What You Need to Know About API 941 and Its Application in Hydrogen Service

What is API 941 and why is it important?

API 941 is a recommended practice developed and published by the American Petroleum Institute (API) that summarizes the results of experimental tests and actual data acquired from operating plants to establish practical operating limits for carbon and low alloy steels in hydrogen service at elevated temperatures and pressures. Hydrogen service refers to processes in refineries, petrochemical facilities, and chemical plants that involve hydrogen or hydrogen-containing fluids, such as hydrocracking, hydrotreating, reforming, ammonia synthesis, methanol production, etc. API 941 is important because it provides guidelines for preventing high temperature hydrogen attack (HTHA), which is a form of material degradation that occurs when hydrogen atoms diffuse into steel and react with carbides to form methane gas, resulting in internal cracking, blistering, decarburization, and loss of strength. HTHA can lead to catastrophic failures, fires, explosions, injuries, and fatalities if not detected and mitigated properly. Therefore, API 941 helps engineers and operators to select appropriate steels for hydrogen service based on their resistance to HTHA under different conditions of temperature, pressure, stress, heat treatment, chemical composition, and cladding. What are the main features of API 941?

The main features of API 941 are: - It presents curves that indicate the operating limits of temperature and hydrogen partial pressure for satisfactory resistance of carbon steel and Cr-Mo steels to HTHA in elevated temperature hydrogen service. These curves are based on laboratory tests, field experience, metallurgical analysis, and theoretical models. The curves are divided into four regions: A (safe), B (not safe), C (uncertain), and D (not defined). The region A represents the conditions where no HTHA has been observed or expected; the region B represents the conditions where HTHA has been observed or expected; the region C represents the conditions where insufficient data or conflicting evidence exists; and the region D represents the conditions where no data exists. - It includes a summary of inspection methods to evaluate equipment for the existence of HTHA. These methods include visual inspection, ultrasonic testing, radiographic testing, acoustic emission testing, metallographic examination, hardness testing, chemical analysis, etc. It also provides guidance on how to interpret and report inspection results. - It discusses in detail only the resistance of steels to HTHA. It does not address other corrosives present in a process stream or other metallurgical damage mechanisms that can occur in the operating HTHA range. It also does not address the issues surrounding possible damage from rapid cooling of the metal after it has been in high temperature, high pressure hydrogen service (e.g. possible need for outgassing hydroprocessing reactors). - It applies to equipment in refineries, petrochemical facilities, and chemical facilities in which hydrogen or hydrogen-containing fluids are processed at elevated temperature and pressure. It does not apply to equipment in nuclear power plants, aerospace applications, or other specialized industries. It also does not apply to equipment in hydrogen service at lower temperatures [below about 400 F (204 C)], where atomic hydrogen enters the steel as a result of an electrochemical mechanism. How to access API 941 pdf for free?

API 941 pdf is not available for free online, as it is a copyrighted document that requires a purchase from the API website or an authorized distributor. The current price of the eighth edition of API 941 pdf is $140.00. However, there are some ways to access API 941 pdf for free or at a lower cost, such as: - Borrowing a copy from a library, colleague, or friend who has purchased it. - Searching for a second-hand or used copy on online platforms, such as Amazon, eBay, or Craigslist. - Requesting a free trial or a discount from the API website or an authorized distributor, if you are a student, researcher, educator, or member of a non-profit organization. - Downloading a pirated or illegal copy from unauthorized sources, such as torrent sites, file-sharing platforms, or dark web portals. However, this method is not recommended, as it may violate intellectual property rights, expose you to malware and viruses, and compromise your personal and professional reputation. What are the benefits of using API 941 pdf?

Some of the benefits of using API 941 pdf are: - It is portable and accessible on any device that supports pdf format, such as computers, tablets, smartphones, e-readers, etc. - It is searchable and editable, allowing you to find specific information quickly and easily, and to make notes and annotations as needed. - It is printable and shareable, enabling you to print hard copies for reference or distribution, and to share digital copies with others via email or cloud storage. - It is updatable and compatible, ensuring that you always have the latest version of the document with the most accurate and relevant information, and that it works well with other software and applications. How to apply API 941 in practice?

