they have usually performed a significant amount of

qualifying and troubleshooting work to optimize electrode preparation for their

equipment. However, where these specifications do not exist or the welder or

engineer would like to change those settings to possibly improve and optimize their

welding, the following guidelines apply:

A. Electrode Taper - This is usually called out in degrees of included angle (usually

anywhere between 14º and 60º). Below is a summary chart that illustrates how

different tapers offer different arc shapes and features:


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Sharper Electrodes Blunter Electrodes

Easy arc starting Usually harder to start the arc

Handle less amperage Handle more amperage

Wider arc shape Narrower arc shape

Good arc stability Potential for more arc wander

Less weld penetration Better weld penetration

Shorter electrode life Longer electrode life

In addition, to demonstrate graphically how the taper selection will affect the size of

the weld bead and the amount of penetration, below is a drawing that shows typical

representations of the arc shape and resultant weld profile for different tapers.

Figure-15-6: Arc Shapes and resultant weld profiles

B. Electrode Tip Diameter - Grinding an electrode to a point is sometimes desirable

for certain applications, especially where arc starting is difficult or short duration

welds on small parts are performed. However in most cases it is best for a welder to

leave a flat spot or tip diameter at the end of electrode. This reduces erosion at the

thin part of a point and reduces the concern that the tip may fall into the weld.

Larger and smaller tip diameters offer the following trade-offs:


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Smaller Tip Larger Tip

Easier arc starting Usually harder to start the arc

Potential for more arc wander Good arc stability

Less weld penetration More weld penetration

Shorter electrode life More electrode life

Tungsten Electrode Grinders and Pre-Ground Electrodes: Using electrodes pre-ground

to requirements or a dedicated commercial electrode grinder to provide electrode

tip quality and consistency offers the following benefits to the user in their welding

process:

1. Improved arc starting, increased arc stability and more consistent weld

penetration.

2. Longer electrode life before electrode wear or contamination.

3. Reduction of tungsten shedding. This minimizes the possibility of Tungsten

inclusions in the weld.

4. A dedicated electrode grinder helps ensure that the welding electrodes will

not become contaminated by residue or material left on a standard shop

grinder wheel.

5. Tungsten electrode grinding equipment requires less skill to ensure that the

tungsten electrode is ground correctly and with more consistency.

Pre-Ground Electrodes: Rather than risk electrode radioactivity issues and also

constantly endure the variability of each operator grinding the electrodes with a

slightly different touch, many manufacturing organizations have chosen to

purchase electrodes pre-ground. In addition, since a small difference in the

dimensions of an orbital electrode can produce a big difference in the weld

results, pre-ground electrodes are the preferred electrode choice to maintain the

consistency of your welding. This low cost option ensures that the electrode

material quality, tip geometry and ground electrode surface input to the welding

process is constant.


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Figur e- 15- 7: Using p re -g rou nd el ec tro des

ensu re t ha t th e el ect rod e ma te rial qual i ty , tip

geom et ry an d gro un d el ect rod e surf ace inp ut

to th e w el ding p rocess i s constant

Consult electrode charts or a pre-ground electrode

supplier to obtain the electrode diameter and tip

geometry that is most suitable for your welding

application.

15.4.11 Welding Parameter Development

Many welding equipment suppliers offer a series of pre-calculated weld programs

for a variety of tube diameters, wall thicknesses and materials. Welders should

always follow an equipment supplier's suggested procedures first, because they

have usually performed a significant amount of qualifying and troubleshooting

work to optimize electrode preparation for their equipment.

However, it is impossible for the equipment suppliers to have welding procedures

for every welding application and there will always exist a trade off in maximum

weld speed possible versus weld quality and repeatability. Where weld parameter

specifications do not exist or the welder or engineer would like to change those

settings to possibly improve or optimize their welding, the guidelines noted below

give information on how to modify the welding parameters for a desired result.

Note: The "rules of thumb" noted below are general guidelines only and will not

apply to every welding application and mix of parameters chosen. Although the

welding parameters are often chosen and changed according to the specific needs

of the application, there are some industry standards that have been developed as

starting points. Experimentation and experience will determine the final weld

parameters.

