Pressure relief devices are used for protection of pressure vessels from over pressure damage. F0llowing key terminology for relief devices helps in better understanding of design and usage of these pressure relief devices.
Maximum Operating Pressure
This is the maximum value of pressure, expected during normal operating of the vessel.
Design pressure for a vessel is taken by adding a suitable margin to the maximum possible pressure during normal operation. This design pressure value is used to determine the minimum thickness for all vessel elements, by applying vessel design rules to the design pressure. Design pressure is less than or equal to the Maximum Allowable Working Pressure (MAWP). When MAWP value for a fabricated value is not available, design pressure value should be used as a conservative replacement for MAWP.
Maximum Allowable Working Pressure (MAWP)
The maximum allowable working pressure is the highest pressure that the vessel can withstand based on actual nominal vessel thickness excluding the allowances provided for corrosion and loadings other than pressure. MAWP has to be at the least equal to design pressure. MAWP is commonly calculated for a fabricated vessel. During fabrication, availability of materials and thickness of material used usually leads to a vessel exceeds the minimum requirements imposed by design pressure. For such a vessel the MAWP is greater than design pressure. For a vessel which only meets the minimum requirements imposed by design pressure, MAWP is equal to the design pressure.
Set pressure of a relieving device on a pressure vessel is the pressure when the relieving device just starts to open.
Maximum allowable accumulated pressure is specified for a vessel protected by a pressure relief device. It is the maximum allowable pressure during discharge from the relieving device. This pressure is commonly expressed as a percentage of Maximum Allowable Working Pressure (MAWP). This percentage is determined by the governing case for opening of the relief valve protecting the vessel. For instance, vessel protected for fire case has a maximum allowable accumulated pressure which is 121% of the MAWP.
Overpressure is similar to pressure accumulation value described above. The difference is that overpressure is reported as a percentage of the set pressure of relieving device. If the relieving device is set to open at Maximum Allowable Working Pressure (MAWP), then overpressure becomes equal to pressure accumulation.
Blowdown is the pressure when the pressure relief valve closes this discontinuing the relieving flow. This pressure value is lower then the set pressure and is commonly reported as a percentage of the set pressure value.