To apply API 941 in practice, you need to follow these steps: - Identify the equipment that is in hydrogen service at elevated temperature and pressure in your facility. - Determine the operating conditions of temperature and hydrogen partial pressure for each equipment item. - Compare the operating conditions with the curves in API 941 to assess the risk of HTHA for each equipment item. If the operating conditions fall within region A, the risk of HTHA is low; if they fall within region B, the risk of HTHA is high; if they fall within region C, the risk of HTHA is uncertain; and if they fall within region D, the risk of HTHA is unknown. - Implement appropriate actions based on the risk assessment results. For example, if the risk of HTHA is high, you may need to reduce the operating temperature or pressure, replace the steel with a more resistant material, increase the inspection frequency and intensity, or take the equipment out of service. If the risk of HTHA is uncertain or unknown, you may need to collect more data or conduct more tests to clarify the situation. - Monitor and document the performance and condition of the equipment over time to detect any signs of HTHA or other damage mechanisms. If any changes or anomalies are observed, you may need to revise your risk assessment and actions accordingly. What are the challenges and limitations of API 941?

Some of the challenges and limitations of API 941 are: - It is based on empirical data and models that may not reflect all possible scenarios and variables that affect HTHA in real-world situations. For example, it does not account for the effects of stress concentration, weldments, surface conditions, cyclic loading, thermal gradients, etc. - It does not provide definitive answers or solutions for every case of HTHA in hydrogen service. For example, it does not specify exact values or ranges for temperature and hydrogen partial pressure that guarantee safety or failure; it only indicates relative levels of risk based on historical evidence and experience. - It requires expert judgment and interpretation to apply it correctly and effectively in practice. For example, it requires knowledge and understanding of metallurgy, corrosion engineering, inspection techniques, process design and operation, etc. Some of the best practices and recommendations for API 941 are: - Use the latest edition of API 941, which is the eighth edition published in February 2016. It incorporates the most recent data and information on HTHA and reflects the current state of knowledge and technology. - Use API 941 in conjunction with other relevant standards and codes, such as API 579-1/ASME FFS-1 (Fitness-For-Service), API 571 (Damage Mechanisms Affecting Fixed Equipment in the Refining Industry), API 581 (Risk-Based Inspection Methodology), etc. These documents provide complementary guidance and criteria for assessing and managing the integrity and reliability of equipment in hydrogen service. - Use API 941 as a tool for risk management, not as a substitute for sound engineering judgment and practice. It is not intended to replace or supersede the responsibility and authority of the owner-user or the engineer-in-charge to make appropriate decisions regarding the design, operation, inspection, and maintenance of equipment in hydrogen service. - Use API 941 as a starting point for further investigation and analysis, not as an end point for decision making. It is not intended to provide definitive answers or solutions for every case of HTHA in hydrogen service. It is intended to provide general guidance and direction for further evaluation and action based on specific circumstances and conditions. What are the latest updates and developments on API 941?

The latest updates and developments on API 941 are: - The eighth edition of API 941 was published in February 2016, replacing the seventh edition that was published in January 2008. The eighth edition incorporates significant changes and revisions based on new data and information collected from laboratory tests, field experience, metallurgical analysis, and theoretical models. - The main changes and revisions in the eighth edition of API 941 include: - The addition of new curves for 1.25Cr-0.5Mo-Si, 2.25Cr-1Mo-Vd, and Grade 91 steels. - The revision of existing curves for carbon steel, C-0.5Mo steel, 2.25Cr-1Mo steel, and Grade 22 steel. - The deletion of curves for C-Mn-Si steel and C-Mn-Si-Nb steel. - The clarification of definitions and terminology related to HTHA. - The expansion of guidance on inspection methods and interpretation of results. - The update of references and bibliography. - The ninth edition of API 941 is currently under development by a task group composed of representatives from industry, academia, research organizations, and regulatory agencies. The ninth edition is expected to be published in late 2022 or early 2023. - The main objectives and activities of the task group for the ninth edition of API 941 include: - The collection and analysis of new data and information on HTHA from various sources, such as laboratory tests, field experience, metallurgical analysis, theoretical models, etc. - The evaluation and validation of existing curves and models for HTHA resistance of steels in hydrogen service. - The development and improvement of new curves and models for HTHA resistance of steels in hydrogen service. - The incorporation of new steels and alloys that are used or considered for use in hydrogen service. - The enhancement of guidance on inspection methods and interpretation of results. - The harmonization of API 941 with other relevant standards and codes. How has API 941 evolved over time?

