A "Reduced Pressure Principal" backflow preventer is the style most commonly required to protect against used water flowing in reverse through the water meter (or service connection) and is typically required by water suppliers. It is preferred because it has 3 elements of protection, consisting of two spring loaded check valves to prevent reversal of flow, and a relief valve to discharge used water before it moves past the upstream check valve.
The following are artist's renderings of a Wilkins Model 975XL2. It is shown in various conditions of operation.
Operating Properly Without Flow
#1 = Number One Check Valve, spring-loaded up to 10 psi.
Dark Blue = High-side pressure, for example, 100 psi. This pushes against the "high" side of the diaphram, closing the Relief Valve.
Operating Properly With Water Flow
When the end user opens a valve (faucet, sprinkler, etc), water flows in the normal direction from the supplier, and into the end user's facility. The flow of the water pushes the spring-loaded check valves open. The tension of the #1 check spring maintainins the pressure difference which creates the Zone of Reduced Pressure. This difference in pressure keeps the relief valve in the closed position while water flows in the normal direction.
No Flow, Debris Fouls the #1 Check
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 pressure to equalize 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, Debris Fouls the #2 Check
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 relief valve spring and the elevated customer side pressure act against the supply side pressure on the relief valve diaphram and push the relief valve 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 relief valve spring overcomes the supply pressure and pushes open the relief valve.
Back-Siphonage, Debris In the #2 Check
A reduction in supply side pressure caused by an incident such as a water main break, or a heavy water demand from activities such as fire fighting, 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.