Q & A > Question Details
What is the basic difference between a thermal and pressure safety valve?
07/03/2021 A: Nishant Panchal, Skywin Valves Pvt. Ltd., mkt@skywinvalve.com
A thermal relief valve is designed to allow a small flow of liquid to
escape from a pipeline. Typically, one will be used on every section of
line beween valves that is exposed to heat - maybe solar if the line is
above ground.

A pressure safety valve is designed to pass large quantities of liquid to
avoid overpressurising. It will be used where there is a possibility of
large volume flows of liquid being forced into a vessel when the discharge
cannot handle them, or where failure of a pressure control loop would allow
pressures to increase above the MAWP.

The set pressure will be laid down by pressure vessel codes applicable to
the plant. In general, the relief valve will be set to the maximum
allowable working pressure for the vessel or pipework being protected. But
check the codes that apply to the design of your plant.
23/02/2018 A: Hiren Panchal, Hyper Valve, hypervalves06@gmail.com
Both the thermal relief valve and pressure relief valve are used in the liquid line in order to avoid pressure in the line. The thermal relief valve relieves the over-pressure in a piping system of fluid due to temperature rise. In most cases, the thermal relief valve is used when a liquid is blocked in a pipe, and the pipe is exposed to sunlight. The pressure relief valve limit the system pressure to a specific set level. Pressure relief valves (safety relief valves) are designed to open at a preset pressure and discharge fluid until pressure drops to acceptable levels.
23/02/2010 A: omsakhti kesavan, GFPC, sakthikesavan@gmail.com
Thermal relief valve or pressure relief valve are used in the liquid line in order to avoid pressure in the line due to the evaporation of the liquid or discharge valve closed scenario, pressure relief valve are kept in the vessel in order to avoid pressurizing, or release pressure to flare when pressure increase due power failure or fire case.
20/07/2009 A: Anilendu Dalal, Petrokemya, anilendu_dalal@yahoo.com
The thermal safety valve relieves the overpressure in a piping/equipment system heating or thermal expansion of fluid due to temperature rise or continuous heat input especially during blocked condition.
Whereas, the pressure safety relief conventionally handle overpressure relief during other process reasons like pump shut off pressure, cooling water failure, reflux failure, control valve failure so on as mentioned for various scenarios in API-RP-521.
The evaluation & calculation for the capacity is provided in the code.
19/07/2009 A: keith bowers, B and B Consulting, kebowers47@gmail.com
A 'thermal relief' valve is very small (1/2" or less usually) and is designed to relieve excess pressure in a section of closed off piping/equipment, that would result if the section were blocked in at normal operating conditions and then would heat up from atmospheric or other conditions while it was blocked in. The quantity of fluid that need be 'relieved' may be very small--less than an ounce.
A PSV (pressure safety valve) is sized and designed to prevent the internal pressure of the protected system from exceeding the maximum allowable value during the absolutely worst credible case equipment incident or human error that would cause the internal pressure to rise above the allowable value. These valves may be quite large in some cases. Relieved quantity may exceed one million pounds an hour.
Mechanical designs are vastly different for the two types of 'pressure relief' valves.