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API Publ 937-A-2005: Evaluation of the Effects of Hydrogen on Materials for Hydrogen Service in Refineries
A Comprehensive Guide to Managing Hydrogen Damage in Refining Equipment
API Publ 937-A (2005), titled “Evaluation of the Effects of Hydrogen on Materials for Hydrogen Service in Refineries”, is a critical publication by the American Petroleum Institute that provides engineers and inspectors with a systematic approach to assessing material suitability in hydrogen-containing environments. This publication consolidates decades of industry experience and research on hydrogen damage mechanisms, offering guidance on material selection, testing protocols, and inspection strategies for refinery equipment such as pressure vessels, piping, and heat exchangers.
Scope and Purpose of API Publ 937-A
API Publ 937-A addresses the evaluation of metallic materials intended for service in refinery environments where hydrogen damage is a concern. Its scope encompasses both new construction and in-service equipment. The primary objective is to provide a consistent methodology for determining the resistance of materials to various forms of hydrogen attack, including low-temperature cracking mechanisms and high-temperature hydrogen attack (HTHA). The publication is not a mandatory standard but serves as a recommended practice to be used alongside relevant API and NACE standards.
Key areas covered include:
- Identification of hydrogen damage mechanisms based on operating conditions (temperature, pressure, hydrogen partial pressure, H₂S concentration, pH).
- Guidelines for material selection during design to minimize susceptibility.
- Methods for evaluating existing equipment through non-destructive examination (NDE) and mechanical testing.
- Criteria for acceptance of materials based on standard test methods such as NACE TM0284 (HIC) and NACE TM0177 (SSC).
- Mitigation measures, including post-weld heat treatment (PWHT), cladding, and corrosion inhibition.
The publication emphasizes a risk-based approach, allowing operators to prioritize inspections and replacements based on damage severity.
Technical Requirements and Material Evaluation Criteria
The core of API Publ 937-A lies in its criteria for material evaluation. It establishes that materials used in hydrogen service must demonstrate adequate resistance to hydrogen-induced cracking (HIC) and stress-oriented hydrogen-induced cracking (SOHIC) in wet H₂S environments, as well as resistance to hydrogen stress cracking (HSC) in high-strength steels and hydrogen embrittlement in other alloys. The publication details specific requirements for:
- Chemical composition: Limits on sulfur and phosphorus content, calcium treatment for sulfide shape control.
- Mechanical properties: Hardness limits, tensile strength requirements, and toughness criteria.
- Testing requirements: HIC testing per NACE TM0284 (typically in Solution A, pH 2.7–4.0), SSC testing per NACE TM0177 (Method A for tensile, Method D for DCB), and hydrogen permeation studies if applicable.
- Acceptance criteria: Maximum crack length ratio (CLR), crack thickness ratio (CTR), and crack sensitivity ratio (CSR) thresholds for HIC resistance.
| Damage Mechanism | Susceptible Materials | Key Parameters | Test Methods |
|---|---|---|---|
| Hydrogen-Induced Cracking (HIC) | Carbon steels (low strength) | H₂S ≥ 50 ppm, pH < 5, water-wet | NACE TM0284 |
| Stress-Oriented HIC (SOHIC) | Low-strength steels with high inclusion content | High residual stress, sour environment | NACE TM0284 + stress |
| Hydrogen Blistering | Carbon steels, low-alloy steels | High H₂S, low pH, metallic inclusions | Visual inspection, UT |
| Hydrogen Stress Cracking (HSC) | High-strength steels, martensitic alloys | Hardness > HRC 22, tensile stress, H₂S | NACE TM0177 |
| High-Temperature Hydrogen Attack (HTHA) | Carbon and low-alloy steels | Temperature > 400°F, high H₂ partial pressure | API RP 941 curves |
Caution: The presence of even trace amounts of H₂S can significantly accelerate hydrogen charging. Always verify the sour service classification per NACE MR0175/ISO 15156 before applying API Publ 937-A criteria.
Implementation Highlights for Field Application
Successful implementation of API Publ 937-A requires a multidisciplinary approach involving materials engineers, corrosion specialists, and inspection personnel. Key implementation steps include:
- Service environment characterization: Accurately determine operating temperature, pressure, hydrogen partial pressure, H₂S concentration, pH, and presence of water. Use worst-case scenarios for design.
- Material documentation and traceability: Verify material test reports for chemistry, heat treatment, and mechanical properties. Ensure compliance with the specified limits.
- Fabrication and welding controls: Implement proper welding procedures (WPS/PQR) that include PWHT when required. Control hardness in weld heat-affected zones (HAZ) to avoid hydrogen cracking.
