Brass 28mm double check valve.
A double check valve is a backflow prevention device designed to protect water supplies from contamination.
A double check valve has two internal, spring loaded, non return valve mechanisms. This employs two operating principles: firstly one check valve will still act, even if the other is jammed wide open. Secondly the closure of one valve reduces the pressure differential across the other, allowing a more reliable seal and avoiding even minor leakage. The screw in the middle of the valve is a test point for monitoring correct operation of the check valve.