Sour Service Valves: What NACE MR0175 / ISO 15156 Actually Decides

ANSI/NACE MR0175-2021 / ISO 15156:2020 does not qualify valves. It qualifies metallic materials, for service in equipment, in H₂S-containing environments, in oil and gas production and natural-gas sweetening plants. The current edition is the fourth, and its three parts are identical adoptions: Part 1 general principles, Part 2 carbon and low-alloy steels and cast irons, Part 3 corrosion-resistant alloys and other alloys.

That sentence is the whole specification problem. A buyer who asks for a "NACE valve" has asked for a document about a material in an environment, without saying which environment.

This article sets out what the standard covers, which cracking mechanisms it addresses, where it stops, why dry hydrogen service is a different question, and what has to go into an enquiry instead.

Read it and you can write a sour-service line that a manufacturer can answer with a document.

What does the standard cover?

Materials for H₂S-containing environments in upstream production and gas sweetening — and it supplements rather than replaces the design code.

The scope statement in the published standard describes general principles and gives requirements and recommendations for the selection and qualification of metallic materials for service in equipment used in oil and gas production and in natural-gas sweetening plants in H₂S-containing environments, where the failure of such equipment can pose a risk to the health and safety of the public and personnel or to the environment. It states that it supplements, but does not replace, the materials requirements of the applicable design codes, standards or regulations.

Two structural points follow, and both are binding on how an enquiry is answered.

It is a materials document, not a valve document. Part 1 sets the general principles for selecting cracking-resistant materials. Part 2 covers carbon and low-alloy steels and cast irons. Part 3 covers corrosion-resistant alloys and other alloys. The valve product standard still governs the valve.

It is written around load-controlled design methods. The scope states that it applies to the qualification and selection of materials for equipment designed and constructed using load-controlled design methods, and directs strain-based design to Clause 5.

Which cracking mechanisms does it address?

All of the H₂S-driven ones, and it names them: sulfide stress cracking, stress corrosion cracking, hydrogen-induced cracking, stepwise cracking, stress-oriented hydrogen-induced cracking, soft zone cracking, and galvanically induced hydrogen stress cracking.

That list is the reason "sour service" cannot be answered with a grade name. Each mechanism has a different driver — hardness and heat-treatment state, microstructure, applied or residual stress, the environment's H₂S partial pressure, pH, chloride content and temperature. The standard's own position, as reproduced in industry guidance, is that the materials it covers are not immune to cracking under all conditions; they are qualified for defined service conditions.

Qualification runs by three routes, per Part 1: from existing lists and tables of qualified materials, from documented field experience, or from laboratory testing. Each requires the material to be described and documented, and the method of selection or qualification to be reported.

Where does the standard stop?

At the refinery gate, at non-metallics, and at anything not defined as an environment.

Refining and downstream are a different document. The scope states that the standard is not necessarily applicable to equipment used in refining or downstream processes. The downstream complement is NACE MR0103 / ISO 17945, metallic materials resistant to sulfide stress cracking in corrosive petroleum refining environments. A refinery unit in wet H₂S service is normally answered under that document, not under ISO 15156.

Non-metallic seats and seals are outside it. ISO 15156 qualifies metals. The elastomer or polymer in the seat, the graphite in the packing and the gasket material are separate selections made against the service conditions and the temperature — which is exactly where this site's seat-temperature work applies.

It does not decide the duty. It qualifies a material for a described environment. Where the environment is not described — no H₂S partial pressure, no pH, no chloride, no temperature — there is nothing to qualify against.

Why is hydrogen service a different question?

Because the cracking in ISO 15156 is sulphide-driven, and dry high-pressure hydrogen damages steel by a different route.

Every mechanism in the standard's list is caused by H₂S: sulphide stress cracking, hydrogen-induced cracking, stress-oriented hydrogen-induced cracking and the rest. Hydrogen embrittlement of a steel in dry, high-pressure hydrogen gas without H₂S is a separate mechanism, addressed by separate material and design practice. So a valve qualified for sour service is not thereby qualified for a hydrogen duty, and the reverse is also true.

