Note#102
Relief Valves
– The required relief area for two-phase flow is typically two to ten times the area for single-phase flow.
– For spring-operated relief valves, small amounts of leakage start to occur at 92–95% of the set pressure.
– A relief devices over-pressure is the pressure increase over its set pressure, usually expressed as a percentage of the set pressure. Pop-acting relief valves do not immediately open completely (to 100% lift). Sufficient over pressure is necessary to achieve full lift.
– if the vessel is protected by multiple relief devices, then one relief device must be set no higher than the MAWP but the others can be set as high as 105% of the MAWP.
