Design Pressure, MAWP and Operating Pressure in a Vessel RFQ

Operating pressure is what the vessel sees in normal running, and maximum operating pressure is the highest it should see in any foreseeable operation. Design pressure is the pressure the engineer uses in the code calculations, set with a margin above the maximum operating pressure. MAWP, the maximum allowable working pressure, is the highest pressure the finished vessel is allowed at the top in its operating position, at design temperature, based on the thicknesses actually used less the corrosion allowance; it is never below the design pressure and often a little above it. In European and UK pressure equipment law the comparable limit is PS, the maximum allowable pressure.

Those numbers then set the rest of the ladder: the relief valve set pressure must not be above the MAWP, the pressure during relief is limited to a small accumulation above it, and the hydrostatic test pressure is a multiple of it.

Why this matters now

A buyer's datasheet is where these pressures first get written down, and small slips are expensive. Leave out vacuum and the vessel may collapse during a steam out. Set design pressure too close to operating and the relief valve will weep. Set it too high and the vessel is thicker, heavier and dearer than it needs to be, and every flange, valve and instrument on it has to match.

The definitions also differ by jurisdiction. Pressure Vessel Engineering, a Canadian design consultancy, notes that in Alberta and Saskatchewan the design pressure must equal the MAWP [1]. A vessel bought from one country for use in another can carry the right number with the wrong meaning.

Five pressures, one vessel

Pressure Vessel Engineering explains that MAWP is governed by the weakest component, reflects hot and corroded conditions, and is shown on the nameplate as a whole number; the maximum allowable pressure in the new, cold condition is a different figure [1].

How much margin to add

There is no single rule, but the textbooks agree on the order of magnitude. Northwestern University's open process design textbook summarises Turton's guidance as a design pressure of 10%, or a fixed margin of roughly 0.69 to 1.7 bar, above the maximum operating pressure, whichever is greater, and Towler's as overdesign of 5% to 10% [2]. Company standards often set their own figures; use them if you have them.

Two practical checks sit on top of the rule of thumb:

• The relief valve needs room. If the set pressure equals the design pressure and the vessel normally runs close to it, the valve may simmer or leak. Ask the valve supplier what gap it needs between operating and set pressure.
• Every upset counts. Pump shut off head, a blocked outlet, thermal expansion of trapped liquid and the static head of liquid at the bottom of a tall vessel can all exceed normal operating pressure. Design pressure must cover the foreseeable cases the relief system will not.

Design temperature gets a margin too. The same textbook cites Turton's allowance of 25 °C above the maximum operating temperature for vessels between about minus 30 °C and 345 °C, and a Towler figure of 50 °F above the maximum and 25 °F below the minimum, differences of about 28 °C and 14 °C [2]. It gives corrosion allowances generally in the range of 1.5 mm to 5 mm, depending on the service [2].

Why MAWP is usually higher than design pressure

The designer sizes each part for the design pressure, then picks the next stock plate or pipe thickness above the minimum. That extra thickness means the finished vessel can usually take a higher pressure, which the designer can calculate as the MAWP. Ward Vessel and Exchanger, a fabricator, puts it simply: MAWP is always equal to or higher than design pressure [3]. It also notes the owner's choice: stamp the higher MAWP to allow for possible higher pressure service later, or rate the vessel only at design pressure and treat the extra thickness as additional corrosion allowance, which can extend its life [3].

Decide which you want and say so in the RFQ. Otherwise each bidder decides for you, and the nameplates on two otherwise identical vessels may differ.

What follows from the MAWP

Relief devices. Under ASME VIII, a single relief device must be set no higher than the MAWP, and accumulation during relief is limited to 10% above MAWP with one device, 16% with several and 21% in a fire case, according to Emerson's relief valve handbook, which reflects the 2013 code edition [4]. For new equipment under the UK Pressure Equipment (Safety) Regulations 2016, pressure must not permanently exceed PS, and a momentary surge may reach no more than 10% above it [5].

Hydrostatic test. ASME VIII Division 1 tests at 1.3 times the MAWP, adjusted by the ratio of allowable stresses at test and design temperature [6], while the UK regulations apply a floor of 1.43 times PS as a general rule for vessels [5]. Our guide to hydrostatic test pressure works through the codes side by side.

Pressure equipment category. Under the European and UK regimes, PS is one of the inputs that decide the category, which in turn decides how much third party assessment the vessel needs. See PED and PER 2016 categories.

What to put on the datasheet

Units are a frequent source of error. Bar gauge and bar absolute differ by about one bar; psi and bar by a factor of about 14.5. Write the unit next to every number. Our pressure vessel datasheet guide covers the other fields.

Practical example

In this illustrative example, a buyer specifies a stainless receiver that normally runs at 6.0 barg, with a maximum operating pressure of 7.5 barg when the feed pump runs against a closed outlet.

Applying the 10% rule gives 8.25 barg; a fixed margin of 0.69 to 1.7 bar gives 8.19 to 9.2 barg. The buyer's own standard uses a fixed margin of 1.5 bar, giving 9.0 barg, which is greater than the 8.25 barg from the 10% rule, so design pressure is set at 9.0 barg, with full vacuum as the external design case because the vessel is steamed out. The relief valve is set at 9.0 barg, which leaves 1.5 bar between maximum operating and set pressure for the valve to stay tight.

The fabricator's calculations, using stock plate, give a calculated MAWP of 10.4 barg. The buyer asks for the vessel to be stamped at 9.0 barg and the extra thickness kept as additional corrosion margin. With one valve and no fire case, the pressure during relief is limited to 9.9 barg, 10% above the stamped MAWP [4]. The ASME hydrostatic test, with a stress ratio of 1, is 11.7 barg [6], but because the vessel is going into service in Great Britain the 1.43 times PS floor of about 12.9 barg applies as a general rule [5].

Had the buyer left the stamping choice open, one bidder might have stamped 10.4 barg and offered a relief valve set there, and the comparison of bids would have mixed two different vessels.

Where ChemCapital fits

ChemCapital sends one structured RFQ with line items and attachments, so the same datasheet, with its pressures, temperatures and stamping choice, reaches every bidder. The AI quotation comparison then extracts price, lead time, materials, certifications and scope from each quote, keeps the source text beside every extracted field, and checks for missing fields and unit mismatches, so a bid quoted in different units is easier to spot before anyone compares prices.

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