- Inspection planning: Use NDE techniques such as manual UT (for HIC/SOHIC detection), acoustic emission, and hydrogen flux monitoring. Establish baseline and periodic inspection intervals based on damage rate.
- Repair and mitigation: If damage is found, the publication provides guidance on grind-out, weld overlay, or replacement based on the extent and location.
Tip: For new equipment, specify clean steels with low sulfur (<0.002%) and calcium treatment to reduce inclusion shape anisotropy, which greatly improves HIC resistance. This is a cost-effective proactive measure.
Success Story: A refinery implemented the API Publ 937-A evaluation criteria for their amine regeneration unit, resulting in a 50% reduction in HIC-related repairs after switching to calcium-treated steel and optimizing the post-weld heat treatment schedule.
Compliance and Quality Assurance Notes
While API Publ 937-A is a recommended practice, it is often incorporated by reference in industry contracts and regulatory permits. Compliance with NACE MR0175/ISO 15156 is mandatory for sour service environments in many regions; API Publ 937-A complements this by providing material evaluation details specific to refinery hydrogen service. Companies should maintain a compliance matrix that maps the publication’s recommendations to their internal specifications. Key compliance aspects include:
- Documentation: Maintain records of all material certifications, test results (HIC/SSC), and inspection findings.
- Personnel qualifications: Ensure that personnel performing material evaluations and NDE are certified to applicable standards (e.g., ASNT SNT-TC-1A for NDT).
- Third-party verification: For critical services, consider independent review of material selection and testing by a qualified third party.
- Continuous improvement: API Publ 937-A is based on 2005 knowledge; operators should stay current with newer editions and supplementary documents from API and NACE.
Danger: Failure to properly evaluate materials for SOHIC can lead to sudden catastrophic failure without prior visible warning. SOHIC cracks propagate rapidly under stress and can cause through-wall leakage or rupture in pressure vessels.
Frequently Asked Questions
Q: What is the difference between API Publ 937-A and NACE MR0175/ISO 15156?
A: NACE MR0175/ISO 15156 specifies material requirements for equipment in sour gas/oil production environments, focused on sulfide stress cracking and other cracking mechanisms. API Publ 937-A is specifically for refinery hydrogen service, covering a wider range of hydrogen damage mechanisms (HIC, SOHIC, blistering) and includes evaluation methods applicable to both new and in-service equipment. They complement each other: NACE MR0175 provides material selection tables, while API 937-A provides detailed evaluation procedures for HIC resistance and other hydrogen damage.
A: NACE MR0175/ISO 15156 specifies material requirements for equipment in sour gas/oil production environments, focused on sulfide stress cracking and other cracking mechanisms. API Publ 937-A is specifically for refinery hydrogen service, covering a wider range of hydrogen damage mechanisms (HIC, SOHIC, blistering) and includes evaluation methods applicable to both new and in-service equipment. They complement each other: NACE MR0175 provides material selection tables, while API 937-A provides detailed evaluation procedures for HIC resistance and other hydrogen damage.
Q: Is API Publ 937-A applicable to existing refinery equipment?
A: Yes, the publication provides guidelines for evaluating materials already in service. It recommends baseline inspections, fitness-for-service assessments per API 579, and damage rate calculations to establish remaining life. It is especially useful when process conditions change (e.g., increased H₂S or hydrogen partial pressure) or when corrosion damage is suspected.
A: Yes, the publication provides guidelines for evaluating materials already in service. It recommends baseline inspections, fitness-for-service assessments per API 579, and damage rate calculations to establish remaining life. It is especially useful when process conditions change (e.g., increased H₂S or hydrogen partial pressure) or when corrosion damage is suspected.
Q: What testing is required to demonstrate HIC resistance per API Publ 937-A?
A: The publication typically requires HIC testing per NACE TM0284 using test Solution A (pH 2.7–4.0) for 96 hours. Acceptance criteria are usually: CLR ≤ 15%, CTR ≤ 5%, CSR ≤ 2%. Additional tests like shear fracture testing may be specified for heavy plates. Weld coupons may also be tested to assess HIC susceptibility in the HAZ.
A: The publication typically requires HIC testing per NACE TM0284 using test Solution A (pH 2.7–4.0) for 96 hours. Acceptance criteria are usually: CLR ≤ 15%, CTR ≤ 5%, CSR ≤ 2%. Additional tests like shear fracture testing may be specified for heavy plates. Weld coupons may also be tested to assess HIC susceptibility in the HAZ.
This article is based on API Publ 937-A (2005) and related references. For the most current requirements, consult the latest edition of the publication. Technical relevance verified as of 2026.