The Reduced Pressure Principal Backflow Preventer is the most common style of meter protection and is most often what the Water Supplier will require. The following are artist’s renderings of a Wilkins Model 975XL2 Lead Free RPP Backflow Preventer to help illustrate operation under various conditions.
Operating properly in a static (no flow) condition
#1 = The Number One Check Valve (spring-loaded, with resilient seal against a seat)
#2 = The Number Two Check Valve (spring-loaded, with resilient seal against a seat)
RV = The Relief Valve (hydraulically operated based on pressure differences acting across the #1 Check Valve)
Dark Blue = Supply Pressure – water delivered by the water purveyor. For example, 100 pounds per square inch (psi). As the diagram shows, this pressure also routes to the “high” side of the Relief Valve Diaphram, pushing the Relief Valve closed.
Light Blue = Zone of Reduced Pressure – the supply pressure is reduced by the #1 Check spring (for example, down to 90 psi). The supply pressure acting on the Relief Valve diaphram closes the Relief Valve against the pressure in the Zone. If the pressure in the Zone equalizes to the supply pressure (within 2 to 3 psi), the Relief Valve spring combines with the Zone pressure to open the Relief Valve discharge. In this way, the Zone controls the opening and closing of the Relief Valve.
Grey = End-User Pressure – water has passed through the #2 Check into the plumbing/distribution system of the end user. The #2 Check slightly reduces the water pressure to, for example, 88psi. However, the water is now subject to pressure variations caused by the end user.
Operating properly in a normal flow condition
When the end user opens a demand valve (faucet, sprinkler, etc), water begins to flow in the normal direction: from the supplier, through the backflow device, and into the end user’s facility. The flow of the water pushes the spring-loaded check valves open and the springs push back, maintaining the difference between the Supply Pressure and Zone of Reduced Pressure. This difference in pressure keeps the relief valve in the closed position.
Zero demand condition with debris fouling the #1 Check Valve
The system has returned to a zero demand condition (no valves open downstream), however debris has become lodged between the #1 Check Valve seat and the sealing disc, preventing the #1 Check from closing all the way. This allows the equalization of pressure between the Supply and the Zone of Reduced Pressure. When this happens, the Relief Valve Spring pushes back against the Relief Valve Diaphram, opening the Discharge.
The Relief Valve is operating properly, but the #1 Check Valve is not, due to the debris preventing the full closure of the Check. Depending on the amount of debris, the Relief Valve can discharge just a slight drip or can open fully and continuously, flowing water at full pressure and volume until the Supply is eliminated by closing the #1 Shut-off Valve.
Back Pressure condition with debris fouling the #2 Check Valve
Debris has become lodged between the #2 check seat and sealing disc. Some source of pressure in the water user’s system (pump, elevation, heat, etc.) is causing the user’s pressure to increase toward or beyond the pressure delivered by the water supplier. Because of the debris fouling the #2 check, that pressure increase affects the Zone. The combined force of the rv spring and the elevated customer side pressure act against the supply side pressure on the rv diaphram and push the rv open until the zone pressure has been restored to a level below the supply side. Water discharges from the relief valve for as long as the customer pressure plus the rv spring overcomes the supply pressure and pushes open the relief valve.
Back-Siphonage condition with debris fouling the #2 Check Valve
A reduction in supply side pressure due to a water main break, “siphoning” of water from nearby areas by a heavy water demand from activities such as fire fighting, etc. causes the customer side pressure to exceed the supply pressure. A fouled #2 Check Valve allows the excess customer side pressure to act on the Relief Valve Diaphram, opening the Relief Valve discharge. Water will discharge until the supply side pressure again exceeds the customer side pressure and closes the Relief Valve.
Special thanks to the folks at Zurn for graciously allowing our use of their diagrams.