Safety valves should be installed wherever the maximum allowable working pressure (MAWP) of a system or pressure-containing vessel is likely to be exceeded. In steam systems safety valves are typically used for boiler overpressure protection and other applications such as downstream of pressure reducing controls.
The safety valve is one of the most important safety devices in a steam system. Safety valves provide a measure of security for plant operators and equipment from over pressure conditions. The main function of a safety valve is to relieve pressure. It is located on the boiler steam drum and will automatically open when the pressure of the inlet side of the valve increases past the preset pressure.
Technical Requirements for Boilers Safety Valves - Weldon
At least one safety valve should be installed for boilers with a rated evaporation capacity lower than 0.5t/h; for boilers with a rated evaporation capacity higher than 0.5t/h at least two safety valves should be installed besides the economizer. Safety valves must be installed at the outlet of the separable economizer steam superheater reheater and start-up separator of the once-through boiler.