Valve Body Material Selection for Sour Service

ted wang
May 31
2 min read

Sour service refers to process environments containing hydrogen sulfide (H2S), which creates a risk of sulfide stress cracking (SSC), hydrogen embrittlement (HE), and stress-oriented hydrogen-induced cracking (SOHIC) in susceptible materials. NACE MR0175/ISO 15156 governs material selection for valves in sour service.

Definition of Sour Service

H2S partial pressure exceeds 0.0003 MPa (0.05 psi) per NACE MR0175
Concentration and pH determine severity of sour service environment
Region 3 (most severe): high H2S partial pressure in aqueous environment
Dry gas service with H2S may be exempt from SSC requirements
Service classification determines allowable materials and hardness

Material Restrictions

Carbon and low alloy steels must have maximum hardness of 22 HRC (250 HB) to resist SSC in sour service. High-strength materials, cold-worked surfaces, and hardened trim components are susceptible to hydrogen damage. Electroless nickel plating and similar coatings are restricted or require qualification in sour environments.

Approved Materials for Sour Service

Carbon steel ASTM A216 WCB (normalized or normalized and tempered)
Low alloy steel limited to maximum carbon content and heat treatment
13Cr martensitic stainless steel restricted to specific H2S partial pressures
Austenitic stainless steels (316, 317) generally acceptable
Nickel alloys (Alloy 625, Alloy C-276) for severe sour service

Hardness Testing Requirements

NACE MR0175 requires hardness testing of all pressure-containing components and trim parts. Hardness measurements are taken on representative locations of the finished valve after all heat treatment is complete. For small components where Rockwell testing is impractical, Vickers or Brinell conversions may be used with appropriate corrections.

Special Considerations

Welded valve bodies require post-weld heat treatment to reduce HAZ hardness
Flame-cut or mechanically stressed areas may need additional heat treatment
Bolting materials must also comply with sour service requirements
Documentation and material traceability are critical for regulatory compliance
Third-party inspection and certification often required for sour service valves