Ask ten suppliers to quote a valve for sour service and most will come back with a price. Ask them which material class they are offering, at what partial pressure of hydrogen sulphide, and with what hardness limits on the bolting, and the field narrows quickly. That narrowing is the point of this article.
Sour service is where oil and gas procurement most often looks correct on paper and fails at inspection. The failure is rarely dramatic. It is a hardness reading three points over the limit, a bolting grade nobody specified, or a certificate that covers the body but not the trim. None of it shows up in a commercial comparison, and all of it shows up months later when the equipment is already at site.
What the standard is actually for
NACE MR0175, published jointly as ISO 15156, exists to prevent sulphide stress cracking in materials exposed to wet hydrogen sulphide. It is a materials standard, not an equipment standard. It does not tell you which valve to buy. It tells you which materials may be used, in which condition, under which environmental limits.
That distinction matters because it means compliance cannot be assessed from a model number. It has to be assessed from the environment, and the environment has to come from you.
The environment defines everything upstream of it
Three variables set the material envelope: the partial pressure of hydrogen sulphide, the in-situ pH, and the chloride content. Temperature and the presence of elemental sulphur modify the picture further. Together these place the service into a region of the standard, and that region decides which alloys are permitted and in what heat treatment condition.
If a quotation states compliance without referencing the environment it was assessed against, it is a marketing claim rather than a technical one. This is the single most useful check a buyer can apply, and it takes one email to apply it.
Where the material chain extends further than people expect
The common error is treating sour service as a wetted-parts problem. It is broader than that, and the items below are where specifications most often fall short in practice.
- Bolting. Fasteners exposed to the environment carry their own hardness restrictions. A body and trim correctly specified with standard commercial bolting is a non-compliant assembly.
- Gaskets and seals. Elastomer selection is driven by the same environment, plus explosive decompression resistance where pressure cycles are rapid.
- Weld overlay and cladding. Base material and overlay are assessed separately. A compliant overlay does not make a non-compliant base acceptable.
- Hardness after fabrication. Welding, cold work and machining all change hardness locally. Compliance is a property of the finished component, not of the material certificate it started from.
- Instrument components. Diaphragms, capillaries and impulse line fittings sit in the same stream and are routinely left out of the material specification altogether.
- Vent and drain routing. Not a material question, but a design one that follows from the same hazard, and one a general purpose package will not have addressed.
What a compliant offer contains
When we evaluate offers for sour service equipment, these are the items that decide whether an offer is comparable to the others or not.
- A statement of the environment the offer was assessed against — partial pressure, pH, chlorides, temperature
- Material class per ISO 15156 for each pressure-containing and pressure-controlling component
- Hardness limits and the test method, with the location of the readings identified
- Bolting specification, stated explicitly rather than left as standard
- Elastomer selection with explosive decompression qualification where required
- EN 10204 3.1 certification as a minimum, with 3.2 third party endorsement where the contract calls for it
- Welding procedure qualification covering the as-delivered condition
An offer missing several of these is not necessarily wrong. It is incomparable, which is worse, because it will usually be the cheapest one in the table.
The practical sequence
The approach that avoids most of this is straightforward, and it is the same approach that shortens lead times as a side effect.
Start from the gas analysis, not the equipment list. If the analysis is old, say so and quote against the design basis instead. An assumed environment stated openly is manageable. An assumed environment left implicit is not.
Write the material requirement into the enquiry. Not “sour service” as a phrase, but the environment and the resulting material class. This alone removes most of the variance between offers.
Review certificates before release for manufacture, not on arrival. A hardness reading over the limit is a manufacturing decision at that stage and a rejected delivery at the other.
Include the boring items in the scope. Bolting, gaskets, instrument wetted parts. They are cheap to specify correctly and expensive to discover afterwards.
Why this ends up being a commercial question
Specifying sour service properly looks like extra work at enquiry stage, and it is. What it buys is a wider supplier base. Once the environment and material class are written down, manufacturers who were previously excluded because they could not interpret a vague enquiry become genuinely comparable — and competitive pricing on engineered equipment comes from having three qualified offers rather than from negotiating hard against one.
If you have a sour service requirement and the gas analysis is incomplete, that is a normal starting point rather than an obstacle. Send us what you have and we will tell you what is missing before quoting rather than after.