What is a Thermal Relief Valve? Difference Between TRV and PRV

Thermal relief valves are also known as thermal safety valves, temperature relief valves, or thermal expansion relief valves. It is a safety device employed in liquid piping and pipeline systems to protect the equipment and system. When the thermal expansion of a liquid creates excessive pressure inside a closed system, the thermal relief valve pops up to release some fluid and bring down the pressure back to an acceptable limit. In general, when liquid heats up, it expands a little. When the temperature increase is quite high, the volume change of the liquid though not by a large percentage may increase the system pressure. To protect against this overpressure situation and to avoid explosion, thermal relief valves prove to be a good device.

Design Consideration for Thermal Relief Valves

The common size for thermal relief valves is relatively small. The usual size of thermal safety valves used for piping and pipeline systems are generally (1”x1”) or (¾”x1”) with a flanged end having a minimum orifice area of 0.110 in².

Depending on the design code requirements a lifting device may or may not be required for thermal relief valves. In general, for air, hot water, or steam services with temperatures in excess of 600C lifting devices are normally specified.

Thermal relief valves are normally not vented directly to the environment. So, some sort of containment method is always suggested.

Thermal relief valves are usually designed based on API 526 or EN 14597 codes.

Applications of Thermal Relief Valves

Thermal safety valves are used for protection in case of excessive temperature for liquid applications in long pipes/pipelines exposed to the environment (sunlight), closed vessels, water heater applications, cooling water return side of heat exchangers between isolation valves, and pumps that recirculate water where heat buildup can be a problem. Piping in transport pipelines or storage areas, that will be regularly blocked during normal operation and can have a pressure rise due to solar heating or heat tracing calls for TRV installation.

The principal benefit of using a thermal relief valve in pump systems is that it allows an operator to run the unit in a bypass mode for an extended period that reducing the chances of premature seal wear on the high-pressure pump.

In general, thermal expansion relief valves are not intended for:

• Process plant piping
• Two-phase flow lines
• Storage or transport piping sections that are not normally shut in for operational or emergency purposes
• Systems that are not fully liquid filled, i.e. less than 95% liquid filled.

Working of Thermal Relief Valves

The working of TRV is mostly similar to pressure relief valves. Even though the name is a thermal safety valve, the pressure increase is the main cause for its working. Under normal operation, the TRV remains closed by the spring force. When the force due to fluid expansion is great enough and exceeds the internal spring force, the valve pops up. Once the pressure reduces, the spring force again closes the thermal relief valve in a position to work smoothly.

Thermal safety valves work in an automatic manner and when the temperature falls below the set point it automatically reset.

Thermal relief valves are usually placed in a remote location without easy access. Also, normally temperature sensors, processors, or solenoid-type sensors are not used to control TRV systems. So, thermal expansion relief valves must be designed to be durable and work properly in case of system failure.

Difference between TRV and PRV

Even though the working mechanism of both thermal relief valves (TRV) and Pressure relief valves (PRV) are almost similar, they have some differences.

• A TRV is usually small whereas PRV can be of big sizes.
• TRV acts when overpressure occurs because of temperature increases and relieves a small quantity of fluid whereas PRV is sized to protect from any kind of overpressure and usually releases large quantities of fluid.
• Safety is the main purpose of both TRV and PRV.