Design pressure is the pressure an engineer chooses up front to size a vessel against; MAWP (maximum allowable working pressure) is the pressure the vessel, as actually built, can be shown to safely withstand. Design pressure is an input to the thickness calculation; MAWP is an output, back-calculated from the real, as-built (often rounded-up) wall thickness. Because plate comes in standard gauges, the as-built MAWP is almost always slightly higher than the design pressure that drove the calculation.
Why the two numbers diverge
A shell or head thickness calculation solves for the minimum thickness that satisfies the code at the chosen design pressure. Real plate is only available in discrete gauges — for example [ELI-REVIEW: ¼-inch steps from ¼ in. up through 2 in., then ¼-inch steps above that] — so the fabricator rounds the required thickness up to the next available plate size. That extra material has spare capacity, and running the thickness formula backward (solving for pressure instead of thickness) on the as-built, corroded thickness gives a MAWP that is equal to or higher than the original design pressure. A vessel is never allowed to have a calculated MAWP below its design pressure; if the numbers come out the other way, the design fails and the vessel needs a thicker wall or a different material.
| Design pressure | MAWP | |
|---|---|---|
| Role in the calculation | Input | Output |
| When it's known | Chosen before sizing | Calculated after plate is selected |
| Typical relationship | The floor | Equal to or above design pressure |
| Where it's found | Design basis / datasheet | [ELI-REVIEW: Vessel nameplate (U-1 form)] |
Where relief valve sizing fits in
Pressure relief devices are set relative to MAWP, not design pressure, because MAWP is the number that describes what the as-built vessel can actually take. A relief valve set point above the vessel’s true MAWP would leave the vessel unprotected; that is one reason the MAWP back-check happens on every vessel, not just the ones where it might matter.
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PRELIM computes required shell and head thickness from your design pressure, rounds to a stock plate gauge, and back-calculates the resulting MAWP, all on screen, free.
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Can MAWP ever equal design pressure exactly?
It can, if the required thickness happens to land exactly on an available plate gauge with no margin left over, but this is uncommon. In most real designs MAWP comes out slightly above design pressure because of stock plate rounding.
What happens if a vessel's MAWP comes out below its design pressure?
That design does not satisfy the code and cannot be built as specified. The engineer has to increase wall thickness, change material, change geometry, or reduce the design pressure requirement, then re-run the check until MAWP is at or above design pressure.
Does corrosion allowance affect MAWP?
Yes. MAWP is normally calculated on the corroded thickness (nominal thickness minus corrosion allowance), not the full nominal plate thickness, since corrosion allowance is metal that is assumed to be gone by the end of the vessel's service life.
Is design pressure the same as operating pressure?
No. Operating pressure is what the system runs at day to day; design pressure includes margin above the highest expected operating condition (including static head and any standstill condition) so the vessel is never operated at or above its design basis under normal conditions.