SELECTION OF SUPPORTS

DATA TO BE COLLECTED TO START DESIGN

1. A complete set of piping general arrangement drawings.

2. A complete set of steel and structural drawings

including the equipment foundation .

3. A complete set of drawing showing the location

of ventilating ducts, electrical trays, instrument

tray etc.

4. A complete set of piping specification and line

list which includes pipe sizes, material of

construction, thickness of insulation, operating

temperatures etc.

5. A copy of insulation specification with densities.

6. A copy of valve and specialty list indicating weights.

7. The movement of all critical equipment connections such as turbines, compressors, boilers, etc.

On collection of the above data, the steps in which the engineer will apply this basic information are as follows.

1. The determination of support location.

2. The determination of thermal movement of the piping at each support location.

3. The calculation of load at each support

location.

4. The selection of the type of support i.e. Anchor Guide, Rest, Constant or Variable spring etc.

5. Checking the physical interference of the

support with structures, tray, ducts equipment’s etc.

Anchors are provided to secure the desired points of piping whereas guides are provided to direct or absorb the same. They shall permit the piping to expand and contract freely away from the fixed points. Sliding or Rest supports permit free movement of piping and shall be designed to include friction resistance along with the dead weight of the piping. Resilient supports are those which support the dead weight throughout the expansion / contraction of the piping.

The ‘primary support’ is the supporting element which is attached or in contact with the piping “secondary support” is the supplementary steel provided to carry the load on the structures.

Fig. 1.1 Fig.1.2

Fig.1.3

Fig.1.4 Fig.1.5

Fig.1.6Fig.1.7

Fig. 1.10

Fig 1.8

Fig. 1.9

2.0 THE DETERMINATION OF SUPPORT LOCATIONSThe support location is dependent on the pipe size, piping configuration, the location of heavy valves and specialties and the structure available for support. The simplest method of estimating the support load and pipe stress due to weight is to model the pipe as a beam loaded uniformly along the length, the length of the beam equal to distance between supports.

There are two possible ways to model the pipe, depending upon the end conditions – the simply supported (pinned end) beam or the fixed end beam.

For a simply supported beam , the maximum stress and support loads are.

Mmax = wl2

8  = wl2

8Z 

F = wl 2

where, Mmax = maximum bending moment, ft-lb

(N-m)         = Bending stress, psi (N/mm2) 

Mmax

w = weight per unit length, lb/in (N/mm)l = length of pipe, in (mm)

 F = force on support, lb (N)

 Z = section modulus in3 (mm3)

For fixed end beam

wl2

Mmax = 12

  wl2

     = 12 Z

 wl

F =2

For either model, the support load remains the same. However, depending upon the model chosen the stress in pipe varies. In actual practice the pipe at the point of support is not free to support fully, since it is partially restrained through its attachment to piping segment beyond the support. If the pipe runs between supports are equally loaded and of equal length, segment end rotation could cancel each other causing the pipe to behave as fixed-end beam. Therefore, the true case lies somewhere between the two beam models. Hence, as a compromise case, the stress is calculated as  

wl2

max =10 Z

 

Hence, support spacing is decided by the formula 10 Z S

L = w whereS is the allowable stress as per the code in psi (N/mm2)

The suggested maximum spans between the supports as recommended by ASMEB 31.1 in Table 121.5 are as follows:

Nominal Suggested Maximum SpanPipe Size Water Service Steam, Gas or

Air ServiceNB Inch M (ft) M (ft)1 2.1 (7) 2.7 (9)2 3.0 (10) 4.0 (13)3 3.7 (12) 4.6 (15)4 4.3 (14) 5.2 (17)6 5.2 (17) 6.4 (21)8 5.8 (19) 7.3 (30)12 7.0 (23) 9.1 (30)16 8.2 (27) 10.7 (35)20 9.1 (30) 11.9 (39)24 9.8 (32) 12.8 (42)

The above spacing is based on fixed bean support with a bending stress not to exceed 2300 psi and insulated pipe filled with water or the equivalent weight of steel pipe for steam, gas or air service and 2.5mm (0.1 inch) sag is permitted between supports.

The selection of supports should consider the following guidelines i) The support should be located as near as possible to concentrated load such as valves, flanges etc. to keep the bending stress to the minimum.

ii)  When changes of direction in a horizontal plane occur, it is suggested that the spacing be limited to 75% of the tabulated values to promote stability and reduce eccentric loadings. Note that the supports located directly on elbows are not recommended since that will stiffen the elbow and no flexibility will be available.

iii)The standard span does not apply to vertical run pipes (risers) since no moment and no stress will develop due to gravity load in the riser. The support should be located on the upper half of

a riser (above the center of gravity) to prevent instability in overturning of pipe under its own weight. Guides may be placed on long vertical risers to reduce pipe sag resulting in excessive pipe deflection. These guides are usually placed in span intervals of twice the normal horizontal span and do not carry any dead weight.

iv)       Support location should be selected near the existing building steel to minimize the use of supplementary steel.  

