Type of Backflow Preventers
What Is Backflow ?
Backflow is the flow back or reversal of the normal direction of flow, either by means of back siphonage or back pressure. Back Siphonage is backflow caused by pressure below atmospheric in the supply system. Back pressure is a pressure higher than the supply pressure.
Types of Backflow Preventers
- Air gaps
- Double Check Valve Assembly (DCVA)
- Reduced Pressure Principle (RP)
- Pressure and Atmospheric vacuum breakers (PVB, SRPVB, AVB)
- Dual Check Valves (DuC)
How Do We Prevent It ?
The most common method of backflow prevention is the air gap. A well-designed and properly maintained air gap is in theory the best means available for protection against backflow. It consists of the unobstructed vertical distance through air between the lowest point of a water supply and the flood level rim of a fixture or vessel. However, an air gap can easily be tampered nullifying their effectiveness and is not always practical. Based on the degree of hazard that a water system poses to the potable water system, different backflow preventers have their specific uses. A professional from Tourangeau Mechanical can help you choose the device that best suits your facility and system design.
The DCVA consists of two independently acting, internally loaded check valves installed as a unit between two tightly closed shut-off valves, with four properly located test cocks. It is effective against back pressure and back siphonage situations and should be only installed on systems with moderate hazards.
The RP consists of two independently acting, internally loaded check valves separated by a reduced pressure zone with a pressure differential relief valve. This device is installed as a unit between two tightly closing shut-off valves with four properly located test cocks. It is effective against backflow caused by back pressure and back siphonage and is designed for use under continuous pressure. The RP is used to isolate severe hazards with the main advantage being there is visible flow if failure occurs. The RP requires proper drainage close by to allow the relief valve’s spillage to be piped to drain.
The Pressure Vacuum Breaker consists of an internally loaded check valve, an internally loaded vacuum relief valve for the downstream side, an inlet and outlet shut-off valve and two properly located test cocks. The Spill Resistant Pressure Vacuum Breaker is like the PVB, apart from an additional internal spill-resistant membrane. The PVB/SRPVB is effective against backflow caused by back siphonage only should not be used if back pressure can develop. They are recommended to isolate minor, moderate and severe hazards.
The Atmospheric Vacuum Breaker performs the same as a PVB except that the AVB is not spring assisted or internally loaded. The AVB acts as a check valve under back siphonage conditions. The AVB is effective against back siphonage only and shall not be used if back pressure can develop. They are restricted to minor and moderate hazards and shall not be installed where it will be in continuous operation for more than 12 hours.
The Dual Check Valve consists of two independently acting soft seated check valves, internally force loaded to the normally closed position. During normal operation, the check valves open permitting flow through the device and close tightly when back pressure or back siphonage occur. The DuC is designed for minor hazards and may be used to provide premise isolation on residential water services or individual outlets. It is recommended the DuC be checked periodically to ensure the device is functioning properly.
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