Detector assembly

A detector assembly is a backflow preventer designed for fire protection service lines. It pairs a full-size mainline assembly, either a double check or reduced pressure principle type, with a small parallel bypass line that carries its own miniature assembly of the same type plus a water meter. Because fire lines normally sit static, the bypass and its meter are sized to register the small flows that the large mainline assembly would let pass unnoticed.

The detector arrangement solves a practical problem: fire service lines are usually unmetered, so without the bypass there would be no way to notice a slow leak, a dripping sprinkler system connection, or someone quietly drawing water from the fire line for unauthorized purposes. Any small, sustained flow moves through the bypass and turns the meter, giving the purveyor and the owner a simple indicator that water is moving on a line where it normally should not be.

Testers and property owners should remember that a detector assembly is really two assemblies, and both must be tested. The mainline checks and the bypass checks each need to hold, and the bypass meter reading is worth recording at every visit, since an advancing reading between tests signals a leak or usage that deserves investigation. Coordination matters on fire lines: impairment procedures and notification to the responsible parties should precede any shutdown for testing.

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