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What is API RP 934-A PDF and Why Should You Care? A Brief Introduction to Materials and Fabrication of Heavy Wall Pressure Vessels for High-temperature, High-pressure Hydrogen Service


API RP 934-A PDF: A Guide to Materials and Fabrication of Heavy Wall Pressure Vessels for High-temperature, High-pressure Hydrogen Service




If you are involved in the design, fabrication, operation, or maintenance of heavy wall pressure vessels for high-temperature, high-pressure hydrogen service, you may have heard of API RP 934-A. This is a recommended practice (RP) published by the American Petroleum Institute (API) that covers materials and fabrication requirements for new 2Cr and 3Cr steel heavy wall pressure vessels. In this article, we will explain what API RP 934-A is, why it is important, what it covers, and where you can find a copy of it in PDF format.




Api rp 934 a PDF


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What is API RP 934-A and why is it important?




API RP 934-A is a document that provides guidance and recommendations for materials and fabrication of heavy wall pressure vessels for high-temperature, high-pressure hydrogen service. It was first published in December 2000 as API 934, and then revised and updated in January 2019 as API RP 934-A. The latest edition also includes an errata published in March 2021.


Heavy wall pressure vessels are defined as those with a shell thickness of 4 in. (100 mm) or greater, and high-temperature is considered to be operating temperatures of 650 F (345 C) and above. These vessels are typically used in hydroprocessing units, such as hydrotreating, hydrocracking, hydrodesulfurization, hydrodenitrogenation, etc., where hydrogen is used as a reactant or a catalyst to upgrade petroleum products. These units operate at high temperatures and pressures to achieve higher conversion rates and product quality.


However, these operating conditions also pose significant challenges for materials selection and fabrication of pressure vessels. Hydrogen can cause various forms of damage to steels, such as hydrogen embrittlement, hydrogen blistering, hydrogen-induced cracking (HIC), stress-oriented hydrogen-induced cracking (SOHIC), etc. These phenomena can reduce the mechanical strength, ductility, toughness, fatigue resistance, and fracture toughness of steels, leading to premature failure or leakage of pressure vessels.


Therefore, it is essential to use appropriate materials and fabrication methods for heavy wall pressure vessels that can withstand high-temperature, high-pressure hydrogen service without compromising safety, reliability, performance, or cost-effectiveness. This is where API RP 934-A comes in handy. It provides best practices and recommendations based on industry experience and research to help engineers and fabricators select suitable materials and fabrication techniques for heavy wall pressure vessels.


What are the main objectives and scope of API RP 934-A?




The main objectives of API RP 934-A are to:



  • Provide guidance on materials selection and fabrication requirements for new heavy wall pressure vessels for high-temperature, high-pressure hydrogen service;



  • Promote consistency and quality in materials selection and fabrication practices among different manufacturers and users;



  • Enhance safety, reliability, performance, and cost-effectiveness of heavy wall pressure vessels;



  • Reduce the risk of failure or leakage due to hydrogen damage or other causes;



  • Facilitate compliance with applicable codes and standards;



  • Support continuous improvement and innovation in materials technology and fabrication methods.



The scope of API RP 934-A covers:



  • Materials selection criteria for conventional steels including standard 2Cr-1Mo (P22) and 3Cr-1Mo (P21) steels; advanced steels which include vanadium-modified grades such as 2Cr-1Mo-V (P23), 3Cr-1Mo-V-Ti-B (P24), and 3Cr-1Mo-V-Nb-Ca (P25) steels; enhanced steels which have higher mechanical properties due to special heat treatments such as ASME SA-542 Grade B Class 4 steel;



  • Mechanical properties testing requirements for base metal, weld metal, and heat affected zone (HAZ);



  • Chemical composition limits for base metal, weld metal, and weld consumables;



  • Heat treatment procedures for base metal, weld metal, and HAZ;



  • Fabrication requirements including design, welding, inspection, and documentation;



  • Application of weld overlay linings using austenitic stainless steel to provide corrosion resistance;



  • Special considerations for heat exchanger shells and channels.



The scope of API RP 934-A does not cover:



  • Vessels with shell thicknesses below 4 in. (100 mm), or operating at temperatures below 650 F (345 C), although this document may be used as a resource with changes defined by the purchaser;



  • Vessels designed, fabricated, certified, and documented in accordance with ASME Section VIII, Division 3, which has much stricter design rules and material testing requirements;



  • Vessels with stainless steel cladding using a roll-bonded or explosion-bonded layer on Cr-Mo base metal, or multilayer vessels;



  • Vessels made of other materials than those covered by this document, such as nickel-based alloys, carbon steel, etc.



What are the materials and fabrication requirements for heavy wall pressure vessels for high-temperature, high-pressure hydrogen service?