This is where the Chinese supply side has been publishing. A hydrogen-service butterfly valve delivery recorded on this site — NPS42 Class 900 units for a coal-chemical methanol-synthesis unit — describes metal materials controlled to ultra-low sulphur and phosphorus content, and states hydrogen permeation reduced by more than 95% against the conventional arrangement, presented as the measure against hydrogen embrittlement risk. That is a hydrogen-service claim, not a sour-service one, and it belongs to a different document than ISO 15156.

Who has put what on the record?

Three entries make up the record: the standard's own scope, the downstream complement in NACE MR0103 / ISO 17945, and the Chinese hydrogen-service delivery reported on this site.

Where does this route stop?

A material qualification is bounded by the environment it was qualified for, and by the state of the material as supplied.

Three limits matter. The qualified condition includes heat treatment and hardness state, not just the alloy designation — a grade is not a qualification. The qualified environment is the one on the record; a change in H₂S partial pressure, pH or chloride moves the valve outside it. And the standard's lists are not a promise of immunity: industry guidance reproducing the standard's own position notes that listed materials are not free from cracking in all conditions.

What does the trend add up to?

The standard is being revised, and the boundary between upstream and downstream material practice is being drawn more explicitly in parallel documents. At the same time, the duties that generate these enquiries are moving: hydrogen service, CCUS and high-parameter coal-chemical units all put a valve into an environment that is not the one a grade was originally qualified for. The practical effect is that the environment description in the enquiry, not the alloy name on the datasheet, is becoming the document that decides.

When does this go to a human?

Five boundaries for sour and hydrogen enquiries: any duty where H₂S is present and its partial pressure is not stated; any hydrogen service; any duty where the required material qualification has to be evidenced rather than asserted; any duty below −101 °C (−150 °F), where automatic coverage ends unconditionally; and any duty above 425 °C (797 °F) or above 538 °C (1 000 °F).

• Any duty where H₂S is present and the enquiry does not state the partial pressure, pH, chloride content and temperature
• Any hydrogen service, and any duty that mixes H₂S with hydrogen
• Any enquiry where a material qualification has to be evidenced by a certificate, a field record or a laboratory report
• Any duty below −101 °C (−150 °F), where this site's automatic coverage ends unconditionally
• Any duty above 425 °C, and any duty above 538 °C without exception

What people assume, and what the standard says

Common questions about sour service valves

What do I actually have to state in the enquiry?

The environment — H₂S partial pressure, pH, chloride content, temperature, and whether water is present — plus the design basis and the applicable document. Without the environment there is nothing to qualify a material against.

Is ISO 15156 or NACE MR0103 the right document?

Upstream production and gas sweetening fall under ISO 15156 / NACE MR0175. Refining and downstream process units in wet H₂S fall under NACE MR0103 / ISO 17945. The two are complements, not editions of one another.

Does a sour-service valve also need a low-emission certificate?

They are separate requirements on separate documents. A sour-service material qualification says nothing about stem leakage to atmosphere.

Can the seat be soft if the body is qualified?

The body material qualification and the seat selection are different questions. The seat has to answer to the medium, the temperature and the shut-off class, which is a separate gate.

What this article does not cover

Note that this article does not select a material for a duty, does not determine whether a valve or an installation complies with any standard or regulation, does not specify heat treatment, hardness limits or welding requirements, and does not provide corrosion or qualification advice. It sets out the scope of ANSI/NACE MR0175-2021 / ISO 15156:2020, the mechanisms it addresses, where it stops, and what an enquiry has to state.

Where the next constraint sits

Material is the third cut, not the first. Where the material question sits in the sequence — after function and after class and temperature — and why the order matters, is set out in Valve Selection Guide: From Function to Class to Material.

Where the duty is sour or hydrogen, two more documents usually arrive with it: a low-emission requirement at the stem, and a seat-leakage or shut-off requirement at the seat. ISO 15848 Explained: Reading BH, CO3 and SSA0 on a Valve Certificate decodes the first, and ISO 5208 Rate A: What a Valve Seat-Leakage Class Does and Does Not Promise sets out what the second actually permits.

The medium view of the same duty — which materials carry sour service and to what temperature: Sour Service Valves (H₂S-Bearing).

Sources