How to Select and Size Safety Valves and Pressure Relief Valves

Safety valves and pressure relief valves ensure safe operation of pressurized systems. Both pressure relief valves and safety valves are used to protect piping systems, equipment, and vessels from overpressure. Since overpressure can lead to damage and even explosions it is important to correctly size and select the valves. In this article we will discuss five criteria that need to be considered when sizing and selecting these kinds of valves:

1. Set pressure
2. Back pressure
3. Capacity
4. Temperature
5. Valve and seal material

Set pressure

The set pressure or response pressure is the pressure at which the valve starts to open. The set pressure is not allowed to exceed the maximum allowable working pressure (MAWP) of the system. This implies that the safety or pressure relief valve should open at or below the MAWP of the system. Furthermore, the MAWP of the system should be at least 10% higher than the highest expected operating pressure assuming normal circumstances. 

It is important to know that safety valves and relief valves operate differently. Safety valves act very quickly when the set pressure is reached, they pop open. Relief valves on the other hand act slower than safety valves. The difference is shown in figure 1. For a more in-depth comparison please read our dedicated article on pressure relief valves vs safety valves. 

Figure 1: Difference between a pressure relief valve and a safety valve.

Back pressure

Back pressure is the pressure on the outlet side of the safety or relief valve. The back pressure may be constant or variable. It can affect the downstream pressure of the valve and cause chatter (rapid opening and closing). This is due to the fact that increased back pressure could lower the set pressure and cause the valve to pop open repeatable. Chatter could damage the valve, thus it is important to keep back pressure in mind. If your system has variable back pressure, ensure that the back pressure doesn’t exceed 10% of the valve’s set pressure. 

Capacity (blow-off capacity)

Pressure relief valves and safety valves need to be able to relieve pressure at a certain blow-off capacity. This value is determined by multiple factors such as the valve’s geometry, media temperature, and the relief discharge area. The blow-off capacity can be expressed as cubic meters per hour (m³/h), gallons per minute (gpm), or pounds per hour (lbs/hr). 

The valve’s inlet and outlet connections should correspond to the size of the inlet and discharge piping. Both the inlet and the discharging piping should at least have the same diameter as the valve’s inlet/discharge connection. If either the inlet or discharging piping is smaller than the valve’s connection it can obstruct the flow. 

Temperature

The media temperature affects the viscosity and thus the flow of the media. This in turn affects the needed blow-off capacity. Furthermore, the temperature needs to be taken into account when selecting the valve and seal material.

Valve and seal material

The used material needs to be compatible with the media and the temperature it is exposed to. If the media is of corrosive nature or has a high operating temperature, stainless steel valves are desired. Tameson has an overview of chemical resistances of materials that you can consult.

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