Corrosion allowance is extra wall thickness added to a pressure vessel’s shell and heads, beyond what the pressure calculation alone requires, to account for material that is expected to be lost to corrosion over the vessel’s service life. It is added on top of the pressure-design thickness, not blended into it, and the vessel’s MAWP is normally calculated on the corroded (thinner) condition so the nameplate rating already reflects the end-of-life, not the as-new, thickness.
Why refrigeration vessels usually carry some allowance
Ammonia and the other refrigerants used in industrial refrigeration are not, by themselves, especially corrosive to carbon steel in a dry, well-maintained closed system. Corrosion allowance in refrigeration vessels is mainly a hedge against things outside the refrigerant itself: moisture contamination, external atmospheric corrosion on insulated or exposed surfaces, and a general engineering margin for a vessel that is expected to stay in service for decades. Stainless steel vessels sometimes carry little or no corrosion allowance, since stainless is chosen specifically for corrosion resistance in the first place.
| Material / service | Typical corrosion allowance |
|---|---|
| Carbon steel, general ammonia refrigeration service | [ELI-REVIEW: commonly in the range of 1/16 to 1/8 in.] |
| Carbon steel, more aggressive or less-controlled service | [ELI-REVIEW: can run higher depending on project specification] |
| Stainless steel | [ELI-REVIEW: often zero, or a small nominal value] |
How it affects the thickness calculation
In the shell and head formulas, corrosion allowance is added after the pressure-required thickness is solved — see the [ELI-REVIEW: UG-27] shell thickness formula for the pressure term itself. On an inside-diameter basis, the allowance also effectively enlarges the pressure-carrying bore the formula sizes against, since the corroded (final) inside radius is what the vessel will actually see once material is lost. Increasing corrosion allowance therefore increases required nominal plate thickness by more than just its own value, though usually only by a small additional amount.
Run it
PRELIM adds your specified corrosion allowance to the pressure-required thickness and reports both the nominal and corroded MAWP, on screen, free.
Open PRELIM →FAQ
Does corrosion allowance apply to both shell and heads?
Yes, typically the same corrosion allowance value is applied to both the shell and the formed heads, since both are exposed to the same internal and external environment.
Is corrosion allowance required by [ELI-REVIEW: ASME VIII-1]?
The code requires the designer to consider corrosion and specify an allowance appropriate to the service; it does not mandate a single fixed number. The value used is an engineering decision based on the specific fluid, environment, and expected service life.
Does more corrosion allowance always mean a heavier vessel?
Generally yes: more corrosion allowance means more nominal plate thickness for the same pressure design, which increases vessel weight and, in most cases, cost. It's a trade-off between service-life margin and material use.
Should corrosion allowance be set the same for every project?
No. It should reflect the specific service conditions, fluid, environment, and owner/engineer specification for that project, which is why ColdCalcs' calculators take it as an explicit input rather than assuming a fixed default.