Understanding Thermal and Pressure Safety Valves for Fluid Flow Systems

The terms thermal safety valves (TSV) and pressure safety valves (PSV) are often called by a few other names. They might be called relief valves, pressure relief valves, or simply, safety valves. Regardless of the name, they perform the task of relieving pressure when a dangerous level has been reached. They operate on different principles, and therefore, protect against different situations.

 

Figure 1. Safety valves perform the task of relieving pressure when a dangerous level has been reached. Image used courtesy of Adobe Stock

 

Formally, a safety valve operates automatically, using the force of the gas or liquid to operate the action of the valve. However, some sources claim that a proper safety valve not only contains automatic operation but a manual override as well, causing them to be slightly different than a standard relief valve. This is the case in some, but not every safety valve, so it is safe to assume that various industries and applications use their own definitions for slight variations in these devices.

 

Characteristics of a Thermal Safety Valve (TSV)

The thermal safety valve, as the name implies, is for cases when thermal expansion of a liquid would create excessive pressure in a closed system. When liquid heats up, it expands but not nearly as much as a gas. Even if the temperature increase is quite dramatic, the change in the volume of the liquid is not by a large percent, nor is it usually very rapid. The more common industry term for this type of valve is the “thermal relief valve.” 

These factors lead to thermal valves with a couple of unique properties. First, they do not need to be very large since the volume increase will be relatively small. Usually, they will have an inlet diameter no larger than 1”. Liquid is also often unsafe to vent directly into the environment, obviously in the case of petroleum products or acids. Even water that has become extremely hot can be an environmental hazard, so some sort of containment method is also critical near these thermal safety valves.

Common applications for protection in case of the excessive temperature of a liquid are closed vessels, long pipes exposed to the environment (sunlight), water heater applications, and pumps that recirculate water where heat buildup can be a problem.

 

Figure 2. Above-ground oil pipelines may utilize a thermal relief valve to protect from overheating caused by the sun.

 

The operation of most of the valves is based on an internal spring allowing the valve to open if the force of the fluid expansion is great enough. This makes the operation quite similar to most common pressure relief components for hydraulic or pneumatic systems.

Since these valves are often placed in areas without each access to a control system, it is uncommon to find these valves controlled by a temperature sensor/processor/solenoid type of system. They must be able to operate in case of system failures, so they are designed to be extremely durable and operate properly without any supply of power or any communication with another part of the system. 

 

Characteristics of a Pressure Safety Valve (PSV)

Just like with the thermal valve, the common industrial name for these kinds of valves is “pressure relief valve”. Unlike the liquid subject of the thermal valve applications, gasses expand to a much larger volume percent change, and they can heat up much quicker. The ideal gas law proved a much higher degree of correlation between temperature and volume and their effects on pressure. A sudden rise in temperature will cause the volume or pressure to increase dramatically, and the excess volume must be vented.

For pneumatic applications that use air drawn from the surroundings, it can usually be vented with no problems. For other gasses, the same containment considerations for liquids must be considered to prevent harm to the environment.

The construction of the pressure valves is similar to the thermal valves, usually using the force to open a spring-loaded valve to vent the excess pressure (Figure 2).

 

Figure 3. PSVs depend on a spring-loaded pressure relief valve that can vent excess pressure. Image courtesy of RomanM82 [CC BY-SA 4.0].

 

Often the spring is adjustable to allow different pressures to open the valve. Because of the more drastic changes, the valves, in this case, can be of quite a large diameter to vent more matter in a very short time.

 

Safety for Fluid Systems

Both PSV and TSVs have the purpose of relieving a system in case of an excess of pressure with extreme durability and reliability. For liquids, small expansion from thermal increases must be vented with small yet reliable valves. For gasses, excess pressure can lead to larger volume changes, so the valves often must allow larger flows.

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*Article was originally published July 11, 2020, revisions have since been made.