How to calculate mawp

Written by brian baer | 13/05/2017
How to calculate mawp
Chemical plants contain pressure-rated vessels that have MAWPs. (Stockbyte/Stockbyte/Getty Images)

MAWP stands for maximum allowable working pressure and is associated with industrial pressure vessels. The American Society for Mechanical Engineers (ASME) dictates construction and calculation codes for pressure-rated vessels. Typically, a pressure rating is determined by the thickness of the metal alloy used and the alloy's tensile strength properties. Initial calculations are done by qualified engineers, and the vessel is constructed in a code-vessel building shop. To ensure the vessel can withstand the designed pressure, it is typically hydrotested, which means it is filled with pressurised water at pressures above the MAWP.

Determine the size and shape of the pressure vessel as well as the maximum required pressure rating. This is typically provided by process engineers who have designed chemical plant processes. For instance, a vessel is required to meet 45.4 Kilogram per square inch (100 psi) and must have a sufficient volume to contain 10,000 gallons of liquid. The inside radius of the cylindrical vessel is 96 inches.

Determine the physical properties of the metal alloy to be used. Assume that a carbon steel material with a tensile strength of 63,800 psi is being used with a thickness of 1/2 inch. The longitudinal seam efficiency (efficiency of the weld) is 0.85, and the safety factor is 2.5.

Calculate the MAWP using the ASME code formula. This formula is P=(TS x t x E)/(R x SF), where P is the MAWP in psi, TS is the tensile strength of the material in psi, t is the vessel wall thickness, E is the longitudinal seam efficiency, R is the vessel interior radius in inches and SF is the safety factor. The formula calculates a MAWP of 113 psi ((63,800 x 0.5 x 0.85)/(96 x 2.5)), which is adequate for the required 100 psi pressure rating.

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