the improvement of swing-check valve
Single swing-Check Valves have been used for a hundred years as a means to prevent backflow. Unfortunately, a simple check valve is not leak proof, and conditions were found where the single check valve was allowing water to back up into the public system. Then double swing-check valves were used, doubling the reliability of backflow prevention.
An improvement in the swing-Check Valve was the addition of weight inside the valve body and hung on the downstream face of the check valve. The additional weight prevented the swing check from floating open and allowing seepage backwards through the valve and into the public side of the piping. A down side to the design of many older check valves is the inability to actually test the valves to verify that the valves are seating positively.
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