What puts a service into sour operation
A service is sour when it contains hydrogen sulphide at a partial pressure high enough to put the material at risk of sulphide stress cracking. The definition is quantitative: a gas stream is normally treated as sour above a defined H2S partial pressure, and the threshold is lower in wet, low pH conditions. In practice the presence of water together with free H2S is what turns a mild stream into a sour one, because the cracking mechanism needs an aqueous environment. NACE MR0175, also published as ISO 15156, sets the material rules for these services. Above that threshold the standard becomes mandatory rather than advisory, and its material rules apply to every pressure-containing and load-bearing part in the valve, not only to the body. Ball valves in sour duty are specified to NACE MR0175 and ISO 15156 essentially as a matter of course.
Hardness and chemistry limits by material
The controlling requirement for carbon and low-alloy steels is hardness. NACE MR0175 caps hardness at 22 HRC for these materials in sour service, and carbon content and alloying elements are restricted as well. Corrosion-resistant alloys are handled by composition limits and minimum alloy content rather than by a single hardness number. Martensitic stainless steels are generally excluded unless they meet tight limits, while duplex, super duplex and austenitic stainless steels are commonly accepted when they satisfy the chemistry rules. Soft seats and seals are not governed by the hardness limits in the same way, but every metal part that carries load is.
Seats, seals, bolting and trim
NACE compliance covers more than the body. Bolting must meet the same hardness limits, because bolts are high-strength parts under load and are particularly prone to sulphide stress cracking. The ball, stem and seat carrier must satisfy both the hardness and chemistry rules. Soft seat and seal materials are chosen for the pressure and temperature range first, then confirmed for H2S compatibility; PTFE, RPTFE and PEEK are common, while elastomers must be selected for the specific stream. Where higher alloy trim is used, duplex or super duplex seats are common choices. J-VALVES builds sour service ball valves to NACE MR0175 and ISO 15156 with hardness verification on the loaded metal parts.
Documentation a sour service order must carry
A sour service order has to be traceable. At minimum the buyer should expect material certificates to EN 10204 3.1 covering composition and heat number. Where the specification calls for it, EN 10204 3.2 is issued with independent verification of the test results, and third-party inspection can be arranged through SGS or TUV. Hardness test reports on the loaded metal parts belong in the package, and the pressure test records should be supplied alongside them. The certificate set must let the operator trace each pressure-containing part back to its heat. J-VALVES issues EN 10204 2.2, 3.1 or 3.2 certificates according to the scope agreed at order stage, and quotations are typically returned within three working hours.
Failure modes in H2S service
The main failure mode is sulphide stress cracking, in which hydrogen enters the steel in a wet H2S environment and a crack grows under stress. It is a sudden, brittle failure that gives little warning, which is why hardness control matters so much. A related risk is hydrogen-induced cracking in carbon steels exposed to wet H2S. Erosion and corrosion can accompany these problems where the service also carries sand or chlorides, which is common in oil and gas production. Failure is usually sudden rather than gradual, and it often starts at a stress concentration such as a thread root, a weld or a sharp change of section. Selecting a material that sits inside the standard is not sufficient on its own; the hardness verification and the certificate chain are what prove the selection was actually implemented in the delivered valve.
Frequently Asked Questions
Is 316 stainless steel NACE compliant? Austenitic stainless steels such as 316 are commonly accepted under NACE MR0175 and ISO 15156, but acceptance depends on the actual composition meeting the standard chemistry rules and on the service conditions. It is not automatic. The material certificate must be checked against the standard, and cold work and hardness also matter for austenitic grades in some sour services.
Does sour service require EN 10204 3.2 certificates? Not always. EN 10204 3.1 is the common requirement and covers composition and heat number traceability with the manufacturer own test results. EN 10204 3.2 adds independent verification of those results by an authorised third party, and it is specified when the client or the project standard demands that extra level. Ask early, because it affects both cost and schedule.
Can carbon steel be used in wet H2S service? Yes, carbon and low-alloy steels are widely used in sour service, but only when hardness is limited, typically to 22 HRC, and the carbon and alloying element content meets the standard restrictions. The material must also suit the temperature and pressure. The material certificate and the hardness test records are what prove compliance.
