Sour Service Valve Specification

What defines a sour service environment?

A process environment is classified as sour under NACE MR0175 / ISO 15156 when it contains H2S and liquid water simultaneously, with the H2S partial pressure exceeding 0.0003 MPa (0.3 mbar absolute) in the gas phase or equivalent dissolved H2S in liquid phase. Wet CO2 environments can also trigger the requirement if they produce carbonic acid (H2CO3) that generates atomic hydrogen. The classification triggers mandatory material restrictions to prevent sulphide stress cracking (SSC), hydrogen-induced cracking (HIC), and stress-oriented hydrogen-induced cracking (SOHIC).

Failure modes in sour service

Three distinct cracking mechanisms operate in H2S environments and each has different material sensitivity:

• -SSC (Sulphide Stress Cracking): Affects high-strength/high-hardness carbon and low-alloy steels. Hydrogen atoms generated by H2S corrosion enter the steel lattice and embrittle it under tensile stress. Prevented by hardness limits (max HRC 22).
• -HIC (Hydrogen-Induced Cracking): Occurs in carbon steels with elongated inclusions (sulfide stringers). Hydrogen accumulates at inclusion-matrix interfaces causing internal blisters and laminar cracking. Prevented by using calcium-treated, HIC-resistant steel (NACE TM0284 tested).
• -SOHIC (Stress-Oriented HIC): A combination of HIC and SSC. Hydrogen blisters link up in a ladder pattern perpendicular to the applied stress — typically at welds. Most severe in high-pressure, high-stress applications.
• -SCC (Stress Corrosion Cracking): Affects austenitic stainless steels and CRAs in chloride-containing sour environments. Prevented by material selection (duplex SS, Alloy 825, or higher CRA grades).

Material selection for valve bodies

Carbon steel A216 WCB is acceptable for sour service provided: it is supplied with PWHT (post-weld heat treatment); the body, bonnet, and all pressure-retaining components have hardness HRC 22 or below; and documentation includes hardness survey records per ISO 15156-2. For refineries and upstream upstream oil/gas, A216 WCB NACE-compliant is the most common choice. Where chlorides are present alongside H2S, or where temperatures exceed 80°C, consider upgrading to A351 CF8M (316 SS) or duplex stainless A890 Gr.4A (Duplex 2205) which offers higher PREN resistance to pitting.

Trim and stem material for sour service

Trim materials must also comply with NACE MR0175. Standard SS 410 (A276 Gr.410) has hardness around HRC 26-30 and is NOT acceptable in sour environments. The correct trim materials are: SS 316 (A276 Gr.316) cold-worked to HRC 22 maximum — this is standard NACE trim for most process valves. For hard-face applications (high-pressure gate valve seats), Stellite 6 (cobalt alloy) is acceptable as NACE MR0175 permits cobalt alloys. 17-4PH stainless steel (A693) in H900 condition has HRC above 40 and is NOT acceptable. Use H1150 or H1150-M condition with HRC 30-35 only where specifically approved — consult ISO 15156-3 for conditions.

• -NACE-compliant trim: A276 Gr.316 (SS316) HRC≤22
• -Acceptable hard-face: Stellite 6 (Alloy 6) per NACE MR0175
• -NOT acceptable: SS410 trim (HRC 26-30), 17-4PH in H900 condition
• -Monel K-500 trim: acceptable per NACE MR0175 in cold-worked or aged condition with hardness limits

Packing and sealing requirements

Standard PTFE chevron packing is acceptable for sour service. However, graphite packing should be avoided in high-pressure H2S service as the combination of graphite and steel creates a galvanic couple that accelerates hydrogen uptake. Live-loaded packing (spring-loaded or Belleville washer arrangement) is preferred for sour service valves to maintain constant sealing load as packing creeps — particularly for ISO 15848-1 fugitive emission requirements which are often co-specified on sour service valves. Soft seat materials (PTFE, RPTFE, PEEK) are compatible with H2S service. Metal seats in high-pressure sour service should use Stellite-faced seats in preference to SS410 hard-faced seats.

Testing and documentation requirements

A NACE sour service valve delivery package must include: EN 10204 3.1 or 3.2 MTCs for all pressure-retaining components. Hardness survey records showing HRC 22 or below at body, bonnet, trim, and stem (typically at 5-10 locations per component). PWHT records confirming heat treatment and temperature. If HIC resistance is required, NACE TM0284 test reports. If H2S sour service qualifies the valve as Class B or A criticality, third-party (TPI) inspection and countersigned EN 10204 3.2 MTCs should be specified.

• -EN 10204 3.1 MTCs with hardness survey records
• -PWHT record (time-temperature chart) for carbon steel
• -NACE MR0175 compliance statement from manufacturer
• -TM0284 report if HIC resistance required
• -Fugitive emission certificate (ISO 15848-1) if specified

Key Specification Decisions

Material Selection Reference

Frequently Asked Questions