Why Backflow Occurs 2

When the fire truck pulls up to a hydrant for firefighting, the fire truck is typically a pumper truck. The pumper fire truck connects to the hydrant and drafts water from the hydrant. In some cases, there is insufficient pressure to adequately control the fire.  The pumper truck operator compensates for low pressure by regulating the flow of water to the fire so as not to dewater the public water distribution system.   This action causes the pressure in the public water distribution system to decrease and possible creating negative pressure in the buildings in the vicinity of the fire.  If cross-connections are not controlled within the domestic plumbing system where they exist, a backflow situation may occur.  Drinking or using water in a building located in the vicinity of a fire can be hazardous to the health of the consumers of water. Not controlling cross-connection where they exist is a public health threat because backflow could occur during an unforeseen fire incident.

Hydrant Flushing reduces the water pressure int he building creating a siphon effect and a cross-connection incident.

This illustration shows several unprotected cross-connections and the impact they will have on the quality of drinking water within a facility when the pressure in the public water distribution system is under a large demand due to hydrant flushing. As you can see in this illustration, the impacts that unprotected cross-connection can have on the potable water system when the hydrant is opened.  The pressure in the domestic potable water system within the building drops and the submerged inlets with no cross-connection protection, the hoses in sinks with no hose bibb vacuum breakers, and the immersed drainpipe without an air gap are now in a siphon state and non-potable water is being pulled into the potable water system.  Anyone consuming water within the building from the drinking fountain will be consuming non-potable water.  The public water system should inform all parties when hydrant flushing will take place. The property owner should validate to the public water system that all cross-connection are controlled pursuant to the know plumbing codes.