NACE MR0175 / ISO 15156 Valves for Sour Service: H2S and SSC Guide

What Is Sour Service and Why It Matters

A fluid is classified as sour when it contains hydrogen sulphide (H2S) above the threshold defined in NACE MR0175 / ISO 15156 Part 2 — broadly, a H2S partial pressure exceeding 0.0003 MPa (0.05 psi) in a multiphase system with free water present. Wet H2S causes two primary cracking mechanisms in steel: Sulphide Stress Cracking (SSC) — hydrogen atoms generated by the cathodic reaction at the metal surface absorb into high-strength, high-hardness steel and cause sudden brittle fracture under tensile stress; and Hydrogen Induced Cracking (HIC) — hydrogen atoms accumulate at internal inclusions and precipitate, delaminating the steel microstructure in a stepwise pattern. Both mechanisms have caused catastrophic valve failures in sour crude, amine treating, sour water stripper, and high-pressure gas gathering applications.

Key NACE MR0175 Requirements for Carbon Steel Valves

• Maximum hardness 22 HRC (237 HBW, 248 HV10) for carbon steel and low-alloy steel pressure-containing parts (body, bonnet, end flanges) — applies to base metal, welds, and heat-affected zones
• AISI 4130 and 4140 alloy steels: permitted only in normalised-and-tempered condition with hardness 22 HRC maximum; quench-and-temper condition to higher hardness is NOT permitted
• Carbon steel ASTM A105 (forgings) and A216 WCB (castings): both are standard NACE-acceptable materials when hardness is verified at or below 22 HRC
• Welding procedure qualification (WPS/PQR) to ASME IX must demonstrate that weld deposits and HAZ meet the 22 HRC limit — post-weld heat treatment (PWHT) is typically required for ASME P1 and P4 group materials
• Hardened valve trim: NACE MR0175 Table A2 lists permitted trim materials — 13Cr (Type 410 SS) in hardened condition is permitted at 22 HRC maximum; Stellite 6 hardfacing is permitted

NACE MR0175 Compliance: Alloy and Stainless Steel Valve Trim

Sour Service Valve Specification Checklist

1. 1Confirm sour service classification: calculate H2S partial pressure = mole fraction H2S multiplied by total system pressure; if greater than 0.0003 MPa in wet service, NACE MR0175 applies
2. 2Specify NACE MR0175 / ISO 15156 compliance on the valve datasheet and purchase order
3. 3Request hardness test reports for all pressure-containing parts and welds — verify results at or below 22 HRC for carbon/low-alloy steel
4. 4Specify HIC resistance testing per NACE TM0284 for body plate or castings in wet sour service
5. 5For austenitic SS internals (316L trim): specify mill annealed and non-cold-worked
6. 6Specify anti-static device for ball valves per API 6D in sour flammable service
7. 7Specify fire-safe design to API 607 for ball, plug, and butterfly valves in sour flammable service
8. 8For offshore and subsea sour service: additionally specify NACE TM0177 SSC testing on the actual heat of steel used for pressure parts

Common Sour Service Applications and Recommended Valve Types

Documentation Requirements for Sour Service Valves

• EN 10204 3.1 or 3.2 Mill Test Report confirming chemical composition (low S and P for HIC resistance) and mechanical properties including hardness
• NACE MR0175 Compliance Certificate issued by the valve manufacturer
• Hardness test records (Brinell or Rockwell) per ASTM E18 for all pressure-containing parts and weld HAZ
• PWHT records (where applicable) confirming temperature, hold time, and cooling rate
• HIC test report per NACE TM0284 (if specified) — CLR, CTR, CSR must be within limits (typically CLR 15% max, CTR 5% max, CSR 2% max)
• SSC test report per NACE TM0177 Method A (if specified for high-strength components)