Backflow preventers are a useful feature that can help you enhance safety in your water supply system. They are mechanical devices that are installed in the water distribution system to prevent the reverse flow of water through a pipe. This helps to eliminate contamination of household waters caused by used or non-potable water in the water distribution system. Backflow may occur from backpressure or backsiphonage. Backpressure occurs when the downstream pressure is higher than the supply pressure. It is caused when water usage exceeds the supply rate. Backsiphonage is caused by negative pressure in the supply line, which creates a vacuum in the water supply system. When the high pressure in the water supply lines suddenly decreases, it may allow dirty water to flow into the supply line. Below are 4 types of backflow devices.
1. Air Gap
This refers to the physical separation of water from the supply pipe to the receiving vessel. The distance between the supply pipe and vessel should be at least twice the diameter of the supply pipe and not less than 1 inch. Backflow is prevented because of the break in pressure between the water inlet and outlet. It is an effective means of protection against lethal hazards in water supply. Air gaps guard against both backpressure and backsiphonage.
2. Reduced Pressure Backflow Prevention Assembly
This device consists of two check valves with an automatic pressure relief valve in between. It is designed in such a way that the area between the two check valves is always maintained at a pressure lower than the water inlet pressure. Normal flow keeps the check valves open but closes the relief valve. Should backsiphonage occur, the relief valve opens to allow air to disable the vacuum. Once it is open, the relief valve also causes contaminated water to flow out of the system during backpressure occurrence. The device guards against both backsiphonage and backpressure. Reduced pressure backflow devices are the most expensive type of backflow devices, but are also highly efficient.
3. Pressure Vacuum Breaker Assembly
This has a check valve loaded with a spring that constricts tightly during pressure drops. An air relief valve found at the discharge end of the check valve also opens when pressure drops to prevent backsiphonage. This breaks the vacuum created by backsiphonage. The pressure vacuum breaker assembly may spew water from under its cap. It is important that you install it where water spillage will not cause problems. A pressure vacuum breaker is more effective against backsiphonage than backpressure. Pressure vacuum breakers are amongst the cheapest types of backflow preventers.
4. Double Check Backflow Prevention Assembly
It is a mechanical device that consists of two single valves contained in one casing. The unit also contains two gate valves that close tightly at each end. Each check valve has a physical plate attached to the top of the pipe by a hinge. The orientation of the hinge corresponds to the intended flow of water. As long as water flows in the right direction, the plate remains open. When water pressure falls or water fails to flow, the valve closes until normal water flow is restored. Should backflow occur, the plate will remain closed since the flow is contrary to the orientation of the hinge. Double check valves help to prevent both backpressure and backsiphonage of water.