A valve which automatically discharges fluid to relieve pressure. It is generally related to pipeline applications and non-compressible fluids—liquids such as water, oil, etc. Immediate full-flow discharge is not needed since a small flow reduces the pressure.
Since the basic parts of relief valves are the same as those of ordinary safety valves, separate definitions are not necessary. Reference should therefore be made to Section 4:1.
The regulations issued by most countries demand that safety valves be installed on all pressure vessels to ensure that their maximum design pressure is not exceeded. These regulations vary considerably but for example on boiler applications they all state that the safety valve or valves must be able to pass the evaporative capacity of the boiler with a minimum rise in pressure. The permitted rise in pressure varies and is dependent upon the regulations concerned.
Maximum design pressure is the highest working pressure at which a vessel may be operated. Working pressure is the pressure at which a vessel is normally operated.
Set pressure is the pressure at which the disk of a safety valve or relief valve starts to discharge flow. Accumulated pressure is the pressure above set pressure at which the disk has reached its full lift for maximum discharge of flow. Accumulation is the difference between set and accumulated pressures. Normally understood as % of set pressure.
Reseating pressure is the pressure at which the disk shuts off the flow.
Blow down is the difference between set and re-seating pressures. Normally understood as % of set pressure.