Why is an alarm check valve installed in fire sprinkler grids?

An Alarm Check Valve is a double-acting non-return valve that allows water to flow forward into the sprinkler lines but blocks backflow. When a sprinkler head opens and water flows, the check valve clapper lifts. This redirects a portion of water to a physical alarm channel, triggering a mechanical water motor gong (a water-driven alarm bell) and pressure switches to alert security personnel.