A plumber is performing a mandatory annual test on a reduced pressure principle (RPZ) backflow prevention assembly. After closing the #2 shut-off valve and opening the #2 test cock, the differential pressure reading across the relief valve is 0 PSI. What does this reading indicate?
- AThe relief valve is fouled or stuck open, indicating a failure.
- BThe #1 check valve is leaking, allowing water to bypass.
- CThe #2 check valve is failed, causing water to flow backward.
- DThe relief valve is functioning correctly and is holding pressure.
Show answer & explanationAnswer & explanation
Correct answer: A. The relief valve is fouled or stuck open, indicating a failure.
During an RPZ test, after closing the #2 shut-off and opening the #2 test cock, the relief valve should open if the pressure in the zone between the check valves drops below 2 PSI differential. A reading of 0 PSI across the relief valve (meaning no pressure differential is being maintained) indicates that the relief valve is either fouled or stuck open and cannot maintain the required differential pressure, which is a critical failure.
Why the other options are wrong
- B. Incorrect. A leaking #1 check valve would cause the pressure in the zone to rise, potentially preventing the relief valve from opening, not resulting in 0 PSI differential across the relief valve.
- C. Incorrect. The #2 check valve is tested differently; this test specifically evaluates the relief valve's ability to maintain a differential pressure and open correctly.
- D. Incorrect. A functioning relief valve should maintain a positive differential (e.g., >2 PSI) or open to relieve pressure if the differential drops below specification, not read 0 PSI under this test condition.
RPZ Backflow Test (Relief Valve)
A specific test procedure for a Reduced Pressure Zone (RPZ) backflow preventer to verify the proper operation of the differential pressure relief valve.
- Relief valve maintains a differential pressure (typically 2 PSI) above the downstream pressure.
- If the differential drops, the relief valve opens to discharge water.
- Failure to maintain differential or opening at the wrong time indicates a malfunction.
Memory trick: RPZ relief: zero pressure means zero protection, a test of trust.