Arc Length

The arc gap setting is dependent on weld current, arc stability and tube

concentricity/ovality. The objective of the welding engineer is to keep the

electrode at a constant distance from the tube surface with sufficient gap to avoid

stubbing out.


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As a "rule of thumb" use a base arc gap of 0.010" and add to this half the

penetration required (usually the tube wall thickness) expressed in thousandths

of an inch. Thus if the tube wall is .030" then a good starting arc gap would be

0.010" + 0.015" = .025". For a wall thickness/penetration requirement of .154" the

arc gap would be 0.010" + .070" = 0.080"

Weld Speed

The weld speed is dependent on flow rate of material to be welded, and wall

thickness. The objective is to weld as fast as possible while still yielding a quality

output.

As a starting point the tungsten surface speed should be 4 - 10 inches per minute

with the faster welding speeds used for thinner wall materials and the slower

welding speeds used for heavy wall thickness. As a good starting point, use 5

inches per minute.

Welding Current

The welding current is dependent on the material to be welded, wall thickness,

weld speed, and the shield gas chosen. The objective is to achieve full penetration,

defect free welds.

As a starting point use 1 ampere current per 0.001" wall thickness if the material

is stainless steel. Thus for a 0.030" wall tubing the average weld current will be 30

amps in the first level.

Weld Current Levels

Orbital welding normally uses multiple levels of weld current to compensate for

heat building up in the tube during the welding process. If the weld current used

to initially penetrate the tubing was held at the same level for the complete weld,

the weld penetration would increase as the weld progressed around the tube,

producing too much penetration.


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Figure15-8: A Typical Weld Program current Profile ( This weld profile shows

a single level of weld time). Orbital welding normally uses a minimum of 4

levels of weld time with each level decreasing in weld amperage as the tube

heats up during the welding process

Normally orbital welding uses a minimum of 4 levels of weld time with each level

decreasing in weld amperage

Starting parameters: Set weld level 4 to be at 80% of weld level 1 amperages. Set

weld level 2 and weld level 3 to gradually decrease the current from level 1 to

level 4.

Figures 15-9 and 15-10 depict a typical weld program current profile for a 10 mm

O.D. SS tube. It may be noted that in the weld program chosen by the welder, the

time for each level is same (Impulse rate) and the average current decreases with

each level.

Arc Pulsing

Arc pulsing involves using the welding power supply to rapidly alternate the weld

current from a high (peak current) to a low (background current) value. This

creates a seam of overlapping spot welds. This technique reduces the overall heat

input to the base material and can also allow for increases in weld speed. This

welding technique brings many benefits to the welding procedure, often


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improving weld quality and repeatability. In some cases materials and weld joints

with poor fit-up that are difficult to successfully weld with a non-pulsed arc can

easily be welded with a pulsed arc technique. The result is improved weld quality

and increased output.

Figure 15-9: Weld program data sheet of a typical weld in RAPP-6


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In orbital welding, arc pulsing also offers another advantage due to the fact that

the gravity pulls the weld puddle in different directions as the weld is created

around the tube. When pulsing at peak current the base material(s) melt and flow

together, at the lower background current the puddle can solidify before becoming

liquid at the next peak current pulse. This diminishes the effect of gravity on the

molten weld, minimizes the weld sagging at the 12 and 6 o clock positions, and

reduces the molten weld puddle running/slumping downhill at the 3 and 9 o'clock

positions and effectively alters the electrode to weld puddle distance. The arc

pulsing technique thus becomes more advantageous as the wall thickness

increases resulting in a larger weld puddle.

Arc Pulsing Parameters: Arc pulsing involves four welding parameters: peak

current, background current, pulse width (duty cycle), and pulse frequency. Here


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again, opinions vary from one welding organization to another and indeed from

welder to welder. Many welders arrive at the same welding result having

somewhat different welding parameters.

It is important to understand how to choose convenient weld development

starting parameters and the effect on the weld by changing each parameter.

The primary objective is to use the benefits of weld pulsation to improve weld

quality and output.

Peak/Background Current Ratios: The peak to background current ratios basically

provides a means for the welding current to pulse from one level to another.

Industry usage generally varies from 2:1 ratios to 5:1 ratios. A good starting point

is to use 3:1 ratios, make the required weld and test other parameters to see if any

benefit can be gained.

