There are two different test procedures for the reduced pressure principle assembly (RP). First I'll discuss the USC procedure with an added option for check valve #2. Then I'll discuss another method used in some areas of the United States.
The nice thing about the RP test, as taught by the Foundation for Cross-Connection Control and Hydraulic Research at the University of Southern California (FCCC & HR @ USC), is that the #1 shut-off valve is left in the open position. Scale and debris are not dislodged from the valve and therefore debris doesn't hang up in check valve #1.
Backflow preventers have been manufactured since the mid-forties. There is a good possibility that you, as a tester, are going to encounter a backflow preventer with the old style gate valves. These valves are double-gated, metal to metal surfaces, and were never intended to be completely drip tight. If your water is scale forming, then you will have difficulty closing these valves completely. A general rule of thumb for closing the large cast iron, non-rising stem gate valves is 3 turns per diameter inch plus 2 or 3 turns to seat the gate. That means that a 8 inch gate valve needs 24 turns to close plus 3 more turns to seat, for a total of 27 turns. If you are only able to turn the stem 25 turns, then most likely, the valve is not shut off completely. A well trained utility employee will learn to count turns as he or she opens and closes gate valves.
Most of the backflow preventer assemblies manufactured before 1986 were equipped with gate valves. It is safe to say that you are going to encounter a backflow preventer with gate valves on it. It also safe to say that you are going to find some of these gate valves that leak. You have to know how to deal with that leaking gate valve. You need a test procedure that will detect that leaking shut-off valve. The test procedures developed by FCCC & HR are very effective for testing the RP.
| If you close the #2 shut-off valve and the relief valve starts to drip, then chances are, check valve #1 is not holding tight. However, until you connect your gauge and check the differential pressure across check valve #1, you are not 100% sure that the leak is in check valve #1. Because check valve #1 is the first obstruction in the waterway of the assembly any debris in the pipe usually lodges in check valve #1. When you test check valve #1 and find it to be holding tight, with a gauge reading greater than 5.0 psi, then you proceed to the next test: the relief valve test. | |
| Water is allowed to enter the zone between the two check valves to increase the pressure and lower the differential (or difference in pressure). When the pressure on the low side of the diaphragm plus the spring tension, exceeds the pressure on the high side of the diaphragm then the relief valve will open and start to let water out. The low control needle valve on the gauge is only opened slightly, never more than 1/4 turn. This slight opening of the low control valve lets water into the zone very slowly and allows the tester to maintain good control over the pressure increase in the zone. With this slow opening process, the needle on your differential gauge will stop at the pressure that the relief valve opens. |  |
| After the relief valve is tested, we move on to the backpressure test of check valve #2. Connect the vent (by-pass) hose to test cock #4. Open test cock #4. Apply supply pressure behind (downstream) check valve #2 by opening the vent (by-pass) control valve. This backpressure can be as much as 10 psi greater than the pressure that is normally downstream of check valve #2. If check valve #2 is holding tight you should get a reading on your gauge slightly less than the original gauge reading of the differential across check valve #1. This difference is due to the compression of the disk in check valve #2. Simply reset your gauge by opening then closing the low bleed needle valve. The needle on your gauge should return to the original differential reading across check valve #1. |  |
A disadvantage of this backpressure test is that the procedure will not indicate whether the spring in check valve #2 is either broken or missing. If the spring is broken or not installed, it is possible that the backpressure test will indicate that check valve #2 is holding tight. There is also the possibility that a larger than normal spring has been installed in check valve #2. The backpressure test does not give you any values on your gauge. You are only testing check valve #2 for leaks.
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In the ASSE Professional Qualifications Standards, Series 5000, there is an optional test procedure for check valve
#2. After you finish the U.S.C. test procedures on the assembly, then:
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The needle on your gauge should drop to a reading greater than 1.0 psi. You are simply measuring the differential across check valve #2 just as you did on check valve #1. This test is accurate provided that shut-off valve #2 is not leaking or that the low control needle valve on your gauge is holding tight.
LEAKING #2 SHUT-OFF VALVE
If the shut-off valve #2 is leaking and the customer is not using water, you will have no indication
that the valve is leaking. If shut-off #2 is leaking and water is moving through the assembly, you
have two situations with which to deal.
A. If the customer is using water while you are testing the reduced pressure assembly, you will get a few indicators:
During the relief valve test, you open the low control valve on your gauge slightly but not more than 1/4 turn. If the amount of water leaking through shut-off #2 is less than the amount of water moving through the low control valve, then you will be able to test the relief valve. If the leak through shut-off #2 is greater than the amount of water passing through the low pressure hose into the zone of the assembly, then the relief valve will not open. The needle on your gauge may drop slightly but will not continue to the opening point. When the needle on your gauge starts to drop but then stops, you have a suspicion that shut-off #2 leaks.
B. It is possible that there is backpressure on the assembly and shut-off valve #2 is leaking. When you connect the vent (by-pass) hose to test cock #4, open test cock #4, and open the vent control valve on the gauge, the needle will pin to the high pressure side of your gauge. You must immediately turn off the vent control valve on your gauge or test cock #4 on the assembly. You have created a cross-connection from the public water supply at test cock #2 to water of questionable quality at test cock #4. Potentially contaminated water has just passed through the vent (by-pass) hose, through your gauge and into the public water supply through your high pressure hose.
There is a variation of the U.S.C. test procedures for the reduced pressure principle assembly used in a few areas of the United States.
If check valve #2 is holding tight, there is no problem. But if check valve #2 leaks then the relief valve will open prematurely. This accidental opening must be recorded as the relief valve test. When the technician actually tests the relief valve after it has already opened, the technician may get an entirely different value. In other words, the technician must record the relief valve opening the FIRST time it opens. There are some models of reduced pressure principle assemblies that have "lazy" relief valves. If you exercise these relief valves prior to the actual test, they usually will pass with a reading of 2.0 psi or greater.
In the field, if check valve #2 fails, the technician will usually stop the testing and disassemble the backflow preventer to repair check valve #2. After the repair, the technician will then test the assembly again. It is possible that the relief valve will now pass because it had been exercised when check valve #2 failed originally. The technician will miss the "lazy" relief valve that should have been noted on the original test as failing. If the same technician were to return a month later and retest this assembly using the U.S.C. procedures, the technician may find that the relief valve is still failing.
The relief valve should always be tested prior to the check valve #2 test. Also, as you can see, the condition of shut-off valve #2 is very critical to the results of your RP test.
Lets try something a little different with the differential gauge. In Figure 5, all three hose are connected to the RP. The high pressure hose is connected to test cock #2, low pressure hose to test cock #4, and the vent hose to test cock #3.
To test check valve #1:
To test the relief valve:
To test check valve #2:
Let's not forget, when restoring service to the customer and you open shut-off valve #2, you are going to give the customer a slug of dirty water. It would be nice to be able to flush that dirty water before it enters the building. In any case, the customer should be warned that they may get a slug of dirty water.