Safety valves and pressure relief valves are mainly used to protect containers, piping systems and equipment from overpressure, which are important for the safety of workers and equipment. Therefore, the correct valve should be selected, otherwise not only the system will be damage, but also may cause an explosion.

The following factors are commonly used in selecting and determining safety valves or pressure relief valves.

1. Connection size and type

Connection type: flange, male or female, or other connections.
Valve size: Must correspond to the size of the inlet and outlet piping, and the inlet and outlet piping connected to the valve must be at least as large as the inlet/discharge of the valve itself.

2. Set pressure

The set pressure is usually in pounds/ per square inch (PSIG) which is the pressure at which the safety valve or relief valve opens.
The set pressure of the valve must not exceed the maximum allowable working pressure of the boiler or other container. This means that the valve must be opened at or below the maximum allowable working pressure of the device. Meanwhile, the device's maximum allowable working pressure should be at least 10% higher than the highest expected working pressure under normal conditions.

3. Temperature

Temperature affects the volume and viscosity of the gas or liquid medium flowing through the system. Temperature is also an important parameter in our selection of valve materials. For example, steel valves can handle higher operating temperatures than bronze or iron. Operating temperature and releasing temperature must be also considered.

4. Back pressure

The back pressure can be constant or variable, which is the pressure on the outlet side of the safety valve due to the pressure in the discharge system. It impacts the setting pressure of the upstream valve and causes it to open repeatedly, which may cause damage to the valve. For installation environments where is variable back pressure, the back pressure should not exceed 10% of the valve set pressure. For installations with a high level of constant back pressure, a bellows seal or pilot valve may be required.

5. Medium

Different types of medium (steam, air, gas, etc.) require different valves. Valves made of stainless steel are preferred for corrosive medium, the structural of valves should be suitable for maintenance. 

6. Required capacity

Safety valves and relief valves must be able to release pressure at a certain capacity. The required capacity is determined by several factors, including the geometry of the valve, the temperature of the medium, and the discharge area.