NACE MR0175 Valves From China: What Sour Service Compliance Must Show

A valve described as "sour service rated" by a supplier and a valve that meets NACE MR0175/ISO 15156 for your service are not necessarily the same thing. The difference matters on every H2S-containing production system.

A certificate that matched the purchase order

An upstream operator ordered ball valves for a sour gas application. The specification sheet said NACE MR0175 compliant. The valves arrived with 316L trim, certified to NACE MR0175, and failed in the field within eight months.

The supplier had not lied. The paperwork was consistent with the purchase wording, but the wording was ambiguous. "NACE compliant" had been treated as "safe for this sour service" without locking down the actual environment and the exact material table entry.

Compliance depends on the environment

MR0175/ISO 15156 does not approve a material for all sour service. For carbon and low-alloy steels, sulphide stress cracking precautions apply once the H2S partial pressure in the gas reaches 0.3 kPa (0.05 psi). Corrosion-resistant alloys such as 316L are acceptable only inside the environmental limits set for their alloy group in ISO 15156-3 Annex A, covering temperature, H2S partial pressure, chloride, pH and elemental sulphur. Outside those limits the certificate does not cover the application, and commercial 316L trim is often the wrong choice.

Four elements the documentation must show

1. Hardness on every wetted and pressure-retaining component. For carbon and low-alloy steels, ISO 15156-2 sets a 22 HRC maximum for base metal and 250 HV for welds and heat-affected zones. It applies to the body, bonnet, stem, closure member and any welded area, measured component by component. A certificate that reports one bulk hardness value without measurement locations is not compliant documentation.
2. Heat treatment on the actual components. The records must reference the same heat numbers as the component markings, not a qualification coupon with no link to the production batch.
3. Overlay procedure qualification. Where Inconel or Stellite overlay is applied to seats or bores, the procedure qualification must include a hardness survey showing the base metal heat-affected zone stays within the limit after the overlay thermal cycle. This step is often missing from valve packages even when the overlay itself is correctly applied.
4. Chemistry that proves the material group. For carbon and low-alloy steels this includes nickel below 1 percent. For corrosion-resistant alloys, the composition must place the grade in the Annex A group being relied on, including PREN where the table uses it. Any low-sulphur limits for hydrogen-induced cracking resistance come from the project specification and must appear on the certificate.

Checks before award

• Confirm the H2S partial pressure, temperature, chloride content and pH at the valve location, not a generic "sour" note.
• Cross-reference those values to the ISO 15156-3 Annex A table for the proposed trim alloy before accepting 316L.
• Where the trim alloy is critical, add a first-article metallurgical review before the batch is released.

The documentation review adds about one day per order. Missing any element risks a material deficiency or a cracking failure in service.

Related: ball valves to API 6D, industrial valves and five red flags before a valve purchase order.

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