MAWP, MAP and design pressure: what each one means

All three are pressures. They are calculated from different thicknesses, for different purposes, and they answer different questions.

Design pressure

What it is: the pressure the purchaser specified when the equipment was ordered. An input, not a result.

The designer takes it, adds temperature, material and geometry, and calculates a required thickness — then adds corrosion allowance and rounds up to a plate size that can actually be bought. So the vessel as built is usually somewhat thicker than the design pressure strictly required.

It never changes. It is a specification, and it is what appears on the nameplate alongside the rated conditions.

MAWP — maximum allowable working pressure

What it is: the maximum pressure the equipment is permitted to operate at, calculated from the actual current thickness less any future corrosion allowance, in the corroded condition, at the design temperature.

This is the number that matters in service, and it is the one that falls as the equipment corrodes.

A new vessel typically has a MAWP above its design pressure, because of that rounding up to a standard plate thickness. As corrosion consumes the extra, MAWP falls. When it reaches design pressure, the margin from rounding is gone. When it would fall below, the equipment must be repaired, replaced, or re-rated to the lower pressure.

Two details that catch people out:

• MAWP is calculated at the top of the vessel in its normal operating position. The bottom of a tall column carries additional static head, and that is accounted for separately.
• MAWP is limited by the weakest component. A vessel with a thick shell and a thin nozzle has the nozzle's MAWP. Calculating for the shell alone and stopping is a common error.

MAP — maximum allowable pressure

What it is: the maximum pressure the equipment could hold in the new and cold condition — full nominal thickness, no corrosion allowance deducted, at ambient temperature.

It is the theoretical ceiling for that geometry and material. Always the highest of the three.

What it is used for: establishing hydrotest pressure, and understanding how much margin existed at the start. It is not an operating limit and nobody operates to it.

Side by side

Worked through

A vessel designed for 20 bar. Calculation says 9.4 mm is required; add 3 mm corrosion allowance and buy 12.7 mm plate.

• Design pressure: 20 bar. Fixed.
• MAP: what 12.7 mm can hold cold — comfortably above 20 bar.
• MAWP at commissioning: based on 12.7 − 3 = 9.7 mm, so a little above 20 bar. There is a small margin from the rounding.
• MAWP after ten years at 0.15 mm/yr: measured thickness 11.2 mm, less 3 mm future allowance = 8.2 mm. Below the 9.4 mm required, so MAWP is now below design pressure.

At that point the options are the usual four: reduce the future corrosion allowance if the inspection interval justifies it, repair, replace, or re-rate to the pressure the remaining wall supports.

Where this connects to fitness-for-service

When a flaw is found, one of the standard outputs of an API 579 assessment is a reduced MAWP — the pressure at which the damaged component still has acceptable margin.

For local metal loss the link is direct: the remaining strength factor scales the acceptable pressure. If the calculated RSF is below the acceptance value of 0.90, the component can be operated at a pressure reduced in proportion, rather than being condemned. That is the same re-rating decision, arrived at through the assessment.

Which is why MAWP, not design pressure, is the number an integrity engineer tracks. Design pressure tells you what somebody wanted in 1987. MAWP tells you what the metal in front of you can actually do.