Two valves can share the same body material, the same class rating and the same test certificate — and only one of them is safe on a sour gas line. The difference is invisible to a visual inspection: it sits in hardness numbers, heat-treatment condition and weld records. NACE MR0175 / ISO 15156 is the standard that governs those differences for oil and gas production service, and it appears in almost every upstream and pipeline tender. Here is what it actually requires from a valve, and how to buy against it without getting caught out.
What counts as sour service
Sour service means exposure to produced fluids containing hydrogen sulphide (H₂S) in oil and gas production and transport. The standard's trigger is not H₂S concentration alone but H₂S partial pressure — the mole fraction of H₂S multiplied by the total system pressure. The working threshold for sulfide stress cracking evaluation in sour gas systems is a partial pressure of 0.3 kPa (0.05 psi). The consequence is counter-intuitive: a lean gas with a few hundred ppm of H₂S can be firmly inside sour scope at pipeline pressure, while a richer stream at near-atmospheric pressure may fall outside it. The standard further grades severity into regions using partial pressure and in-situ pH, which is why RFQs for sour duty should state the actual service conditions, not just the word “sour.”
One scope note: MR0175/ISO 15156 is written for upstream and midstream production environments. Refinery wet-H₂S service is covered by a different document, NACE MR0103/ISO 17945. Quoting MR0175 on a refinery hydroprocessing line, or MR0103 on a wellhead manifold, is a specification error — name the right standard for the environment.
The failure mechanism: sulfide stress cracking
Sulfide stress cracking (SSC) is a form of hydrogen embrittlement. The corrosion reaction between wet H₂S and steel generates atomic hydrogen at the metal surface, and H₂S poisons the recombination of those atoms into harmless hydrogen gas. The atomic hydrogen instead diffuses into the steel, where it embrittles hard microstructures. Under tensile stress — applied or residual — the material cracks in a brittle mode, often quickly and with no visible warning. Three facts drive the standard's logic: harder material is more susceptible, which is why hardness limits are the core control; cracking severity is worst near ambient temperature, so a valve in a cool piping run can be at higher risk than the same steel in hot service; and weld heat-affected zones and cold-worked spots are classic initiation sites, which is why welding and manufacturing condition are controlled, not just base material grade.
What MR0175/ISO 15156 actually controls
The standard has three parts: Part 1 sets principles, Part 2 covers carbon and low-alloy steels, Part 3 covers corrosion-resistant alloys (CRAs). For carbon and low-alloy steels — the A105, A216 WCB and A182/A217 alloy grades that make up most valve bodies — the headline requirement is a maximum hardness of 22 HRC (approximately 237 HB), achieved through controlled chemistry and heat treatment. Welds must be qualified with hardness surveys across weld metal and HAZ, and post-weld heat treatment is the usual route to bringing welded regions under the limit. For CRAs, compliance is conditional rather than automatic: each alloy family carries environmental limits — H₂S partial pressure, chloride content, temperature, pH — plus condition requirements such as solution annealing for austenitics and specific age-hardening treatments for precipitation-hardened grades.
| Material family | Key controls for sour service |
|---|---|
| Carbon / low-alloy steels (A105, WCB, F5, WC6) | Max 22 HRC; specified heat-treatment condition; weld hardness surveys; PWHT as needed |
| Austenitic SS (F316, CF8M) | Solution-annealed, substantially free of cold work, max 22 HRC; chloride and temperature limits apply |
| Duplex SS | Permitted only within defined H₂S/chloride/temperature envelopes per Part 3 |
| Exposed bolting | Controlled-hardness grades — A193 B7M / L7M studs with A194 2HM nuts are the usual selection |
| Springs and small trim parts | Standard spring steels excluded; cobalt or nickel alloys are the typical compliant choice |
Which valve parts the requirement applies to
- Pressure boundary: body, bonnet or closure, and any welded pressure-containing joints. These see full process exposure and are the first-order concern.
- Bolting: the standard distinguishes exposed from non-exposed bolting. Studs that can contact the sour environment — buried, submerged, under insulation, or inside a leaking flange envelope — must be controlled-hardness grades such as B7M/2HM. Standard B7 studs are commonly harder than the sour limit.
- Trim and internals: stem, ball or wedge, seats, and especially springs — the classic weak point in check valves and seat-loaded ball valves, because ordinary spring steel is far too hard for sour duty.
- Process-wetted parts govern: a handwheel does not need NACE compliance; anything the sour fluid can reach does.
How to specify it on a purchase order
- State “NACE MR0175 / ISO 15156 (latest edition)” as a line-item requirement — not a footnote — and identify the scope: pressure-containing parts, process-wetted trim, and exposed bolting.
- Give the service definition where you have it: H₂S partial pressure, temperature, chloride level. It lets the supplier confirm CRA suitability under Part 3 instead of guessing.
- Call out bolting explicitly: A193 B7M studs with A194 2HM nuts where bolting is exposed. This is the most commonly missed line on sour-service POs.
- Require the compliance statement to appear on the mill test certificate, alongside actual hardness values — see EN 10204 3.1 vs 3.2 MTCs for what each certificate level commits the maker to.
- For welded valves, require WPS/PQR qualified with hardness surveys and PWHT records where applied.
Common mistakes
- Assuming stainless steel is automatically compliant. It is not. A 316 component that is cold-worked, or a CF8M casting outside its solution-annealed condition, can exceed the hardness limit — and Part 3 imposes chloride and temperature ceilings on austenitics regardless of hardness.
- Forgetting the bolting and springs. A NACE-compliant body with standard B7 studs and a carbon-steel check-valve spring is a non-compliant assembly.
- Wrong standard for the environment. MR0175 for production facilities; MR0103 for refinery process units.
- No edition stated. The standard has evolved; state the edition or “latest” so the certificate and the PO can be reconciled at receipt.
- Accepting a brochure claim instead of a certificate. “NACE trim available” in a catalogue is not evidence that your valves were built to it.
Documentation to demand
At minimum: an EN 10204 3.1 MTC (3.2 for critical lines) recording actual hardness results and heat-treatment condition for pressure-boundary parts; a statement of MR0175/ISO 15156 compliance on the certificate or a separate declaration referencing the PO; PWHT charts and weld hardness surveys for welded construction; and bolting certificates showing the controlled-hardness grade. On ball valves for pipeline duty, tie this into the broader spec the same way you would tie in testing and design standards — the API 6D ball valve selection guide covers where NACE sits among the other line items. For what the base material grades themselves mean, see the forged valve materials guide.