Sour service — process media containing hydrogen sulfide — changes valve material selection more than almost any other operating variable. The governing framework is NACE MR0175, published in harmonized form as ISO 15156, and getting it wrong does not show up as a performance issue on day one. It shows up as sudden brittle failure of a hardened component months or years into service.
In the presence of H2S and water, high-strength steels can crack through sulfide stress cracking (SSC). The susceptibility scales with material hardness and applied tensile stress: a 22 HRC valve stem will survive conditions that crack a 35 HRC spring pin. Carbon steel pressure boundaries are usually the easy part; the trap is in the small hardened parts — stems, seats, fasteners, springs — where hardness is often the price of wear resistance.
ISO 15156 splits materials by environment severity — H2S partial pressure, pH, temperature and chloride content define the region. Within those regions it sets acceptance limits: maximum hardness for carbon and low-alloy steels, restricted annealed conditions, and qualification requirements for corrosion-resistant alloys. Common outcomes for valve trim include:
A "NACE certificate" from a vendor is only as good as the traceability behind it. Procurement teams should require hardness test reports tied to heat numbers for the components the standard governs — not a blanket declaration. For critical service, witness hardness verification at the factory. The incremental cost is trivial next to the cost of an in-line stem failure on a sour gas gathering system.
Define the H2S partial pressure and pH from process data, not from a service label. Assign the ISO 15156 materials class, select trim against that class, and write the hardness requirements into the purchase specification explicitly. A specification that says "sour service per NACE MR0175" without component-level hardness limits will be met with the cheapest interpretation available.