This is a diagram of one of the units I will be tested on today to see if I know what the fuck I'm doing. This is called a Reduced Pressure Principle Assembly, commonly known as an RP, or simply, "backflow preventer." All of the units I test in the field will be of this type, although they may look slightly different than the one depicted -- they still operate on this "three zone" principle. The blue-shaded areas indicate the zones. The direction of flow here is left-to-right; from left to right, zones are the supply (darkest blue), which has the be the first thing connected after the water meter, the "zone," or as I think of it, "the buffer," and the lightest-blue area is the discharge, which is connected to the building's plumbing.
Testing the unit makes use of a test gauge, which has to be calibrated and certified annually by an approved agent. That date is recorded on every test result form, along with the make/model and serial number of the test gauge.
The test gauge is typically a relatively simple (if expensive) device. It has one large meter on it which indicates the differential pressure between the various zones depending on which test cocks and which valves on the gauge are open. Many also have a smaller meter that show the supply pressure in PSI.
The first test conducted on an RP is the relief valve opening point. For the unit to work (and pass inspection), if the pressure on the supply side ever drops lower than 2 PSI compared to the pressure in "the zone/buffer," the relief valve must open, allowing the contents of the "buffer" to flow out the bottom of the unit.
To test this, first all test cocks must be flushed. Open #4 test cock slightly, then open and close #1 test cock, open and close #2 test cock, open and close #3 test cock, and finally, close #4 test cock. Connect the low-side (or lower-pressure) hose of the test gauge to test cock #3, and the high-side hose to test cock #2. Place the bypass hose from the test gauge in the drain, or leave free where water will not damage anything. Open the #3 test cock, and then, open the low-side valve on the test gauge and the bypass to purge all air from the test gauge. Air will compress; water, not so much, so it's important that no air is in the system being tested, including the test gauge. Then, open the #2 test cock slowly, and once fully open, open the high-side valve on the test gauge to flush air from that line, as well. Close the high-side valve, the low-side valve and the bypass on the test gauge. The gauge is now "set up," and the first test can begin.
To test the relief valve opening point, begin by shutting off the #2 cutoff valve on the RP. Then, open the high-side valve on the test gauge. Putting one hand underneath the relief valve discharge port, slowly and slightly open the low-side valve on the test gauge until the meter begins to drop. Once water -- even one drop -- is detected at the relief valve discharge port, close the low-side valve on the test gauge, and record this reading as the relief valve opening point. It must be 2 PSID (pounds per square inch difference) or greater in order to pass. If the value here is less than 2 PSID, or no water is ever detected, the test can stop here, as the unit fails the inspection.
The second test is to evaluate the the #2 checkvalve holds against backpressure from the discharge zone. To do this, open the bypass valve briefly to purge any air from the gauge, and then close it. Connect the bypass hose to #4 test cock, and open #4 test cock. Loosen the fitting on #3 test cock (low-side) to reestablish setup pressure between the two zones. Then, tighten the fitting. Open the bypass valve on the test gauge, allowing supply pressure to routed between #2 checkvalve and #2 shutoff valve. If the gauge reading drops and stays steady, or simply stays steady, the #2 checkvalve is holding against backpressure ("held tight"). If the gauge reading continues to fall at any rate, #2 checkvalve is leaking, and the unit fails the test.
The third test is to measure the pressure drop across the #1 checkvalve. To do this, again loosen the low-side hose fitting from #3 test cock and re-tighten to reestablish setup pressure. Observe the gauge reading. If the gauge reading drops and stays steady or simply stays steady at a pressure 3PSI or greater than the relief valve opening point (first test number recorded), then #1 checkvalve is holding tightly enough to minimize "spitting" from the relief valve. If the reading drops to the relief valve opening point, then the #1 check is leaking, and the unit fails.
Next test is to see if the #2 shutoff valve is also holding tight when closed. To test, close the #2 test cock. Observe the gauge. If the reading stays steady from the previous test, the #2 shutoff is holding tight, and passes. If the gauge drops, the #2 shutoff is leaking, and the unit fails.
The final test is to check the pressure drop across #2 checkvalve, which must be 1PSID or greater. TO do this, close all test cocks, and remove the bypass hose from #4 test cock. Remove low-side hose from #3 test cock, and connect to #4 test cock. Remove high-side hose from #2 test cock and connect to #3 test cock. Place bypass hose in drain again. Close all test gauge valves, and turn on both #4 and #3 test cocks. Open low-side valve and bypass valve on test gauge to purge air. Open high-side valve on test gauge, then close. Close low side valve on test gauge. Observe gauge reading and record. If 1PSID or greater, the pressure differential is sufficient, and the unit passes its final test. If lower than 1PSID, #2 checkvalve must be serviced, and the unit fails.
Close all test cocks and remove all gauge fittings, and open all test gauge valves, and drain the hoses appropriately. The test gauge is ready to be stored at this point. Turn on #2 shutoff valve to restore water service. Complete the test report, and sign, date and record tester's certificate number. Follow the controlling authority (usually the local utility)'s procedure for recording and indicating test results (mark/punch tags, file paperwork, obtain customer signature, etc).
Testing complete. Should take approximately 10-15 minutes to perform all five tests (although this is not a requirement for passing the practical test).
The second unit I have to test on today is called a double-check assembly; the test procedure is exactly the same except for Test #1 on the RP, as the DC has no relief valve. The reason I will never be testing those in the job is because those are almost exclusively used in fire-suppression systems ("sprinklers"), and we do not test those at all. That's someone else's job! :)
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