The Engineer Speaks on Design Pressure, MAWP, Operating Pressure, Set Pressure
There has been a lot of confusion among engineers, even internationally on the subject of Design Pressure, MAWP, Operating Pressure and Set Pressure. So today I put on my thinking hat to come up with the following;
Design Pressure is the pressure that design engineer decides that serves as the basis of all engineering calculations, choice of appendages and related equipments and material specifications. The design pressure takes into account the safety factors, corrosion allowance, design life and other design related factors.
At testing most codes require the TEST PRESSURE to be 1.5 times the DESIGN PRESSURE.
Working Pressure is the pressure that the system is configured to operate safely and in a continuous manner without limitations.
MAWP – Maximum Allowable Working Pressure is the pressure above or below the operating pressure allowable for the SYSTEM to operate, limited by duration or temperature or age or level of corrosion or enhanced by material availability during fabrication. It is set by the weakest equipment or subsystem. It is therefore variable and can be higher than the Design Pressure. The MAWP is set by the fabricator/manufacturer or system integrator. It is set after the SYSTEM is COMPLETED. It is to be reviewed at inspection intervals.
SET Pressure is the setting of the Pressure Safety Valve. Pressure relief valves must have sufficient capacity when fully opened to limit the maximum pressure within the vessel to 110% of the MAWP. (ASME VIII). I am of the opinion that it is good practice to put the SET PRESSURE at MAWP.