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PART UIG

THE ASME CODE FOR IMPREGNATED

GRAPHITE PRESSURE VESSELS

Presented by: Ed Soltow

Engineering and Design Manager

SGL Carbon Technic LLC

Member – ASME Standards Committee on Pressure Vessels (BPV VIII)

Chairman – ASME Subgroup Graphite Pressure Equipment

Chairman – NBIC Subgroup Graphite Pressure Equipment

Prepared for

36th

Annual Phosphate Fertilizer & Sulfuric Acid Technology Conference

Sheraton Sand Key Resort 1160 Gulf Boulevard, Clearwater Beach, FL 33767

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ABSTRACT

Impregnated graphite has been used for more than 60 years in the construction of

chemical processing equipment. This equipment has consisted primarily of heat

exchangers because of impregnated graphite’s tremendous corrosion resistance and

thermal conductivity. Until recently, there was not an ASME standard for this type of

equipment and most manufacturers followed either their own internal standards and

practices or the not so comprehensive German Code, AD Merkblatter. Because of this

lack of a mandatory standard the quality of this equipment could vary significantly from

one manufacturer to another.

In 1998 a special working group was commissioned to develop rules for the

design and construction of impregnated graphite pressure equipment by ASME. This

group reported to ASME Standards Committee BPV VIII and in November of 2008, after

more than ten years of exhaustive and collaborative effort, the document they had been

working on “Part UIG”, was approved unanimously by ASME. Part UIG was published

in July 2009 as part of ASME Section VIII Division I, which is the mandatory Code of

construction for pressure vessels in forty of the United States and all of Canada. These

rules became mandatory in all of these locales on January 1st 2010.

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INTRODUCTION

There are many variations and types of impregnated graphite heat exchangers and

pressure vessels which are used for primarily chemical processing applications. For this

paper the primary focus is going be on the impregnated graphite shell and tube type heat

exchanger, specifically the phosphoric acid evaporator. This is the most commonly used

and essential piece of impregnated graphite equipment in phosphoric acid production. It

is very typical for the producer of phosphoric acid to have several of these evaporators

located in various stages and to have spare evaporators as well. It is also very common to

attempt to make them identical or nearly identical in order to make interchangeability

possible.

The impregnated graphite evaporator has been used for many years and over time

the various manufacturers have added their own design variations to this somewhat

standard piece of equipment. This led to a great deal of inconsistency in the equipment

and was one of the motivating factors that led to the desire to develop rules for the

construction of impregnated graphite pressure vessels. The logical choice for the

development of these rules was the American Society of Mechanical Engineers (ASME)

as they already had globally recognized standards for metallic and FRP pressure vessels.

Since the majority of the shell sides of evaporators were already being ASME Code

stamped according to ASME Section VIII Division I (Rules for Construction of Pressure

Vessels) this just reinforced this choice.

When the ASME Special Working Group for impregnated graphite pressure

equipment was developed, it was clearly recognized by all parties involved

(manufacturers, users, jurisdictions, inspection agencies and consultants) that these rules

were necessary. It is a fact, that impregnated graphite equipment is used in some of the

most hazardous and corrosive chemical services in existence and it was understood that

these rules would help to make this equipment more reliable and safe. These rules have

had this effect since “Part UIG” was published as part of the 2009 addenda to the 2007

edition of the ASME Section VIII Division I Code.

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SCOPE, EQUIPMENT AND SERVICE LIMITATIONS

The rules of Part UIG are applicable to any impregnated graphite pressure vessel

or pressure vessel part and are to be used in conjunction with the rules contained within

Section VIII Division I, where they are applicable as well. In addition impregnated

graphite vessels may not be constructed under the rules of U-1(j) (miniature pressure

vessels) or UG-90(c)(2) (multiple duplicate).

The equipment that theses rules apply to is limited to:

- shell and tube heat exchangers

- bayonet heat exchangers

- cylindrical block heat exchangers

- rectangular block heat exchangers

- plate heat exchangers

- cylindrical vessels

Impregnated graphite equipment is limited to a maximum internal or external

design pressure of 350 psi, a minimum design temperature of -100 °F and a maximum

design temperature of 400 °F.

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MATERIAL CONTROL, CERTIFIED MATERIAL

SPECIFICATIONS AND MATERIAL PROPERITES

The raw materials (graphite and resin) used to produce certified material must be

traceable to their source and grade. For these raw materials, any combination of a

specific grade of graphite and resin used within a specific controlled impregnation

process requires a unique certification and qualification. This qualification is

accomplished through the testing of various properties for which the results must at least

meet the values specified in Table UIG-6-1 “Properties of Certified Materials”. This

qualification is documented in a Certified Material Specification (CMS) and the test

results are documented in a Certified Material Qualification (CMQ).

The cement that is used to bond impregnated graphite components together must

also be qualified and certified as well. The qualification is documented in a Certified

Cement Specification (CCS) and the test results are documented in a Certified Cement

Qualification (CCQ). The test results must also meet the values specified in Table UIG-

6-1.

