Valve Sour Gas Service: NACE MR0175/ISO 15156 Material Requirements

Sour gas (H2S-containing) service causes sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC). This guide covers valve material selection per NACE MR0175/ISO 15156.

Sour gas contains hydrogen sulfide (H2S). The severity of sour service is determined by partial pressure of H2S. NACE MR0175 defines three regions: Region 1 (mild), Region 2 (moderate), and Region 3 (severe).

Carbon and low-alloy steels: Maximum hardness 22 HRC (Region 1 and 2). For Region 3, additional restrictions apply including limits on yield strength.

Austenitic stainless steels: Maximum hardness 33 HRC. Must be solution-annealed. Sensitized stainless steel is susceptible to SSC.

Duplex stainless steels: Hardness limit 28 HRC. Must be heat-treated per manufacturer recommendation.

Body: ASTM A352 LCB/LCC (low-temperature carbon steel, normalized) for Region 1-2. CF3M (316L) for Region 3 or corrosive environment.

Trim: 13Cr (UNS S41000) is acceptable up to 22 HRC. For higher corrosion resistance, use Alloy 625 (Inconel 625) cladding or solid Alloy 625 trim.

Gaskets: Use flexible graphite or PTFE. Avoid elastomeric gaskets unless specifically qualified for sour service.

All welds on wetted parts must be post-weld heat treated (PWHT) to reduce hardness. PWHT temperature: 593-621degC for carbon steel, holding time 1 hour per 25mm of thickness.

Hardness testing: Measure hardness after PWHT. Weld hardness must not exceed 22 HRC. Document all hardness test results in the valve material test report.

Hydrogen-induced cracking (HIC) occurs in the base metal, not the weld. Use HIC-resistant steel plates (per NACE MR0103) for vessel and valve bodies in wet H2S service.

HIC test: Immersion in acidic H2S solution for 96-720 hours. Acceptable criterion: no blisters visible under 10x magnification, CLR (crack-length ratio) <15%.

Yes, if the hardness is below 33 HRC and the material is solution-annealed. However, 316 is susceptible to pitting in chloride-containing sour gas. Consider Alloy 625 cladding for severe service.

SSC (sulfide stress cracking) occurs under tensile stress and is related to hardness. HIC (hydrogen-induced cracking) occurs without applied stress and is a base-metal phenomenon. Both are prevented by proper material selection per NACE MR0175.

Ted Wang

Wechat/Whatsapp: +86 18267833722

Email: sales@wofervalve.com

Website: www.wofervalve.com