In this section, we will briefly summarize some of the key materials and fabrication requirements for heavy wall pressure vessels according to API RP 934-A. For more details, please refer to the original document.


Materials




The materials covered by API RP 934-A are conventional steels including standard 2Cr-1Mo and 3Cr-1Mo steels; advanced steels which include vanadium-modified grades such as 2Cr-1Mo-V, 3Cr-1Mo-V-Ti-B, and 3Cr-1Mo-V-Nb-Ca steels; enhanced steels which have higher mechanical properties due to special heat treatments such as ASME SA- 542 Grade B Class 4 steel.


The conventional steels have good strength, toughness, and weldability at moderate temperatures and pressures; but they may suffer from temper embrittlement or HIC at higher temperatures and pressures. The advanced steels have improved strength, toughness, and resistance to temper embrittlement or HIC at higher temperatures and pressures; but they require more stringent heat treatment and welding procedures. The enhanced steels have superior strength, toughness, and resistance to temper embrittlement or HIC at higher temperatures and pressures; but they are more expensive and require special heat treatment procedures.


The mechanical properties testing requirements for base metal, weld metal, and HAZ include tensile strength, yield strength, elongation, reduction of area, hardness, impact toughness, fracture toughness, creep strength, fatigue strength, etc. The tests should be performed according to ASTM standards or equivalent methods. The test specimens should be representative of the actual material condition and orientation. meet the minimum or maximum values specified by API RP 934-A or ASME Section VIII, Division 2, as applicable.


The chemical composition limits for base metal, weld metal, and weld consumables include carbon, manganese, silicon, phosphorus, sulfur, chromium, molybdenum, vanadium, titanium, boron, niobium, calcium, nitrogen, etc. The chemical composition should be determined by product analysis or ladle analysis according to ASTM standards or equivalent methods. The chemical composition should comply with the limits specified by API RP 934-A or ASME Section VIII, Division 2, as applicable.


The heat treatment procedures for base metal, weld metal, and HAZ include normalizing and tempering (N&T), quenching and tempering (Q&T), post weld heat treatment (PWHT), stress relief heat treatment (SRHT), etc. The heat treatment parameters such as temperature, time, cooling rate, etc. should be controlled and monitored according to API RP 934-A or ASME Section VIII, Division 2, as applicable. The heat treatment should achieve the desired microstructure and mechanical properties of the materials.


Fabrication




The fabrication requirements include design, welding, inspection, and documentation of heavy wall pressure vessels according to API RP 934-A or ASME Section VIII, Division 2, as applicable.


The design requirements include design pressure and temperature, design stress intensity values (SIFs), design fatigue curves (DFCs), design margins (DMs), design loadings and load combinations (DLLCs), design by analysis (DBA) methods such as elastic analysis or elastic-plastic analysis (EPA), etc. The design should ensure that the pressure vessel can withstand the operating conditions without exceeding the allowable stress limits or causing excessive deformation or damage.


The welding requirements include welding procedure specifications (WPSs), procedure qualification records (PQRs), welder performance qualifications (WPQs), welding consumables selection and storage, welding parameters control and monitoring, preheat and interpass temperature control and monitoring, post weld cleaning and inspection, etc. The welding should ensure that the weld metal and HAZ have adequate strength, toughness, and resistance to hydrogen damage.


The inspection requirements include nondestructive examination (NDE) methods such as radiographic testing (RT), ultrasonic testing (UT), magnetic particle testing (MT), liquid penetrant testing (PT), etc.; destructive testing methods such as bend testing (BT), macroetch testing (MET), hardness testing (HT), etc.; dimensional inspection methods such as calipers, micrometers, gauges, etc.; visual inspection methods such as surface condition check, weld appearance check, etc. The inspection should ensure that the pressure vessel meets the acceptance criteria for quality, workmanship, and integrity.


The documentation requirements include material test reports (MTRs), welding procedure specifications (WPSs), procedure qualification records (PQRs), welder performance qualifications (WPQs), heat treatment records, inspection reports, certificates of compliance, etc. The documentation should provide evidence of compliance with API RP 934-A or ASME Section VIII, Division 2, as applicable.


Weld overlay linings




The weld overlay linings are applied to the interior surfaces of heavy wall pressure vessels to provide additional corrosion resistance. The weld overlay linings are typically made of austenitic stainless steel such as Type 304L or Type 316L. The weld overlay linings should have a minimum thickness of 0.125 in. (3 mm), and a maximum dilution of 15 %.


The weld overlay linings should be applied using gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), or submerged arc welding (SAW) processes. The welding consumables should be compatible with the base metal and the overlay metal. The welding parameters should be controlled and monitored to ensure proper deposition rate, penetration, and fusion.


