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Technical Articles
October 10, 2018

The What, How, Where and Why of Relief Valves

I. WHAT styles of Relief Valves are available?

A. In a shaft design (Series RVT), a non-sticking solid PTFE shaft passes through three (3) U-cup seals which effectively isolates the liquid from the spring. An elastomer seat at the end of the shaft seals across the valve orifice.

B. In a diaphragm design of either PTFE or an elastomer the diaphragm becomes both the seal isolating the spring as well as the seal across the valve orifice.

Angle (90°) Porting Relief Valves have historically been the most popular design. Design and performance are the same. The benefit of a 90° angle pattern would be for simplifying the piping scheme by using the Relief Valve body in lieu of a 90° elbow. In-line valves can be installed anywhere in a straight piping run.

II. HOW do they operate?

All styles are normally-closed and begin opening once the set pressure is reached. They use spring force to push down upon a shaft or diaphragm. The pressure setting is done manually by turning the adjusting bolt clockwise to raise the pressure setting, and counter-clockwise to lower it. This simply varies the compressive force of the internal spring across the valve orifice. The more it is compressed, the higher the set pressure will be.

When the inlet pressure reaches the set pressure, the force created by the inlet pressure is equal to the force exerted by the spring. Liquid then begins to trickle through the valve. As inlet pressure continues to increase, the valve opens farther, allowing more flow.

CAUTION: The RVT, RVD, RVDM, and RVDT series are NOT “POP SAFETY” valves and should not be used in applications requiring such valves.

III. WHERE would I use a Relief Valve?

In addition to relieving excess/dangerous pressure from closed-top vessels or piping systems, these normally-closed valves provide system control benefits as shown. Depending upon the function performed, they are given different names.

relief app

1. – “Pressure Relief Valve”- to protect a system (e.g. pump, pipe segment or tank)
       from excessive pressure (in excess of the set point).

2. – “Back Pressure Regulator”- to provide a means of retaining desired system
       pressure to points of use in upstream line(s).

3. – “Pressure By-Pass Valve”- to protect a pump from ‘dead-heading’ by enabling
       the flow to by-pass an obstruction.

4. – “Back Pressure Valve”- to provide back pressure directly on the discharge
       of a pump to enhance its performance.

5. – “Anti-Siphon Valve”- to prevent unwanted chemical
       siphoning through a pump; when negative pressure at a lower elevation could 
       create a siphon and drain a tank. The valve is set to open at the desired pumping 
       pressure, but seals tightly when a vacuum occurs downstream.

IV. WHY would I use a Relief Valve versus some other comparable valve?

As shown in the application diagram above, this is a very versatile and widely utilized valve. Its compact size, ease of setting and re-adjustment and repeatability, all contribute to its popularity. There are Air-Loaded (aka. Dome Loaded) Relief Valves selectively available but require numerous accessory items (air regulators, filters, lubricators, etc.) for their operation.


For futher assistance with your specific application involving Relief Valves please contact our Technical Sales Dept. at (973) 256-3000 or E-mail us at info@plastomatic.com