Flow switches work only in wet piping systems. As the name stipulates, they feature a reed or paddle that gets in the way of the water and moves when it flows. But in dry systems where water quickly replaces compressed air, when the sprinklers are activated, this paddle can be damaged or broken - thus, the restriction to wet piping systems.
For smaller pipe sizes (usually below two inches), these switches can take the form of a "TAPT". They have a threaded inlet and a protruding paddle that connects to a threaded fitting. Others have a paddle and saddle with a horseshoe screw. When tightened, this U-bolt holds the sensor in place over a cut hole in the tube.
Paddle and vane type flow switches must face the water flow. However, they can lock in a pipe that runs vertically or horizontally. A standard location for these switches varies depending on where the water enters the system.
Flow switches are activated when the flow exceeds a predetermined rate. However, they can also use a mechanism to delay these signals, preventing false alarms due to pressure surges.
They typically feature two electrical switches: one that signals the fire alarm control panel and another that sounds a bell or another audiovisual alarm. The flow switches connected to a control panel must have a non-silent circuit, which means that the panel cannot disable the alarm signal from the switch.
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