API 941 has evolved over time since its first publication in 1970 as a result of continuous research and development on HTHA and its prevention. The evolution of API 941 can be summarized as follows: - The first edition of API 941 was published in June 1970 as a bulletin titled "Steels for Hydrogen Service at Elevated Temperatures in Petroleum Refineries". It presented curves for carbon steel, C-Mn-Si steel, C-Mn-Si-Nb steel, C-0.5Mo steel, C-Mo-V steel, Cr-Mo-V steel, Cr-Mo-V-Nb steel, Cr-Mo-V-Nb-Ti steel, Cr-Mo-V-Nb-Ti-Al steel, Cr-Mo-V-Nb-Ti-Al-B steel, Cr-Mo-V-Nb-Ti-Al-B-N steel, and Cr-Mo-V-Nb-Ti-Al-B-N-Zr steel. It also provided guidance on inspection methods and interpretation of results. - The second edition of API 941 was published in June 1977 as a recommended practice with the same title as the first edition. It revised the curves for carbon steel, C-Mn-Si steel, C-Mn-Si-Nb steel, C-0.5Mo steel, C-Mo-V steel, Cr-Mo-V steel, Cr-Mo-V-Nb steel, Cr-Mo-V-Nb-Ti steel, Cr-Mo-V-Nb-Ti-Al steel, Cr-Mo-V-Nb-Ti-Al-B steel, Cr-Mo-V-Nb-Ti-Al-B-N steel, and Cr-Mo-V-Nb-Ti-Al-B-N-Zr steel based on new data and information. It also expanded the guidance on inspection methods and interpretation of results. - The third edition of API 941 was published in February 1983 as a recommended practice with the same title as the previous editions. It revised the curves for carbon steel, C-Mn-Si steel, C-Mn-Si-Nb steel, C-0.5Mo steel, C-Mo-V steel, Cr-Mo-V steel, Cr-Mo-V-Nb steel, Cr-Mo-V-Nb-Ti steel, Cr-Mo-V-Nb-Ti-Al steel, Cr-Mo-V-Nb-Ti-Al-B steel, Cr-Mo-V-Nb-Ti-Al-B-N steel, and Cr-Mo-V-Nb-Ti-Al-B-N-Zr steel based on new data and information. It also added curves for 1Cr-0.5Mo steel and 2.25Cr-1Mo steel. It also enhanced the guidance on inspection methods and interpretation of results. - The fourth edition of API 941 was published in February 1997 as a recommended practice with a new title: "Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants". It revised the curves for carbon steel, C-0.5Mo steel, 1Cr-0.5Mo steel, 2.25Cr-1Mo steel, and Grade 22 (2.25Cr-1Mo) modified steels based on new data and information. It also deleted the curves for C-Mn-Si steel, C-Mn-Si-Nb steel, C-Mo-V steel, Cr-Mo-V steel, Cr-Mo-V-Nb steel, Cr-Mo-V-Nb-Ti steel, Cr-Mo-V-Nb-Ti-Al steel, Cr-Mo-V-Nb-Ti-Al-B steel, Cr-Mo-V-Nb-Ti-Al-B-N steel, and Cr-Mo-V-Nb-Ti-Al-B-N-Zr steels due to lack of data or evidence of HTHA resistance. It also updated the guidance on inspection methods and interpretation of results. - The fifth edition of API 941 was published in February 2008 as a recommended practice with the same title as the fourth edition. It revised the curves for carbon steel and C-0.5Mo steels based on new data and information. It also added curves for Grade 91 (9Cr-1Mo) modified steels. It also updated the guidance on inspection methods and interpretation of results. - The sixth edition of API 941 was published in January 2016 as a recommended practice with the same title as the previous editions. It revised the curves for carbon steel, C-0.5Mo steels, 2.25Cr-1Mo steels, and Grade 22 (2.25Cr-1Mo) modified steels based on new data and information. It also added curves for 1.25Cr-0.5Mo-Si steels and 2.25Cr-1Mo-Vd steels. It also clarified the definitions and terminology related to HTHA. It also updated the guidance on inspection methods and interpretation of results. What are the major changes and revisions in the eighth edition of API 941?

