American Backflow Prevention Association
Southern California Chapter

How Does the RP Work?
Part Two

Submitted by Jim Purzycki, Orange County Director

In our first article we pressurized an RP and saw what happens to each of the three components (1st check, 2nd check & relief valve). In this article we are going to begin to see how an RP reacts when the two hydraulic conditions of backpressure and backsiphonage are applied. Let us assume we have a proper working RP and suddenly apply backpressure to the outlet side of our RP.

In our illustration, the inlet pressure is 100 PSI and the outlet pressure is 85 PSI. Let us apply backpressure to our RP. Backpressure is when a greater pressure is generated on the outlet side of the assembly than the inlet pressure. This condition can happen for many reasons, pumps, thermal expansion etc. etc. If the starting outlet pressure (85 PSI) increase to105 PSI and the second check is working properly, the 2nd check closes and keeps the 105 PSI pressure from migrating into the area (90 PSI) between the 2 check valves.

Even if we have a working 2nd check and backpressure is applied we can get discharge from our relief valve. A condition called disc compression can cause discharge from a properly working RP. When backpressure occurs, this increase in pressure placed on the downstream side of the 2nd check causes the 2nd check disc to embed farther into the 2nd check seat. The volume of water in the body between the 2 checks is being squeezed as the 2nd check disc embeds farther into the seat. Water is a not a compressible fluid in these pressure ranges, so this squeezing of this water causes an increase in pressure in the area between the 2 check valves. If this increase in the pressure between the 2 checks, which started at 90 PSI in our illustration, is equal to the inlet pressure minus the relief valve loading (100 – 2.0 = 98), the relief valve will open.

In our field test procedures, when we perform our 2nd check test of an RP we are simulating a backpressure condition by bringing the higher inlet pressure (100 PSI) around to test cock 4 (85 PSI). If you remember from your field test procedures, when an apparent 2nd check failure is observed, you are required to open your low side bleed valve on your test kit. This will draw the elevated pressure from the area between the 2 check valves, while the second check disc stays embedded into the 2nd check seat from the applied backpressure. When the low bleed is opened, you are reestablishing the pressure in the area between the2 checks back to its normal pressure of 90 PSI while the elevated 100 PSI is maintained after the second check. The non-recognition of disc compression when performing a field test is one of the most common failures by a backflow assembly tester. Once the relief valve discharges when testing the second check, you must open the low bleed one more time to determine if the 2nd check is actually working or not. A disc compression scenario may happen and the tester may incorrectly assume the 2nd check is not working. This error has happened more frequently with the inline movable design of check valves in relation to the fixed poppet style.

Let us see what happens when we apply backpressure to a non-working 2nd check. Once the pressure begins to increase on the outlet of our assembly, the 2nd check cannot maintain the separation of pressures between the inlet and outlet of the 2nd check and the pressure will equalize on both sides. As the pressure increase begins (from 85 PSI in our illustration) the area between the 2 checks will also increase. Remember the area before the 2nd check is where our low pressure is applied to the low pressure side of our relief valve elastic element. As the pressure increases above our starting pressure of 85 PSI, and goes to the point equal to the inlet pressure minus the relief valve opening (100 PSI – 2.0 PSI = 98) of 98 PSI, the relief valve will open.

In our next article we will apply backsiphonage to our RP and see what happens.



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Comments to
Revised 06/19/2005 6:47 PM