Difference Between Safety Valve and Relief Valve

Safety Valve vs Relief Valve

Whenever a gas or a liquid is used as the working fluid of a machine, whether it is large or small, it is transported under pressure. Sometimes the pressure in these systems and interconnecting piping can be so large that a breakage can result in catastrophic damages, even loss of human lives. This was a main cause of failures in the steam operated systems in 19th century, such as large boilers. To regulate the pressure within the system and the piping, devices had to be introduced to reduce the pressure automatically by allowing the escape of the working fluid in the system when the system reaches its critical limit.

Safety valves and relief valves are two types of devices belonging to the category pressure relief valves (PRV), and the operation is based on using the static inlet pressure to actuate the device.

More about Safety Valve

A pressure relief valve controlled by the inlet static pressure opens completely when reaching the critical pressure, and it is known as a Safety Valve. Opening of the valve is accompanied by a popping sound caused by the sudden opening, and that is characteristic to this type of valves.

Safety valves are commonly used in systems using compressible gasses, such as steam and air as the working fluid. When connected to the pressurized system (ex. A boiler), the valve is pressed against a spring loaded mechanism by the static pressure within the system. When the internal pressure exceeds the critical value the disc separates from the seat to expose the pressure to a larger surface area of the valve disc. This larger area causes a larger force exerting on the spring mechanism, and as a result, the valve opens completely.

The valve on a pressure cooker is an example of a safety valve.

More about Relief Valve

A pressure relief valve used in liquid systems with the same role as the safety valve is known as a relief valve. Its main function is to control or limit the internal pressure of the system or vessel and prevent the system reaching the critical limit due to process upset, instrument or equipment failure, or fire. In contrast to the safety valves, relief valves open gradually.

Relief valves are used in low capacity systems and systems with low thermal duty. They can also be used in pump systems.

What is the difference between Safety Valve and Relief Valve?

• Safety valves are used in gas systems, and relief valves are used in liquid systems.

• Safety valves open with a characteristic popping sound while relief valves open gradually.