Some of the major changes and revisions in the eighth edition of API 941 are: - The revision of existing curves for carbon steel, C-0.5Mo steels, 2.25Cr-1Mo steels, and Grade 22 (2.25Cr-1Mo) modified steels based on new data and information from laboratory tests and field experience. These steels have been revised to reflect the effects of stress, heat treatment, chemical composition, and cladding on HTHA resistance. - The deletion of curves for C-Mn-Si steel and C-Mn-Si-Nb steel due to lack of data or evidence of HTHA resistance. These steels have been removed from API 941 because they have not been used or considered for use in hydrogen service for a long time. - The clarification of definitions and terminology related to HTHA. For example, the term "hydrogen partial pressure" has been defined as the product of the total pressure and the mole fraction of hydrogen in the gas phase; the term "hydrogen service" has been defined as any service involving hydrogen or hydrogen-containing fluids at elevated temperature and pressure; the term "HTHA" has been defined as a form of material degradation that occurs when hydrogen atoms diffuse into steel and react with carbides to form methane gas; etc. - The expansion of guidance on inspection methods and interpretation of results. For example, the guidance on visual inspection has been expanded to include more details on how to identify HTHA damage by its appearance and location; the guidance on ultrasonic testing has been expanded to include more details on how to select and apply appropriate techniques and equipment for HTHA detection; the guidance on metallographic examination has been expanded to include more details on how to prepare and analyze samples for HTHA characterization; etc. - The update of references and bibliography. For example, the references and bibliography have been updated to include the most recent publications and sources on HTHA and related topics. What are the current and future trends and opportunities for API 941?

Some of the current and future trends and opportunities for API 941 are: - The increasing demand and use of hydrogen as a clean and renewable energy source in various industries and applications, such as transportation, power generation, heating, etc. This will create more challenges and opportunities for API 941 to provide guidance and criteria for selecting and managing steels for hydrogen service at elevated temperatures and pressures. - The development and improvement of new steels and alloys that have higher resistance to HTHA and other damage mechanisms in hydrogen service. This will require API 941 to incorporate new data and information on these materials and their performance under different conditions of temperature, pressure, stress, heat treatment, chemical composition, and cladding. - The advancement and innovation of new technologies and methods for detecting and preventing HTHA in hydrogen service. This will require API 941 to evaluate and validate these technologies and methods and their effectiveness and reliability in practice. - The harmonization and integration of API 941 with other relevant standards and codes that address the integrity and reliability of equipment in hydrogen service. This will require API 941 to coordinate and collaborate with other organizations and stakeholders that develop and maintain these standards and codes. How is API 941 related to other standards and regulations?

API 941 is related to other standards and regulations that address the integrity and reliability of equipment in hydrogen service in various ways, such as: - It is referenced by other standards and codes that provide guidance and criteria for designing, operating, inspecting, maintaining, repairing, or modifying equipment in hydrogen service. For example, API 579-1/ASME FFS-1 (Fitness-For-Service) references API 941 as a source of information for assessing the risk of HTHA in equipment in hydrogen service; API 571 (Damage Mechanisms Affecting Fixed Equipment in the Refining Industry) references API 941 as a source of information for identifying HTHA as a damage mechanism in equipment in hydrogen service; API 581 (Risk-Based Inspection Methodology) references API 941 as a source of information for determining the probability of failure due to HTHA in equipment in hydrogen service; etc. and Alloy Steel Pipe for High-Temperature Service) influences API 941 by providing specifications for steels that are commonly used in hydrogen service; ASME BPVC Section II (Materials) influences API 941 by providing specifications and properties for steels that are commonly used in hydrogen service; ASME BPVC Section IX (Welding, Brazing, and Fusing Qualifications) influences API 941 by providing qualifications and procedures for welding steels that are commonly used in hydrogen service; etc. - It is compliant with other standards and regulations that provide requirements and obligations for ensuring the safety and health of workers, the public, and the environment in hydrogen service. For example, OSHA 29 CFR 1910.119 (Process Safety Management of Highly Hazardous Chemicals) requires API 941 to be followed as a recognized and generally accepted good engineering practice for preventing HTHA in equipment in hydrogen service; EPA 40 CFR Part 68 (Chemical Accident Prevention Provisions) requires API 941 to be followed as a recognized and generally accepted good