The first thing to determine is the type of pressure relief valve the customer is 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 over-pressurization. 

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 relive 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 that its design allow, the maximum discharge flow is reached. These types of relief valves are use to control flow as well as pressure. Lets 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 restricted flow piping system.

Let's 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. You can view or download a copy here by clicking on the "Technical Bulletin" button. Since in our example the pipe size is 3/4" of an inch lets 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 flow only 13 gpm, 7 gpm less than the required 20 gpm.

Since the 3/4" valve won't work because it's flow rate is too low, we'll next try a 1" size valve. From the chart we can see that at a 10 psi overpressure a 1" relief valve set at 40 psi has the capability to 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.