The weld overlay linings should be heat treated after application to relieve residual stresses and improve corrosion resistance. The heat treatment parameters such as temperature, time, cooling rate, etc. should be controlled and monitored according to API RP 934-A or ASME Section VIII, Division 2, as applicable.


The weld overlay linings should be inspected using nondestructive examination (NDE) methods such as liquid penetrant testing (PT), ultrasonic testing (UT), or eddy current testing (ECT) to detect any defects such as cracks, porosity, lack of fusion, etc. The inspection should ensure that the weld overlay linings meet the acceptance criteria for quality, workmanship, and integrity.


Heat exchanger shells and channels




Heat exchanger shells and channels are included in the term \"pressure vessel\" for the purposes of API RP 934-A. They are subject to the same materials and fabrication requirements as other heavy wall pressure vessels for high-temperature, high-pressure hydrogen service.


However, there are some special considerations for heat exchanger shells and channels that need to be addressed. These include:



  • The thermal expansion mismatch between the shell and the tube bundle may cause excessive stresses or deformation in the shell; therefore, the shell design should account for this effect;



  • The tube-to-tubesheet joints may be susceptible to hydrogen damage due to high stress concentration; therefore, the tube-to-tubesheet joints should be designed and fabricated to minimize this risk;



  • The tubes may be subject to fouling or corrosion due to process fluids; therefore, the tubes should be selected based on their compatibility with the process fluids;



  • The shell-side fluid may contain hydrogen sulfide or other corrosive agents; therefore, the shell-side fluid should be analyzed and treated to reduce its corrosivity;



  • The heat exchanger may experience thermal cycling or transient conditions; therefore, the heat exchanger should be designed and operated to avoid thermal shock or fatigue.



Conclusion




In this article, we have explained what API RP 934-A is, why it is important, what it covers, and where you can find a copy of it in PDF format. We have also summarized some of the key materials and fabrication requirements for heavy wall pressure vessels for high-temperature, high-pressure hydrogen service according to API RP 934-A.


API RP 934-A is a valuable resource for engineers and fabricators who are involved in the design, fabrication, operation, or maintenance of heavy wall pressure vessels for high-temperature, high-pressure hydrogen service. It provides best practices and recommendations based on industry experience and research to help them select suitable materials and fabrication techniques for these vessels.


However, API RP 934-A is not a substitute for applicable codes and standards such as ASME Section VIII, Division 2. It is also not a comprehensive guide for all aspects of materials selection and fabrication of heavy wall pressure vessels. Therefore, it is important to consult with qualified experts and follow the relevant codes and standards when designing and fabricating heavy wall pressure vessels for high-temperature, high-pressure hydrogen service.


FAQs




What is the difference between API RP 934-A and ASME Section VIII, Division 2?




API RP 934-A is a recommended practice that provides guidance and recommendations for materials and fabrication of heavy wall pressure vessels for high-temperature, high-pressure hydrogen service. It is not a mandatory code or standard that has legal authority or enforcement power. ASME Section VIII, Division 2 is a code that specifies mandatory rules and requirements for design, fabrication, inspection, testing, certification, and documentation of pressure vessels. It has legal authority and enforcement power in many jurisdictions where it is adopted by law or regulation.


What are the advantages and disadvantages of using vanadium grade steels for heavy wall pressure vessels?




Vanadium grade steels are advanced steels that have improved strength, toughness, and resistance to temper embrittlement or HIC at higher temperatures and pressures than conventional steels. They also have lower carbon content and higher alloying elements than conventional steels. Some of the advantages of using vanadium grade steels are:



  • They can reduce the wall thickness and weight of pressure vessels;



  • They can increase the operating temperature and pressure limits of pressure vessels;



  • They can extend the service life and reliability of pressure vessels;



  • They can reduce the risk of failure or leakage due to hydrogen damage or other causes.



Some of the disadvantages of using vanadium grade steels are:



  • They are more expensive than conventional steels;



  • They require more stringent heat treatment and welding procedures than conventional steels;



  • They may have lower availability or longer delivery times than conventional steels;



  • They may have higher susceptibility to cracking during fabrication or service than conventional steels.



How can hydrogen embrittlement be prevented or mitigated in heavy wall pressure vessels?




  • of API, where you can buy a hardcopy or a secure PDF file for $146.00. You can access the website here: https://www.api.org/products-and-services/standards/individual-standards/api-rp-934-a;



  • The online store of Techstreet, where you can buy a hardcopy, a secure PDF file, or a printed edition plus PDF file for different prices. You can access the store here: https://www.techstreet.com/standards/api-rp-934-a?product_id=2033451;



  • The online store of SAI Global, where you can buy a hardcopy or a PDF file for different prices. You can access the store here: https://infostore.saiglobal.com/en-au/standards/api-rp-934-a-2019-96190_saig_api_api_2704882/.




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