In case of pipeline running in Multiplan, the support load is determined by applying a method called ‘weight balancing’. This method involves breaking the larger piping system into smaller segments of pipe with supports, which are modeled as free bodies in equilibrium and solved statically.

PIPE SUPPORT DESIGN AND ENGINEERING

In case of concentrated loads, the support should be placed as close as possible. When change in direction occurs, it is considered a good practice to keep the span to 75% of the tabulated values.

For the illustrated problem, the following vertical movements are known, Point A – 50 mm up, Cold to Hot

Point B –35 mm up, Cold to Hot

The above data is as furnished by the manufactures of equipment.

H3 - 0 mm Cold to Hot

STEP 1Calculate the expansion at point C and D by multiplying the Coefficient of expansion by the vertical distance of each point from the position of zero movement on the riser CD.

3.0 x 7.62 = 22.86 mm up at point C

6.1 x 7.62 = 46.48 mm down at point D

The calculation of the loads for hangers involves dividing the system into convenient sections. A free body diagram of each section should be drawn to facilitate the calculation with simple arithmetic solution to the problem.

DISTRIBUTION OF VERTICAL MOVEMENT TO INTERMEDIATE POINTS ON HORIZONTAL LEG

Step II

Make a simple sketch between two adjacent points of known movement

(Refer Case3 of ‘Distribution of movements’)

The vertical movement at hanger location can be calculated by proportioning the same.

03.247850

14.2769501 X

Vertical movement of H1 = 22.86 + 24.03 = 46.89

Say 47 mmi.e. 47 mm up

Vertical movement of H2 = 22.86 + 6.74 = 29.60

Say 30 mm i.e. 30 mm up

mmX 74.67850

14.2719501

Step III

Make the sketch of piping between the points B and D, extending the piping to a single plane as shown.

Case 1

1535048.4615350)2548.46(7504 X

= -42.99 mm say –43 mmVertical movement at H4 =43 mm down

= -19.70 mm say –20 mmVertical movement at H5 = 20 mm down

1535048.4615350)2548.46(57505 X

= -3.41 mm say –3 mmVertical movement at H6 = 3 mm down

= -20.81 mm say 21 mmVertical movement at H7 = 21 mm up

1535048.4615350)2548.46(92506 X

1535048.4615350)2548.46(144507 X

For easy reference, when selecting the appropriate hanger, let us make a simple table of hanger movement.Hanger Number Movement (mm)

H1 47 up H2 30 up H3 0 H4 43 down H5 20 down H6 3 down H7 21 up

The first step in the solution is to prepare a table of weights

Description Weight Weight of Total Weight Weight Insln (Ca Si) Used in calculation

150NB Sch 67.5 Kg/m 17.0 Kg/m 84.5Kg/m 84.5 Kg/m160 pipe

150 NB Sch 24.0 kg 8.0 Kg 32 Kg 32 Kg160 900 BWLR Elbow

150 NB BW 725.0 kg 37.0 Kg 762 Kg 762 Kg1500 Ib class Gate Value

Taking moments about H1,

m x kg. = kg.m

0.15 x 25.4 = 3.81

0.60 x 762.0 = 457.20 787.4 461.01

461.01Reaction at the point A =

0.9

= 512.23kg

Reaction at the hanger H1 = 787.4 - 512.2 = 275.17 kg.

Fig. 4.1: DISTRIBUTION OF LOAD BETWEEN EQUIPMENT CONNECTION A & H1

422.5Reaction at the point H1 & H2 =

2

= 211.25 kg

Fig. 4.2: DISTRIBUTION OF LOAD BETWEEN H1 & H2

Taking moments about H3 M x Kg. = Kg-M

0.00 x 234.15 = 0.00

0.0832 x 32.00 = 2.66

1.0895 x 145.42 = 58.44

411.57 161.10

161.10Reaction at H2 =

1.95 = 82.62 kg Reaction at H3 = 411.57 - 82.62

= 328.95 kg.Fig.4.3: DISTRIBUTION OF LOAD BETWEEN H2 & H3

The various distances to the center of gravity of the bend can be calculated using the formula as below

2R Sin /2 A =

R ( 1- Cos )B =

R Sin C =

Applying the above formula for the distance of CG from the center of the arc for 150 NB LR elbow.

