The term backflow means any unwanted flow of used or non-potable water or substance from any domestic, industrial or institutional piping system into the pure, potable water distribution system. The direction of flow under these conditions is in the reverse direction from that intended by the system and normally assumed by the owner of the system.  

 

Backflow may be caused by numerous specific conditions; but, basically the reverse pressure gradient may be due to either a loss of pressure in the supply main called backsiphonage, or by the flow from a customer's pressurized system through an unprotected cross-connection, which is called backpressure.  

 

Thus the term backflow covers both a backsiphonage condition and a backpressure condition.

A reversal of flow in a distribution main-or in the customer's system-can be created by any change of system pressure wherein the pressure at the supply point becomes lower than the pressure at the point of use.

When this happens in an unprotected situation the water at the point of use will be siphoned back into the system; thus, potentially polluting or contaminating the remainder of the customer's system. It is also possible that the contaminated or polluted water could continue to backflow into the public distribution system.  

 

The point at which it is possible for a non-potable substance to come in contact with the potable drinking water system is called a cross-connection.   

To prevent backflow from occurring at the point of a cross-connection a backflow prevention assembly must be installed. However, it is important the backflow prevention assembly match the particular hydraulic conditions at that location and is suitable to protect against the degree of hazard present.  

 

The particular type of backflow preventer appropriate for specific needs will be discussed later.  

First, it is necessary to understand some basic hydraulics which govern the principles of backflow and cross-connection control.

Y. H. Kim/PE, C&T Corp.