Hi,

Can anybody please tell what is the purpose of thermal relief valves & how the selection is done? How are these valves different from the normal safety valves?

 

Hi
Thermal relief valve is to relief the pressure created by increasing in the liquid volume cuased by an increase in temperatuer.

For general applicaiton, we select NPS3/4 X NPS1 relief valve size. More information, please refer API 521, sec. 3.14

 

A thermal relief valve is used when a liquid is blocked (i.e. a line or pipe between two block valves) in a pipe, and the pipe is exposed to sunlight. Although liquid does not expand much, since it fully fills the pipe, the forces developed can seriously stress the pipe. There is no difference between a safety relief valve for any other purpose and a thermal relief valve. However, it is usually small in size, "D" orifice. For a screwed valve this would be 3/4 x 1 inch.

Sizing a thermal relief valve is generally neglected since the smallest orifice size available is generally considerably (10 to 100 times) greater than the required capacity. However, if sizing is required, see API 520 and API 521.

 

Hi,
We use thermal relief valves on heat exchangers in case they are blocked in and the temperature of one side causes the fluid in the other to expand. these valves are typicaly small 1/2-3/4" off the shelf, preset type. The selection is simple just get one that will relieve before the design pressure is exceeded. Not like a safety relief valve that is the subject of a considerable study.

Roy