Why Backflow Occurs 4

Service Shut-down – How can shutting down a service line create a cross-connection incident?  The domestic water system must be under pressure at all times in order to prevent a reversal of water flow in the piping system. Cross-connection incidents will not occur if there is a positive pressure in the public water system and throughout the domestic water system. Cross-connection incidents occur when there is backflow. Backflow occurs when the pressure downstream in the piping system is greater than the pressure upstream in the piping system or the pressure downstream in the piping system is sub-atmospheric. When the service line is closed, a static state occurs downstream of the shut-off valve. The domestic water system pressure will be influenced by atmospheric pressure. As long as no one uses the water downstream of the closed service shut-off valve no problem exists.  It would be like hold your finger on the top of the straw filled with a liquid. The water in the straw will stay in the straw as long as the top end of the straw is not exposed to atmospheric pressure. Once the liquid in the straw is exposed to atmospheric pressure the liquid in the straw will drain out. This is similar to closing the service line valve.  As long as the piping system is not exposed to atmospheric pressure, the water will stay in the piping system. However, if an indirect unprotected cross-connection exists, atmospheric pressure would be introduced.  If someone opened a tap or faucet in an area lower than the unprotected cross-connection, non-potable water would flow into the potable water system.  Backflow prevention device installed on service lines will have a similar effect as closing a valve on the service line. For this reason, all cross-connection shall be controlled no matter where they are located. In order to prevent a cross-connection incident when closing the service line valve, the public water system shall establish a service shut-off policy. The policy should state that before a service line valve is closed, the building owner shall be notified and the building be evacuated or a cross-connection survey shall be conducted of the facility and all cross-connections be controlled before the service valve is closed.