Reduced Pressure Principle Backflow Prevention Device Assembly Rate of Flow and Pressure Loss
As water is flowing through the RPZ, a force is needed to open the check valves since they are spring loaded. That force exerted on the check valves is expressed in pounds per square inch (PSI). The greater the flow in gallons per minute (GPM), the more the check valves will open. As the gallons per minute increases, the pressure loss through the assembly is greater. A ¾-inch RPZ in a static state with a 0 GPM flow rate will have a pressure loss of at least 8.0 PSI. The same RPZ at a flow rate of 20 GPM will have a pressure loss of approximately 12.5 PSI. If the flow rate for this RPZ were 40 GPM, it would have a pressure loss of approximately 14 PSI, which exceeds the American Water Works Association standards for this size RPZ. This concept of pressure loss relationship to flow rate is important to understand because at 0 GPM the check valve should be closed drip tight and the pressure loss indicated on the graph is due to the tinsel strength or force of the springs. When testing a backflow prevention device assembly, the test shall be completed during a static state since that is when the check valves should be closed.
