Car Wash RPZ Relief Valve Discharging Continuously
An automated car wash has a 25 mm chemical-injection line feeding a final-rinse arch. The potable supply also serves two hose bibs and an irrigation zone. A reduced-pressure-zone (RPZ) assembly is installed immediately downstream of the meter. During annual testing, the RPZ relief valve discharges continuously. The downstream system has a 300 L hot-water storage tank with a check valve on the inlet and no expansion tank. The site's static supply pressure is 620 kPa; the RPZ is rated at 700 kPa. The tester proposes removing the RPZ and replacing it with a double-check assembly because 'the RPZ keeps dumping water'. The site also has a below-grade irrigation valve box that can flood during rain.
1. Classify the cross-connection hazard and decide whether replacing the RPZ with a double-check assembly is acceptable.
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Chemical injection into a rinse line is a high-hazard cross connection because a backflow event could introduce chemicals into the potable supply. An RPZ assembly is designed for high hazards and provides protection against both backpressure and backsiphonage. A double-check assembly is suitable only for low hazards; it does not provide the same relief function and is not acceptable for chemical injection. Removing the RPZ would reduce protection and would likely violate the local cross-connection programme. The correct approach is to retain the RPZ, find out why the relief valve is discharging, and repair or rebuild the assembly. The below-grade irrigation box adds a flooding/backsiphonage risk, so the irrigation zone should also have appropriate protection, but that does not justify downgrading the main RPZ.
2. Diagnose the likely causes of continuous relief valve discharge and explain how thermal expansion could contribute.
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Continuous discharge from an RPZ relief valve usually means the first check is not holding tightly, the second check is leaking, or debris is holding a check open. It can also occur if the relief valve itself is fouled or out of adjustment. The hot-water tank has a check valve on its inlet and no expansion tank. When the water heats, thermal expansion has nowhere to go, so downstream pressure rises. That pressure can back up against the RPZ checks and cause the relief valve to open. The supply pressure of 620 kPa is below the 700 kPa rating, so overpressure from the main alone is less likely, but it should still be verified. The tester should inspect and rebuild both checks, clean the relief valve, and confirm whether thermal expansion is raising downstream pressure. Installing a properly sized expansion tank is part of the correction.
3. State the corrective actions and retest requirements before the assembly is returned to service.
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Do not replace the RPZ with a double-check. Isolate and drain the assembly, then inspect and rebuild the first and second checks and the relief valve using manufacturer parts. Flush the strainer and remove debris. Install a thermal expansion tank on the hot-water system, and verify that the check valve arrangement does not create a closed-loop expansion problem. Check the irrigation zone for an approved backflow method and raise or protect the below-grade valve box so it cannot be submerged. After reassembly, test the RPZ using the manufacturer's procedures: verify the first-check pressure drop, the second-check holding pressure, and the relief-valve opening point. Record the results and notify the water authority if required. Only return the assembly to service after it passes and the discharge has stopped.