A Dual Check Valve works the same as a normal check valve. The only difference is that there are two places where water can be stopped instead of one. A check valve is used to keep the flow of liquid moving in one direction and not to back flow up the pipe. It is used frequently in connection with city water supplies and sewer systems. The device in the check valve that keeps the water from flowing backward can be made up of a ball, disk, flap or some other obstacle that will block the flow of water.
Flap Check Valves
These check valves have a flap or disk that allows water to flow in the approved direction. When water flows from the unwanted direction, it forces the flap against the opening and prevents flow. This is the worst kind of double check valve because the flap can rest on top of flowing water--especially if the valve is old.
Ball Check Valves
With a ball Double Check Valve, there is a ball inside the shell that sits up tight against the side of the valve where the water will be coming from. In most cases, there is a spring that holds the ball tight up against the hole, creating a seal. When water is forced through the valve, it pushes the ball into the spring so that it can come through the opening and out the other side. When the water is turned off, the ball settles back against the hole and keeps water from flowing back the other way. A dual valve would have two areas within the valve with balls sealing them shut.
Cone Check Valves
Some valves have a cone-shaped plug sitting up against the valve opening. The cone works in much the same way as the ball check valve. This generally is called a lift check valve. The cone is the only moving object inside the valve. When the water goes in, the plug is lifted and the water flows around. When the water is shut off, the cone is force into place by the backflow, and the valve is sealed off.
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