The limits the system pressure to a specific set level. Pressure relief valves (safety relief valves) are designed to open at a preset pressure.
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, 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 relief valves 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.