Pressure relief valve
Also known as: safety valve, relief valve
Short definitionA valve whose spring gives way and releases refrigerant when pressure reaches the limit the vessel can withstand. It is the last component that prevents a burst when the other protections have failed.
On a normal day this valve never opens, because the high-pressure switch stops the compressor long before. If there is a fire, if the switch fails, or if a vessel full of liquid is shut in on both sides, the job falls to it: the disc lifts, refrigerant is discharged, and it reseats once the pressure drops. It sits on receivers, oil separators and large suction accumulators. In CO₂ plants every section that can be isolated gets its own.
The set pressure cannot be higher than the PS pressure on the vessel nameplate, and the pressure switch is set so that it does not exceed 90% of that. Discharge capacity is calculated to EN 13136; the general rules are in EN 378. The outlet pipe is run outdoors, without any reduction, to a place where nobody is present. No shut-off valve goes between vessel and relief valve, and the seal is left alone. A valve that has opened once usually weeps, so it is replaced with a new one.