Materials Selection in High-Temperature Hydrogen Services in Refinery

High-temperature hydrogen services pose unique challenges to industrial systems, demanding meticulous materials selection and stringent operational practices. Module C of API RP 941, the guiding document for such applications, provides essential guidelines to ensure the integrity and safety of equipment operating in these conditions. This article delves into the key considerations outlined in Module C, highlighting their significance and implications.

High-Temperature Hydrogen Services Materials Selection

1. Materials Selection: A Critical Foundation

The cornerstone of any high-temperature hydrogen service is the selection of appropriate materials. This is determined by API RP 941, which mandates that materials selection accounts for factors like hydrogen partial pressure and operating conditions. In this context, it’s crucial to include a safety margin of 50°F and 50 psia to the maximum operating conditions for materials selection, ensuring robustness in the chosen materials against potential challenges.

2. Piping: Valve Integrity and Inspection

Piping systems are vulnerable points in high-temperature hydrogen services. To mitigate risks, specific standards are set forth:

• All valves shall adhere to API Trim Number 5, ensuring compatibility with hydrogen service.
• Rigorous quality control is prescribed for valve bodies through Level 2 Radiographic Testing (RT) in accordance with MSS SP54. This ensures structural integrity and helps identify potential defects.
• Piping constructed using 1 1/4Cr-1/2Mo and 2 1/4Cr-1Mo materials must undergo post weld heat treatment, irrespective of their process or steam applications. This treatment bolsters the mechanical properties of materials and enhances their resistance to potential degradation.
• It’s important to note that temper-resistant 2 1/4Cr-1Mo filler metals are prohibited for piping construction due to their potential impact on structural integrity.

3. Vessels: Fabrication and Appropriate Materials

Vessel fabrication in high-temperature hydrogen services follows specific guidelines:

• The fabrication of vessels using 2 1/4Cr-1Mo materials adheres to API RP 934-A, ensuring standardized fabrication practices that enhance the durability of vessels.
• Materials such as 2 1/4Cr-1Mo are not recommended for vessels in cyclic services like coke drums due to their susceptibility to damage.
• Depending on factors like shell thickness and operational temperature, vessel fabrication shall adhere to API 934-C for 1 1/4Cr-1/2Mo materials, ensuring safe and compliant construction.
• For low alloy steel vessels with a thickness exceeding 4″, it’s imperative to utilize steels that meet or exceed the quality of 2 1/4Cr-1Mo steels, enhancing the longevity of equipment.

4. Thermal Fatigue: Managing Temperature Dynamics

Mixing high-temperature streams with lower-temperature counterparts can result in thermal fatigue, especially for materials like 300 series stainless steels. To mitigate this risk:

• A ΔT (temperature difference) limit of 50°C is set for stainless steels, ensuring safe operational practices.
• Redesigning thermal injection points to foster effective mixing centers is crucial for preventing thermal fatigue.
• Coinjecting streams is recommended over counter-current injection, as the latter can heighten the risk of equipment failure during plant operations.

Conclusion

Navigating high-temperature hydrogen services necessitates a holistic approach that encompasses materials selection, rigorous fabrication standards, and a comprehensive understanding of thermal dynamics. By adhering to the guidelines outlined in Module C of API RP 941, industries can ensure the reliability, safety, and longevity of their equipment in challenging operating environments.

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