H₂S in a process stream doesn’t just create a safety hazard — it triggers an entirely different set of material requirements that affect every single piece of equipment in your facility. Here’s what you need to know.
What is Sour Service and When Does NACE MR0175 Apply?
NACE MR0175 / ISO 15156 applies when your process stream meets both of these conditions simultaneously:
Condition 1: H₂S partial pressure ≥ 0.05 psia (0.3 kPa) AND
Condition 2: Aqueous phase present (liquid water)
Both conditions must be met. Dry gas with H₂S = no NACE. Wet CO₂ only = no NACE. Wet H₂S above threshold = full NACE.
The 0.05 psia threshold seems low — and it is. At a system pressure of 100 psig (114.7 psia), just 0.044 mol% H₂S triggers sour service requirements. On projects like Mustang Pad in Arctic Alaska and many Gulf of Mexico facilities, this threshold is almost always met.
What NACE MR0175 Actually Controls
The standard defines requirements to prevent three types of environmental cracking caused by H₂S:
- Sulfide Stress Cracking (SSC): Affects high-strength ferritic and martensitic steels. The primary concern for carbon steel equipment and fasteners.
- Hydrogen Induced Cracking (HIC): Affects carbon steel in wet H₂S environments. Blistering and internal cracking along lamination planes.
- Stress Oriented Hydrogen Induced Cracking (SOHIC): A subtype of HIC near stress concentrations like welds.
Critical Material Requirements for Carbon Steel
| Requirement | Value | Why It Matters |
|---|---|---|
| Max hardness — base metal | 22 HRC / 248 HV | Higher hardness = higher SSC susceptibility |
| Max hardness — weld metal | 22 HRC / 248 HV | PWHT mandatory for most carbon steel welds |
| PWHT required | Yes — mandatory | Regardless of thickness |
| CE (Carbon Equivalent) | ≤ 0.43 | Controls base metal hardenability |
Internals and Trim in Sour Service
This is where many engineers make specification errors. It’s not just the vessel shell — all wetted internals must comply:
- Mist eliminator mesh pads: Must be 316L SS or Alloy 825 — NOT standard 304 SS
- Level gauges / displacers: Internals including floats must be NACE-compliant alloys
- Valve trim: Stellite overlays on hard-faced trim need separate evaluation
- Fasteners: B7M studs and 2HM nuts (not standard B7/2H) — this is missed on virtually every project at first issue
- Spring materials in PSVs: Inconel 625 springs required in most designs
On the Mustang Pad Arctic EPF project, we had 30 review comments come back on the first technical offer. A significant portion were NACE-related — internals material specs that needed upgrading. Getting NACE right at first issue saves weeks of back-and-forth with the client reviewer.
Master Sour Service Engineering
Our NACE module covers SSC, HIC, SOHIC, material selection, hardness testing, and PWHT requirements with real project examples.
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