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Don't forget that the relief valve spring is continually trying to open the relief valve, which the inlet pressure is keeping closed, and that a properly working relief valve can only open when the pressure downstream of the 1st check plus the relief valve spring load is greater than the upstream pressure to our RP.

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When this area is pressurized, it will also pressurize the low pressure side of our relief valve.The higher inlet pressure (100 PSI) is placed on the high pressure side of the elastic element in the relief valve, and the lower pressure past the 1st check (90 PSI) is placed against the low pressure side of the elastic element.2nd Check Valve Once this area between the two checks and the low pressure side of the relief valve is pressurized, the pressure will now cause the 2nd check to open.Print this Article How Do Backflow Prevention Assemblies Work?In order to repair any assembly, RP, DC, PVB or SVB, it is important that the repair technician first understand how the backflow prevention assemblies are supposed to work, so that when they are not working, the problem can be properly identified.This pressure will build up on the high pressure side of the elastic element, which will deflect and cause the relief valve stem to compress the relief valve spring, and move the relief valve disc to seal against the relief valve seat.

The function of the relief valve spring is to constantly try to open the relief valve.

For our illustration, we will give the 2nd check a 5.0 PSI loading.

The upstream pressure (100PSI) of the RP in our illustration is reduced by the combined pressure load of the 1st and 2nd check spring (100 - 10 - 5 = 85PSI), producing our downstream pressure of 85 PSI into our piping system.

Reduced Pressure Principle Backflow Prevention Assembly (RP) To understand what happens inside an RP, let us flow water through a generic backflow prevention assembly.

An RP consists of inlet and outlet shut-off valves, four properly located test cocks, a first and second check valve component, and a relief valve component.

Let us hook up our RP to a water source, which delivers 100 PSI, and begin to pressurize the backflow prevention assembly.