How it is set
The construction code fixes the pressure design thickness: ASME Section VIII for pressure vessels, ASME B31.3 for process piping, API 650 for atmospheric storage tanks. Structural or mechanical minimums may add to it. The corrosion allowance is then added on top, chosen by the owner or the designer from the expected corrosion rate in the service multiplied by the intended design life, and recorded on the nameplate or line list. The allowance is a design assumption, not a guarantee; the actual corrosion rate is only known once the equipment is in service and measured.
Minimum required thickness and remaining life
In service, the reference point becomes the minimum required thickness: the thickness the equipment needs to remain safe at its design conditions, calculated from the governing code. The margin between the measured thickness and that minimum is what is left of the corrosion allowance.
API 510 for pressure vessels and API 570 for piping use the same logic:
- Measure thickness repeatedly at fixed condition monitoring locations.
- Derive corrosion rates from those readings — a long-term rate from the original thickness and a short-term rate from recent readings — and use the rate that best reflects current conditions.
- Calculate remaining life as the margin above minimum required thickness divided by that corrosion rate.
- Set the next inspection interval as the lesser of one-half the remaining life and the maximum interval the code allows.
When the margin runs out, the options are repair, replacement, re-rating to less severe conditions, or a fitness-for-service assessment under API 579-1/ASME FFS-1 to evaluate the actual damage.
What an allowance does not cover
An allowance assumes reasonably uniform metal loss. It offers little protection against mechanisms that concentrate damage:
- pitting and crevice corrosion, which perforate the wall while average thickness remains high;
- localized attack at dead legs, injection points and elbows;
- cracking mechanisms such as stress corrosion cracking and hydrogen-induced cracking, which do not remove wall thickness at the surface.
Where these mechanisms are credible, the allowance must be backed by appropriate material selection, coatings or linings, and inspection methods able to find the damage.