To understand what happens inside an RP, let us flow water through a generic RP (see illustration). An RP consists of an 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 produces 100PSI and begin to pressurize the assembly.

As the inlet shut-off is opened, water enters the inlet of the assembly ahead of the 1st check valve. Once in this area the water enters a relief valve sensing line. Some sensing lines are external hose or pipe and some utilize an internal passageway. The water travels through this sensing line to the elastic element in the relief valve. The elastic element is either a diaphragm or a rolling diaphragm. This pressure will build up on 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. If the pressure after the 1st check rises to where it is a minimum of 2.0 PSI less than the inlet pressure ahead of the 1st check then the 2.0 PSI loading from the relief valve spring would cause the relief valve to open. In a properly working relief valve, the opening point can be anywhere from 2.0 – 4.0 PSID depending on the manufacturer, model and size. Once the water pressure has closed the relief valve, then the pressure will increase to the next point which will cause the first check to open and allow the water to travel past the first check. The water pressure will be reduced by the amount of pressure it takes to open the first check. The 1st check of an RP can have a loading of anywhere between 5-15 PSI depending on the make, model and size. In our illustration we will assume our relief valve spring generates a 2.0 PSID and the first check spring has a 10.0 PSI Loading. Once the 1st check opens, water will travel past and pressurize the area between the 1st and 2nd checks. 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 and the lower pressure past the 1st check (90 PSI) is placed against the low pressure side of the elastic element. 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. The loading of the 2nd check spring will be anywhere between 1-5 PSI depending on the make model and size. The pressure after the 2nd check will be reduced by the amount of pressure it takes to open the 2nd check. For our illustration, we will give the 2nd check a 5.0 PSI loading.

The inlet 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 = 85 PSI) producing our downside pressure of 85 PSI. The relief valve spring is continually trying to open the relief valve which the inlet pressure is keeping closed. A properly working relief valve can only open when the pressure after the 1st check plus the relief valve opening point is greater than the inlet pressure of our RP. In our illustration the pressure past the 1st check (90 PSI) must increase to 98.1 PSI so that the value of the relief valve opening point (2.0 PSID) and the pressure after the 1st check (90 PSI) is now greater (98.1 + 2.0 ? 100 PSI) than the inlet pressure.

A properly working RP will have the relief valve pressured closed and the check valves will modulate between an opened and closed position to fill the water demand of the plumbing system. The two conditions that can cause an RP to not work in a normal condition are backpressure and backsiphonage. In the next article we will see what happens inside the RP when backpressure and backsiphonage are present.