Once a grade of impregnated graphite or cement is certified some of its properties

must be retested every 3 months in order to maintain the material certification and to

ensure that the impregnation process is still under control.

All impregnated graphite used in the construction of pressure vessels is

documented with a Certified Material Test Report (CMTR). This is similar to the

Material Test Report (MTR) that one would expect to receive with metallic plate, pipe

and forging materials.

Table UIG-6-1

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DESIGN

LOADINGS – It is required that all UG-22 loadings such as internal or external

design pressure, wind, seismic, piping, static head, thermal expansion etc. be taken into

consideration when designing a impregnated graphite pressure vessel. In addition to

these loadings UIG-22 strongly recommends the use of bellows for graphite connections.

ALLOWABLE STRESS – For tensile loadings the design factor is 6.0 and the

maximum allowable stress value used for design is the average value at the design

temperature stated in the CMQ minus 20% divided by the design factor of 6.0. The

metallic ASME Section II materials used for Section VIII construction have a design

factor of 3.5 on tensile stress. When compared to each other it is clear that the allowable

stress used for the design of impregnated graphite pressure vessels is even more

conservative than metallic pressure vessels.

CYLINDRICAL SHELL THICKNESS – For internal pressure calculations the

appropriate UG-27 or Appendix 1 formula shall be used for design using a joint

efficiency of 1.0. This is a significant testimony to the strength of the properly designed

cemented joint. In comparing welded metallic equipment the welded joint has a joint

efficiency of 0.7 unless additional radiography is performed. For spot radiography a joint

efficiency of 0.85 is possible and a joint efficiency of 1.0 can only be obtained with full

radiography. The reason for the joint efficiency of 1.0 (without additional non-

destructive examination} is that the properly designed, certified cement joint is actually

stronger than the base material itself. This has been, and continues to be confirmed by

testing. For external pressure calculations the appropriate UIG-28 formula must be used

for machined cylinders or extruded tubes. Because of the relatively low allowable stress

of impregnated graphite the diameter to thickness ratios of cylindrical shells designed by

these formulae are such that full vacuum ratings and even high external pressure ratings

are easily possible.

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TUBESHEETS – The calculations for tubesheets follow the rules of UHX-13

which some additional steps and modifications to account for the differences between

metallic and impregnated graphite heat exchangers. Prior to the development of UIG

most manufacturers used the TEMA bending formula or a variation of it to calculate

tubesheet thickness. Because ASME felt that the TEMA formulae for tubesheet

calculation were not conservative for any shell and tube heat exchanger (even though

they had been used successfully for many years) they developed part UHX (Rules for

Shell and Tube Heat Exchangers), which became a mandatory part of Section VIII

Division I in 2004. Because UIG is part of Section VIII Division I, it was mandated that

impregnated graphite use the same methodology for tubesheet calculation, which is now

contained in UIG-34(b). The end result is that the required thickness for tubesheets has

increased.

Previously, it has been and continues to be the practice of some manufacturers of

impregnated graphite equipment to use metal shrouds as a method to treat the graphite

(which is the corrosion resistant material) like a liner and reduce the thickness of tube

sheets. This was done primarily to reduce the cost of equipment. This practice can still

be done, but no credit can be taken for the shroud and the graphite tubesheet must be

designed according to Code rules as a pressure part. In reality the shroud does present

some unique challenges that are not present with the stand alone graphite tubesheet. Due

to the difference of 3 to 1 in the rate of thermal expansion for steel versus impregnated

graphite, the thermal growth of the shroud presents sealing and mechanical failure issues.

In addition it is also necessary to create and maintain a seal between the shroud and the

tubesheet itself which is difficult to do and nearly impossible to service. Using this

shroud no longer presents any cost savings (actually, it is presents cost increases) and the

additional challenges previously mentioned with little or no benefit.

TEMA Tubesheet Formula

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UIG-34 Tubesheet Configurations for UHX 13

LETHAL SERVICE

It is possible to certify an impregnated graphite heat exchanger for Lethal Service

in accordance with ASME Code rules. There are some additional requirements to satisfy

this, but this certification is not necessary for the phosphoric acid evaporator.

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FABRICATION AND PROCEDURE AND PERSONNEL

QUALIFICATION

Each manufacturer is responsible for the quality of the materials, processes and

personnel used by their organization. Only qualified cementing procedures can be use for

the design of pressure containing or structural joints. This cementing can only be

performed by certified cementing technicians. Each cementing technician is assigned a

unique identification symbol to identify his work and the manufacturer must maintain a

continuity record for each cementing technician. This practice of certifying cementing

procedures and personnel is very similar to the qualification of welding procedures and

welders.

CERTIFIED MATERIAL, CEMENT, PROCEDURES AND

TECHNICIANS

CERTIFIED MATERIAL SPECIFICATION (CMS) – The CMS includes the

raw materials and processes necessary to produce certified material. It includes all

essential and non essential variables and tolerance ranges. The tested properties include

tensile strength, tensile strength at elevated temperature, flexural strength, compressive

strength, coefficient of thermal expansion and coefficient of permeability.

