Pressure Relief Valves PSV

Pressure relief valves are provided to cater to two main conditions of the process -normal conditions and emergency conditions. Normal conditions relate to the designed operation of the process which emergency conditions can be caused by either (1) external fire conditions, (2) failure of reflux or cooling, (3) Failure of the power supply, (4) failure of steam supply, (5) heat exchanger failure (6) introduction of incompatible materials, (7) thermal expansion with outlets closed. 

Because these causes of overpressures are considered random and infrequent, the pressure relief capability has to be automatic and constantly available. 

Where relief valves are provided on liquid storage tanks or vessels, where there is a possibility of liquid release, i.e., liquid slug, careful evaluation of the release disposal system (e.g., flare header) needs to be undertaken. In some cases, a liquid slug may block a header from releasing pressure and defeat the purpose of the pressure release system. 

---