Relief valve
The hydraulically operated valve between the checks of a reduced pressure principle assembly that discharges water to atmosphere when internal pressure conditions become unsafe.
In backflow prevention, the relief valve is the differential pressure relief valve found in the zone between the two check valves of a reduced pressure principle assembly. It continuously compares supply pressure against the pressure in the zone. As long as the zone stays sufficiently below supply pressure, the relief valve stays closed. When that margin collapses, because a check is leaking or pressure has reversed, the relief valve opens and dumps zone water out through a port to the atmosphere.
The relief valve is what elevates the RP above a simple pair of checks. Rather than trusting two mechanical seals to be perfect, the design assumes they may leak and provides a safe exit path for any water that should not be there. The discharge is intentionally visible, turning an internal failure into an external symptom a building occupant can notice and report. This fail-safe behavior is why RPs are accepted on connections where a failure could threaten health.
Testers and property owners should learn to read the relief valve's behavior. Brief discharge during pressure fluctuations can be normal, while steady dripping usually points to a fouled first check and continuous heavy discharge suggests more serious trouble, such as reversal or relief valve damage. Never plug or pipe the relief port shut to stop a leak; doing so defeats the assembly's core safety feature and conceals the very failure it was designed to reveal.
Related terms
| Reduced pressure principle assembly (RP) | A testable backflow prevention assembly using two independently acting check valves with a pressure differential relief valve between them, suitable for high hazard applications. |
| Check valve | A valve that allows water to flow in one direction and closes to prevent flow in the reverse direction. |
| Differential pressure gauge | The calibrated field test instrument that measures pressure differences across the components of a backflow prevention assembly to verify they are holding. |
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