CERTIFIED CEMENT SPECIFICATION (CCS) – The CCS includes the raw

materials and processes necessary to produce certified cement. It includes all essential

and non essential variables and tolerance ranges. The tested properties include tensile

strength at room and elevated temperature.

CERTIFIED CEMENTING PROCEDURE SPECIFICATION (CPS) – The

CPS includes the raw materials and processes necessary to manufacture items using

certified materials and cement. It includes all essential and non essential variables and

tolerance ranges. The tested properties include tensile strength at room temperature.

CERTIFIED TECHNICIAN QUALIFICATION (CTQ) – Only qualified

cementing technicians can be used in the production of Code parts and vessels. They are

responsible for proper joint preparation, cleaning of parts to be joined, mixing cement,

applying cement and curing the joint. Technicians shall be requalified if they have not

engaged in the production of graphite pressure vessels for 6 months or if there is any

reason to question their ability to produce a sound joint.

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Typical Test Specimens used for Material, Cement, Cementing Procedure, and

Personnel Qualification

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REPAIR OF MATERIALS

Materials may be repaired using qualified procedures provided that the

concurrence of the Authorized Inspector is first obtained for the method and the extent of

the repairs. Only certified materials that meet the specified properties can be used for

repairs.

In addition, the National Board Inspection Code (NBIC) has rules for the repair,

routine repair, alteration and in service inspection of impregnated graphite equipment.

Some of the rules included are tube plugging, tube replacement and repairing fractures.

As is the case with the ASME rules, only certified personnel and materials can be used

for NBIC “R” stamped repairs.

Typical NBIC Repair Method

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INSPECTION AND TESTS

VISUAL EXAMINATION – All parts, materials, finished joints and completed

vessels must be visually examined by the manufacturer using a procedure qualified

according to ASME Section V, Article 9 (Visual Examination). Visual examination is

the only NDE utilized for impregnated graphite. Radiography, Ultrasonic Examination

and Acoustic Emissions testing do not yield results of any value.

ACCEPTANCE STANDARDS AND DOCUMENTATION – All surfaces

shall be free of any laminations, spalling or cracks and if present they must be repaired,

although cracks in tubes cannot be repaired and shall be rejected. For tubes scratches are

limited to 1/32” in depth and for all other material 1/8” in depth. Unacceptable

discontinuities may be repaired by removing them entirely using a qualified procedure.

However, cracks and voids cannot be repaired by adding cement only.

PRESSURE TESTS – Completed pressure vessels shall be hydrostatically tested

according to UG-99 except that the test pressure shall not be less than 1.5 times (1.75 for

lethal service) the design pressure for the graphite side of the equipment in a multi-

chamber pressure vessel. This testing is more conservative than the 1.3 times design

pressure requirement for Section VIII Division one metallic pressure vessels.

MARKINGS AND REPORTS

Each impregnated graphite pressure vessel or part shall be marked in accordance

with UG-116 with the exception that the letter “G” shall be stamped below the

Certification Mark and the “U” designator. The appropriate data report (U-1, U-1A or U-

2A) as specified in UG-120 shall be filled out. In addition to this the supplemental U-1B

data report for graphite must be filled out, attached and referenced on the applicable data

report specified in UG-120.

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Sample Nameplate Markings

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SUMMARY

The ASME rules for the construction impregnated graphite pressure vessels and

the NBIC rules for the repair of this equipment were long overdue. They have ensured

the maximum safety and reliability of this type of equipment when these rules are

applied. It is up to the user of this equipment to follow jurisdictional and OSHA

guidelines to ensure that this is the case when procuring or repairing this type of

equipment.

For the phosphoric acid evaporator this may present some dimensional challenges

due to the amount of older and not so conservatively designed equipment in use. Because

most users have multiple, nearly identical evaporators it is important to them to have drop

in replacements. Even with the much more conservative ASME Code rules this is still

possible in most cases. It may take the reduction of some tube length or the relocation of

the condensate nozzle in some cases in order to accommodate the thicker ASME

tubesheets. However, these modifications should be inconsequential when compared to

the benefit of having a piece of equipment that is constructed according to the ASME

standard.

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REFERENCES

ASME Boiler and Pressure Vessel Committee on Pressure Vessels, American Society of

Mechanical Engineers, 2010 ASME Boiler and Pressure Vessel Code Section VIII

Division I, 2011a Addenda, July 1st, 2011

ASME Standards Technology, American Society of Mechanical Engineers, Impregnated

Graphite for Pressure Vessels, 2005

Tubular Exchanger Manufacturers Association, Inc. (TEMA), Standards of the Tubular

Exchanger Manufacturers Association, Eighth edition

The National Board of Boiler and Pressure Vessel Inspectors, The National Board

Inspection Code (NBIC), 2011