How to Select a Pressure Relief Valve

How to Select a Pressure Relief Valve

Don't make the mistake of trying to pick the right relief valve for your application based on just the size of the pipe in the system or the set pressure. Do it this way and you are almost guaranteed to be spending time trying to figure out why the valve is not doing what it is supposed to do.

The first thing to determine is the type of pressure relief valve you are looking for. There are two common types of relief valves. One is the safety or pop off valve that opens completely when the set pressure is reached, diverting all, or almost all, of the flow out the discharge port of the valve. An example of this type of relief valve, made of metal, is found on hot water heaters to protect them from damage should steam form inside the heater and cause overpressurization.

The other common type of relief valve, and the one made by Hayward in PVC plastic, gradually diverts the flow out of the discharge port as the pressure is relieved. This type of valve is often used not only to relieve pressure but also to control the flow in the system.

In this type of valve a spring pushes a piston down onto a seat. As the pressure exceeds the set pressure, the piston is moved off its seat and the flow is gradually diverted out the discharge port. More flow is diverted as the system pressure continues to increase above the set pressure. When the piston is pushed as far up into the valve body as its design allows, the maximum discharge flow is reached. These types of relief valves are used to control flow as well as pressure. Let's look at an example of sizing a valve for this type of application.

One application that we frequently come across for pressure relief valves is to protect a pump from operating against a closed-off system or a restricted flow piping system.

Let's now say you have a 3/4" piping system that requires a flow rate of 20 gpm at 40 PSI. The pump in the system is rated at a nominal 50 PSI at 20 gpm. A Hayward plastic relief valve is to be installed in the system to prevent the pump from having to operate against a closed system should a valve downstream from the pump be closed.

The first thing to do is to look at the relief valve sizing chart for Hayward relief valves.

Since, in our example, the pipe size is 3/4", let's start there and see if a 3/4" valve will be the best choice. From the chart we can see that a 3/4" valve set at 40 PSI (the required system pressure) will flow 13 gpm at 10 PSI overpressure (the set pressure of 40 minus the pressure output of the pump of 50 PSI). This size valve will not work. It will permit a flow of only 13 gpm, 7 gpm less than the required 20 gpm.

Since the 3/4" valve won't work because its flow rate is too low, we'll next try a 1" valve. From the chart we can see that, at a 10 PSI overpressure, a 1" relief valve set at 40 PSI will permit flow up to 27.5 gpm. Since this is more than the 20 gpm that is required, the 1" valve is the one to use.

Remember: all applications are different. Examine all the application parameters before reaching a decision on a valve.