C = R Sin

= 229.0 x 1 / 2

= 145.8mm

Distance of the CG form the center line of the straight pipe = 229.0 - 145.8 = 83.2 mm

Taking moments about H4 M x Kg. = Kg_M

0.2605 x 44.0 = 11.46 0.6668 x 32.0 = 21.34

0.750 x 496.1 = 372.08

572.1 404.88

404.88Reaction at H3 =

0.750

= 539.84 kg

Reaction at H4 = 572.1 - 539.84

= 32.26 kg.

Fig. 4.4: DISTRIBUTION OF LOAD BETWEEN H3 & H4

422.5

Reaction at the point H4 & H5 =

2

= 211.25 kg.

Fig. 4.5: Distribution of Load Between H4 & H5

Taking moment about H6

M x Kg. = Kg-M

0.5 x 105.6 = 52.8

1.60 x 126.75 = 202.80

2.275 x 63.4 = 144.2

268.5 356.2

356.2Reaction at H5 =

2.5 = 142.48 kg

Reaction at H6 = 268.5 - 142.48 = 126.02 kg

Fig. 4.6: Distribution of Load Between H5 & H6

Taking moment about H6

M x Kg = Kg-M

2.60 x 439.4 = 1142.44

5.35 x 25.4 = 135.89

5.80 x 762.0 = 4419.60

1226.8 5697.93As the nozzle B is relieved of load

5697.93Reaction at H7 =

5.2

= 1095.76 kg

Reaction at H6 = 1226.8 - 1095.76

= 131.04 kg.

Fig. 4.7: DISTRIBUTION OF LOAD BETWEEN H6 & H7 TO MAINTAIN ZERO REACTION AT NOZZLE B

•When vertical displacement occurs as a result of thermal expansion it is necessary to provide a flexible support which apply supporting force throughout the contraction and expansion cycle of the system.

•Flexible hangers are two types :

• Constant Spring • Variable Spring.

O

O

Y

= = Sin Sin Sin

Considering, Y Z=

Sin Sin

YSinSin =

Z Since Y SinX

XSin Z

Y Substituting in Eqn. =

SinSin

Y i.e. = X/Z Sin

YZ

X Sin

YZ Sinor X =

The Load ‘L’ is suspended from the lever at point ‘A’ and at any point within the load travel range the moment of the load about the main lever-pivot ‘P’ is equal to the load times its moment arm.Thus load moment =L (WSin), where WSin is the load moment arm.

The spring is attached to one of its ends to the fixed pivot “B”. The free end of the spring is attached by means of a rod to the lever-pivot ‘D”. This spring arrangement provides a spring moment about the main lever-pivot “P” which opposes the load moment and is equal to the spring force, “F’ times its moment arm.

Thus spring moment )( YZSinFFX

Where X is the spring moment armThe spring force “F’ is equal to the spring constant “K” times to the spring deflection “E”

Thus F = KE

To obtain PERFECT constant spring, the load moment must always equal to spring moment.

)( SinYZKEMomentSpring

SinKEYZSinLW

By proper design and are made equal

The spring and the rod are so designed that the spring deflection “E” always equals the distance “”Between pivots “B” and “D”

Therefore LW = KYZ

KEYZLWTherefore

WKEYZLor

This equation holds true for all position of load within its travel range and “K”, “Y”, “Z” and “W” remain constant. It is therefore true that perfect constant support is obtained.

But due to spring hysteresis, bearing friction, sliding friction of moving parts and manufacturing tolerances, it is not normally possible to keep constant effort throughout the travel range. The deviation is kept very minimum by using PTFE washers and bushes at all pivot points and life time lubricated antifriction bearings.

But due to spring hysteresis, bearing friction, sliding friction of moving parts and manufacturing tolerances, it is not normally possible to keep constant effort throughout the travel range. The deviation is kept very minimum by using PTFE washers and bushes at all pivot points and life time lubricated antifriction bearings.There are different models of constant springs available based on the type of supporting arrangement. These are manufacturer specific and generally as below.

a) Spring located horizontally with the supporting structure above and the supported pipe line below the spring called model “H” by the manufacturers.

b) Spring located horizontally with the supporting structure below and the supported pipe line also below the spring called model “E” by M/s Sarathy and Model “M” by M/s Myricks.

c) Spring located horizontally with the supporting structure below and the supported pipe line above the spring called model “F” by M/s Sarathy and Model “S” by M/s Myricks.

d) Spring located vertically with the supporting structure above and the supported pipe line below the spring called model “V” by the manufacturers.

e) Spring located vertically with supporting structure above and the supported pipe line below the spring called model “P” by M/s Myricks.

