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. The pump in the system is rated at a nominal 50 psi at 20 gpm.