PART UIG THE ASME CODE FOR IMPREGNATED GRAPHITE PRESSURE VESSELS

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 36 th Annual Phosphate Fertilizer & Sulfuric Acid Technology Conference Sheraton Sand Key Resort 1160 Gulf Boulevard, Clearwater Beach, FL 33767 1

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 1 st 2010. 2

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. 3

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. 4

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 5

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 nondestructive 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. 6

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 7

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. 8

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. 9

Typical Test Specimens used for Material, Cement, Cementing Procedure, and Personnel Qualification 10

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 11

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 multichamber 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. 12

Sample Nameplate Markings 13

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. 14

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 1 st, 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 15