Thermal relief valves you will find for instance at oil pipelines (hot countries) in tankfarms.
On hot days you can get pressure build up in pipelines with crude oil because of the gas formed. To protect the line from overpressure a small relief valve (thermal relief valve) is placed to vent some oil back in to a storage tank. Steven van Els
SAvanEls@cq-link.sr
Thermal relief valves are also used on chemical transfer lines that may be valved in. They are also used in regular (non-hot) countries. Some materials have a high coeffient of expansion per degree and could burst a pipe of not relieved. Just the sunlight on the pipe can cause the pressure to rise. The amount relieved at a time may be very small as the valve may only burp.
Fire relief is for equipment (i.e. tank, exchanger) that is being heated by a fire incident.
Thermal Relief Valve (TRV) is to protect the system (normally pipeline/piping and heat exchanger) due to thermal expansion. TRV is required for the blocked in piping which contain LIQUID and the volume of liquid contains is more than 500 l AND it is exposed to the environment higher than the operating temp of the contain fluid. E.g. if the operating temp of blocked in fluid is 25 and ambient temp is 35, then TRV is required. Otherwise, TRV is not required. TRV is also required if the system is heated by mean of heat tracing (either by steam or by electric).
Size of TRV normally is 1/2" x 1" or 3/4" x 1". 1 deg C temp increase would increase press of up to 100 psi!.
PSV for fire is normal PSV (spring type or pilot type) and with the overpressure of 21 % (unless not permitted by local code). PSV for fire is to take into account the external heat due to fire (pool fire or else) into the system. Please refer to API RP 520 pt 1 and 521.
Thermal Relief typically refers to protection from minimal expansion within a nominally "solid" liquid filled vessel due to ambient conditions, i.e. sunlight. Such Valves may or may not be ASME Code Stamped for Sec. VIII, Liquid Service rated at 10% overpressure. Fire Protection valves are typically added in addition to the required ASME Sec. VIII required relief for 10% overpressure. Fire Protection Valves are rated at 21% overpressure. They may be protecting Vapor or Liquid Vessels, whereas Thermal Relief is typically Liquid and often the Tube Side of a Heat Exchanger.
