Sour Service Valve Materials: NACE MR0175 and H2S Cracking Prevention
Quick Answer
Sour service means the fluid contains hydrogen sulfide (H2S). Under NACE MR0175 and ISO 15156, valve materials must stay below defined hardness limits so they do not fail by sulfide stress cracking. Carbon and low-alloy steels need controlled hardness, and trim must also comply.
Why H2S Is a Problem
Atomic hydrogen generated in H2S environments can enter steel and cause cracking, especially at high hardness. The risk rises with H2S partial pressure and applied stress. Sour-service rules exist to keep materials below the cracking threshold.
Key Material Rules
- Body and bonnet steels: limit hardness per the standard, typically verified after welding and heat treatment.
- Trim: hard-facing such as Stellite is allowed only within the hardness and process envelope defined by the standard.
- Bolting: controlled grade and heat treatment.
Our petrochemical material selection guide and materials and flanges reference give compatible grade tables.
Specifying a Sour-Service Valve
- Determine H2S partial pressure from the fluid data.
- Confirm whether NACE MR0175 applies.
- Specify NACE-compliant body, trim, and bolting.
- Require hardness test reports for welds and trim.
Common Mistakes
- Assuming a standard carbon steel valve is automatically NACE-compliant.
- Overlooking trim hardness after hard-facing.
- Skipping post-weld heat treatment verification.
References
This guide references NACE MR0175 (now ISO 15156) and general refinery material practice. Consult the current edition and the project specification.
Frequently Asked Questions
What is sour service?
A fluid environment containing hydrogen sulfide (H2S) that can cause sulfide stress cracking in susceptible steels.
What does NACE MR0175 control?
It sets material and hardness limits so valve steels do not crack in H2S service.
Does NACE apply to trim?
Yes. Hard-faced trim such as Stellite must stay within the standard's hardness and process envelope.
Is stainless steel exempt from NACE?
Not automatically. Austenitic stainless can still be subject to cracking mechanisms; compliance depends on the service and standard.
How do I confirm a valve is NACE compliant?
Specify NACE MR0175 in the purchase order and require hardness test reports for body welds and trim.