A Safety Valve is a valve which opens automatically to prevent a predetermined gauge pressure being exceeded and which recloses after decrease in pressure. Overpressure in closed vessels or systems is a physical phenomenon to be avoided to preserve the integrity of personnel and properties. Overpressure in a closed system may be majorly caused by three common causes such as blocked discharge, excessive heating by external fire or by thermal expansion.
The case of blocked discharge might be caused by a human error with blocked valves closed while the full system input flow continues; the case of external fire to the system would result in the liquid or gas expansion causing an overpressure and, finally, the thermal expansion is caused by the reaction of the external heat exerted on to the full of liquid pipe or pressure vessel, being non compressible liquids there would be tremendous internal forces inside the system.
Safety Valves, that use to be Pressure Relief Devices in case of liquids, are the last possibility, when all the other protection devices fail, to protect a system against unacceptable pressure. They can be made under diverse design codes, materials of constructions and engineering suited to the most convenient applications.