Sour service compliance is a materials statement, not a sticker. If you cannot show it, the valve is priced as standard service.
What sour service means
Sour service means the process contains hydrogen sulphide at a partial pressure and pH that can cause sulphide stress cracking in hard, high-strength steels. NACE MR0175, now also published as ISO 15156, sets out which materials are acceptable and under what conditions.
For a valve, compliance means the body, bonnet, stem, trim, bolting and any internal components are made from materials that meet the standard, and that the hardness limits have been respected in manufacture and any weld repair.
How to tell whether a valve is compliant
Look for the marking. Compliant valves are frequently stamped NACE MR0175 or the equivalent, and the material certificates will state compliance. Some manufacturers use a distinctive body marking or a separate tag.
If the marking is absent, the valve is not automatically non-compliant, but it is unproven. In a surplus transaction, unproven means priced as standard service, because verifying the metallurgy and hardness costs money.
Why it changes the price
Sour service valves are bought by oilfield, pipeline and refinery operators who need compliant equipment and cannot substitute. The pool of compliant surplus is small, so the premium can be substantial, often well above the difference in material cost.
That premium disappears the moment the documentation does. A NACE-marked valve with certificates is a different asset from an identical valve with the marking ground off.
Common misunderstandings
Stainless steel is not automatically compliant. Some stainless grades are acceptable only within limits, and hardness control still applies. Duplex and super duplex are often used in sour service but still require the correct specification and verification.
A valve that ran in sour service is not necessarily a sour service valve. Service history and material compliance are separate questions, and buyers ask about both.
What to send
Send the body stamping, the nameplate and any material certificate. If the valve is marked for sour service, photograph the marking specifically. If you have a material test report, send the page that states the standard and the hardness results.
If none of that exists, say so plainly. An honest standard-service lot is easy to price; an optimistic sour-service claim is not.
Questions sellers ask
Is all stainless steel NACE compliant?
No. Compliance depends on grade, condition and hardness, and must be demonstrated.
Do you buy non-compliant valves?
Yes, at standard-service pricing. Compliance is a value factor, not a purchase condition.
Can compliance be verified after the fact?
Sometimes, through material testing, but the cost usually outweighs the benefit on a single valve.