Why Backflow Occurs 5

When you close a valve suddenly, the water in the pipe stops abruptly and there is a sudden and rapid change in the velocity of the water creating a hydraulic shock wave in the piping system. The hydraulic shock wave is accompanying by a banging noise. This is known as water hammer. The hydraulic shock wave is accompanied by a pressure surge that can damage the piping system and create maintenance problems for backflow prevention devices. Any time you have a change is pressure there is the possibility of having a cross-connection incident. The maximum working pressure of backflow prevention device assembly is 175 pounds per square inch (psi).  According to the published literature, the pressure surge created by a fast closing valve can be as high as five (5) times the system working pressure.  If the system pressure is 60 psi the pressure resulting from the effects of the water hammer would be 300 psi which is must greater than the design operating pressure of backflow prevention device assembly. Water hammer can be devastating to the second check valve of a backflow prevention device assembly. A water hammer can also be created the starting a liquid flow suddenly.  Water hammer occurs in almost all water distribution systems and the placement of backflow prevention device should be assessed to prevent the occurrence of water hammer. The public water system and maintenance persons should also understand the impact of closing valves because this activity can cause the pressures in the public water distribution system or domestic water system to increase suddenly and drastically. This increase in pressure could create a cross-connection incident and be a public health threat.