Difference Between Pressure Regulating Valves and Pressure Relief Valves

 

 

Our company is an authorized representative and distributor for Wanner Engineering, a manufacturer of sealless positive displacement pumps and pressure regulating valves.

The terms pressure regulating valves and pressure relief valves are often used interchangeably although actually are two different products. Following is a comparison of each valve type highlighting their differences in design and application.

Pressure Regulating Valves: Purpose
A pressure regulating valve provides a mechanical method of controlling flow and pressure to the downstream process by continuously bypassing excess flow (with the pump sized accordingly for this purpose).

Mechanics
The valve body has three ports, an inlet, outlet and bypass port. The inlet ports are 180° apart and the bypass port is 90° offset from the inlet/outlet ports.

Within the machined body there is an orifice located directly over the bypass port. Located above the bypass port is a spring-loaded valve plunger, the tension of the spring is adjustable with a cap screw at the top of the valve. Spring tension dictates the pressure required to compress the spring and lift the valve off its seat.

Setting the Pressure
With the pump running, slowly tighten the cap screw, increasing the compression of the spring against its seat until the desired maximum discharge pressure is reached.

Operation
The pumps discharge pressure will not exceed the set pressure because any excess flow will compress the spring further, allowing excess flow to bypass. If the discharge is dead-headed the pressure regulating valve will respond accordingly bypassing 100% of the pumps flow rate, protecting the pump and system components from exceeding the set pressure. When the pressure requirements are lower than the set pressure, less fluid is bypassed.

It is possible that the set pressure is higher than the operating pressure and in such case no flow is bypassed. The reason we suggest sizing your pump to enable some bypass flow is that the system pressure is regulated smoother then if the pressure needs to spike enough to unseat the valve before settling into bypassing flow.

If the discharge is to spray nozzles or a filter system, the amount of flow bypassed will vary. In the case of worn spray nozzles, less flow will be bypassed and spray nozzle pressure (and performance) will be maintained. In the case of clogged filtration, the pressure will not exceed the set amount and the differential pressure across the filter will increase as less flow passes through the filter and more is bypassed once the set pressure is reached.

Pressure Relief Valves: Purpose
A safety device to protect the pump and system from overpressure.

While a pressure regulating valve also performs this function, the pressure relief valve in normally closed and opens only when the system pressure exceeds the set pressure, opening just enough to reduce system pressure and then reset itself.

Although the basic design is very similar, the distinction is that a safety valve is used for emergency protection and not designed for continuous regulation of pressure because the flow through it is not proportional, rather “opened or closed”.

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