Pulse Frequency: The pulse frequency is dependent on spot overlap required. Good

starting parameters are to attempt for a 75% spot overlap. Pulse rate for thin wall

tube is often equal to the weld speed in ipm (5 ipm = 5 pps) {pps: pulse per

second}

Pulse Width: The pulse width (the percentage of time spent on the peak current) is

dependent on heat sensitivity of material and available current from power

supply. Higher heat sensitivity requires lower pulse width % on peak current.

Standard pulse widths are often 20% to 50%. A good starting parameters would

be to set a pulse width of 35%.

Welding Parameter Development Example for 1" Tube/.030" Tube Wall

Thickness:

1. Arc Length/Gap = .010" + (0.5 x penetration required)

Starting Parameters: .010" + (0.5 x .030") = .025"

2. Weld Speed = 5 ipm surface speed

RPM = ipm/(3.1415 x dia.)

Starting Parameters: 5/(3.1415 x 1") = 1.59 RPM

3. Welding Current Levels

Level 1 = 1 amp per .001" of wall thickness for level 1 current

Level 4 = 80% of Level 1 current

Levels 2 and 3 gradually decrease the current from Level 1 to Level 4

Starting Parameters:


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Level 1 Peak Current = .030" wall thickness = 30 amps

Level 4 Peak Current = 30 amps x 80% = 24 amps

Level 2 Peak Current = 28 amps

Level 3 Peak Current = 26 amps

Background Current will be 1/3rd of peak current. Pulse width/duty cycle is 35%

4. Tungsten Electrode Diameter & Tip Geometry - Use your equipment

manufacturer's specifications or consult your pre-ground electrode supplier

The above data gives starting parameters. On completion of the first test weld, the

parameters will be modified to obtain the final result desired.


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16.0 REFERENCES AND SUGGESTED READING

1. An installer’s pocket guide for Swagelok tube fittings: Swagelok instruction manual.

2. ANSI/ASME B31.1-1980: Power piping

3. ISO-2186-1973 (2001): Fluid flow in closed conduits- connections for pressure signal transmissions between primary and secondary elements.

4. SA 655: Specification for special requirements for pipe and tubing for nuclear and other special applications.

5. TAPP-3&4/60610/93/B/4670 dated 9th June, 1993: Design note on Class-I Instrument

tubing.

6. TAPP-3&4/60610/93/B/4671 dated 9th June, 1993: recommendation note on impulse connections and their installations

7. Eric Lundin, “Is your bend good enough”, The Fabricator.

8. ISA-S 67.02.02(1996): Nuclear safety related instrument sensing line piping and tubing.

9. ASME PTC code 19.5: Flow measurement.

10. ASME PTC code 19.2: Pressure measurement.

11. PB-E-344: Specifications for SS tubes

12. PB-E-146: Specifications for SS compression type twin ferrule (flareless)tube fittings.

13. PB-M-23: Specifications for Seamless Copper tubing for 500 MWe

14. PB-E-44: Specifications for Brass compression type (flareless)tube fittings

15. ANSI/ASME B1.20.1-1983, Pipe Threads, General purpose (inch)

16. IS-1239 (part-II)-1982, Specification for mild steel tubes, Tubular and other Wrought

steel fittings

17. ASME Section-III- Rules for construction of Nuclear Power Plant Components; Division-I-Subsection NB: Class 1 Components

18. BS-4368-Part-I-1972; Carbon and Stainless Steel Compression Couplings for Tubes

19. “Fundamentals of orbital tube welding”, Pro-Fusion technologies, Inc.

20. ANSI/ASME B1.20.4-1976, Dryseal Pipe Threads


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21. “Tools and Manufacturing Engineer’s Handbook: A Reference Book for Engineers,

Managers and Technicians”, Tom Drozda, Charles Wick.

22. “Perry’s chemical Engineers’ Handbook”, Robert H. Perry, D.W. Green.

23. Brochure of Tubes and Tube fittings, Parker Hannifin, USA.

24. “TIG Welding Basics-for TIG Welders, by a TIG Welder”, Jody Collier.

25. Swagelok SWS Orbital tube welding machine training notes.

26. Swagelok Manual on Orbital tube welding machine SWS D-100.

instrumentation tubing and their connections-nirbhay gupta

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