Shutoff valves
The isolation valves at the inlet and outlet of a backflow prevention assembly that allow it to be tested, serviced, and replaced.
Shutoff valves are the full-flow isolation valves installed immediately upstream and downstream of a backflow prevention assembly, and they are considered part of the assembly itself rather than optional accessories. They allow the assembly to be isolated from the system for testing, repair, or replacement without draining large sections of piping. Resilient seated designs are favored because they close drip tight, which matters when a test procedure depends on truly stopping flow.
During a field test, the downstream shutoff valve plays an active role: closing it creates the static conditions needed to measure whether each check holds. A shutoff valve that leaks through its seat can make a good assembly appear to fail, or mask a marginal component, so testers often have to diagnose valve leakage as part of interpreting odd gauge behavior. Worn or damaged shutoffs are one of the most common practical obstacles to completing an accurate test.
Property owners should resist the temptation to treat assembly shutoffs as everyday control valves for the building. Frequent operation wears the seats, and leaving one partially closed starves downstream systems. Owners should also know that these valves are typically required to stay accessible and operable, since an assembly that cannot be isolated cannot be tested or serviced. If a tester reports a leaking shutoff, repairing it protects the validity of every future test.
Related terms
| Test cocks | Small valved ports on a backflow prevention assembly that give a tester gauge access to the pressure zones inside the assembly. |
| Double check valve assembly (DC) | A testable backflow prevention assembly consisting of two independently acting spring-loaded check valves, used for non-health hazard applications. |
| 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. |
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