1. First select the basic model best suited for piping layout and the physical structure available for mounting.2. Establish the total travel by giving a positive allowance of about

20% to the calculated actual travel and in no case less than 25 mm in order to allow for a possible discrepancy between calculated and actual piping movement.i.e. Total travel = actual travel + Over travel

3. Use the selection table supplied by manufacturer and locate the total travel required at the corresponding table.

4. Move along the line until load nearest to the operating load to be supported is located such that the load fits within a reserve range of ± 10% of the average of the maximum and minimum loads

specified.5. If the total travel lies between the two indicated figures, the loads

between the successive travels can be incorporated.6. The corresponding hanger size can be read from the respective

column.

HOW TO SELECT A CONSTANT SPRING SUPPORT

The following data is required to be specified while inquiring/ordering for a constant spring,

i. The exact Hot or Operating load required to be

ii. supported during the working condition.iii. Hydrostatic test load.iv. The total travel and its erection.v. The direction of travel, either upwards or

downwards from the erected position.vi. The set pin locking position (Top, Middle, Bottom

or as required).vii. The basic model.viii. Requirement of bottom accessory components such

as rods, clamps etc.

• Any hazardous environmental conditions.• any special finish on the body such as galvanizing etc.• Tag or Identification number.

5.2.1 How to select the series?

5.2.2 How to determine the type?

5.2.3 How to determine size?

5.2.2 How to determine the type

The type of variable spring hanger to be used depends upon the physical characteristics required by the suspension problem I.e. available head room, pipe to be supported above the spring or below the spring etc. The type should be selected from the seven standard types available. (See sketch for types A through G)

5.2.1 How to select the series

The selection of the hanger series shall be done to limit the supporting force within the allowable range. In choosing between the series VS1, VS2 and VS3 it must be ensured that the calculated movement will fall within the working load range. The series VS1 has the maximum variation in supporting force and hence is not a competitive selection but an invention of necessity where head room is not sufficient to use VS2.Good engineering sense combined with available space and reasonable economic considerations should ultimately determine which series of variable spring hangers should be used.

5.2.3 How to determine size

For determining the size of the hanger the load deflection table shall be referred. In order to choose the proper hanger size the data required is the actual load or the working load (also called the hot load) and the amount and direction of the pipe line movement from cold to hot .

Locate the hot load in the table. To determine the cold load, read the spring scale up or down for the amount of expected movement. The chart must be read opposite from the direction of pipe movement. The load arrived is cold load.

If the cold load falls outside the working load range of hanger selected, relocate the hot load to the adjacent

column and find the cold load. When both the hot and cold loads are within the working range of a hanger, the size of the hanger is the number found at the top of the column.

Should it be impossible to select a hanger in any series such that both loads fall within the working range, consideration should be given for a constant spring hanger. Once selected, the percentage load variation shall be checked as follows:

Travel x Spring Rate x 100Load Variation Percentage =

Hot load

This should be within 25% as specified in the code.

SPECIFICATION FOR ORDER The following data is required to be specified while

inquiring/ordering for a variable spring:

I. II. The exact hot or operating load required to be supported

during the working condition.III. Hydrostatic test load.IV. The calculated vertical movement andV. The direction of travel, either upwards or downwards from the

erected position.VI. The hanger series, type and size.VII. The allowable percentage variation of load from cold to hot.VIII. Requirement of accessory components such as rods, clamps

etc.IX. Any hazardous environmental conditions.X. Any special finish on the body such as galvanizing etc.XI. Tag or Identification number.

COMMISSIONING OF SPRING SUPPORTS

2.5.1 Securely attach the spring to the building structure by identifying and locating at each support point in accordance with hanger installation drawing. The location should be such that the hanger should be perpendicular in the hot or operating position/the

load should act vertical.2.5.2 Make sure the moving parts are unobstructed.2.5.3 The locking should not be disturbed till complete

erection is over. The lock that makes the support work as a rigid support during erection, hydrostatic

testing or chemical clearing etc.2.5.4 The locking pins must be removed after the hanger

is fully loaded to put the piping systems into operation. In case of top mounted support, this lock

shall be freely

removed by the hand after adjusting the distance between the hangers and pipe by rotating the turn buckle.In case of foot mounted supports the load flange is rotated till it touches equipment/pipe being supported. Then the threaded bush with hexagonal sides is rotated so that it moves up and the load is gradually transferred on to the support. The preset pin becomes loose when the pipe load becomes the preset or factory calibrated load. The support is then ready for use.2.5.5 Once the preset pin is removed the support allows

movement up or down by the specified amount of travel in accordance with the expected pipe movement.

2.5.6 When the line is in operation, carefully check the support for its free movement. Generally no further

adjustment is necessary. In case of any adjustment, the same shall be achieved by turning the threaded bush with hexagonal sides in case of foot mounted support or the turn buckle in case of top mounted support.

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