A Frozen RPZ Vault After a Cold Snap
A school's 3-inch RPZ is installed in a below-grade vault with a steel cover. After a week of temperatures near 10°F, custodial staff report water pooling in the vault and the relief port discharging steadily. The downstream building pressure is normal at 55 psi. The vault has no heater and the drain is a 2-inch floor drain that appears partially blocked with debris. The assembly passed its annual test eight months ago. The school needs domestic water restored before students return in three days.
1. Diagnose the most likely failure and explain how freezing produces it.
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The most likely failure is freeze damage to the relief valve or check valve components. Water trapped in the assembly or sensing lines expands when it freezes, cracking the relief valve seat, diaphragm, or check valve disc. The result is a relief port that discharges continuously because the seat cannot seal.
The pooling water and steady discharge are consistent with a damaged relief valve. Normal downstream pressure at 55 psi suggests the checks may still hold, but the relief valve is passing. The blocked floor drain compounds the problem by allowing water to stand, which can refreeze and cause further damage. The absence of a heater in a vault subject to freezing is an installation deficiency.
2. Outline the repair and restoration plan, including what must be verified before returning to service.
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- Isolate and drain the assembly, then open the vault and inspect for cracked components.
- Replace damaged parts - relief valve seat, diaphragm, O-rings, and any cracked check components - using manufacturer parts.
- Clear the floor drain and verify drainage capacity.
- Add freeze protection: a thermostatically controlled heater, insulation, and a properly fitting insulated cover, or relocate the assembly indoors.
- Retest the assembly after repair: check differentials, relief opening point, and air-in-water.
- Document the repair, parts used, and test results, and notify the purveyor if required.
Do not return to service until the assembly passes a full field test and the vault is protected from freezing.
3. What installation and maintenance changes would prevent recurrence?
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Install a freeze-protection strategy appropriate to the climate: a thermostatically controlled heater with a low-temperature alarm, insulation on the assembly and piping, and a vault cover that seals. Ensure the drain is sized and maintained. Add an annual pre-winter inspection to the maintenance schedule, including drain clearing and heater verification. Consider relocating the assembly above grade inside the building if the vault cannot be reliably protected. Update the site survey record to note the freeze risk and the corrective actions.