Backflow preventers are designed to stop the reverse flow of water in a plumbing system. Picture the Smith family washing their stinky dog in a backyard tub. The Smiths are absorbed in the task and don't realize that the discharge end of the hose has been left in the tub with the water running. At that moment, a delivery truck knocks over a nearby fire hydrant. Water in the system rushes toward the broken pipe, siphoning the dog's bath water back into the family's home. When the water crews restore service, the Smiths make lemonade with water from their kitchen sink. There are many backflow prevention devices that could have protected the Smiths' lemonade. A hose bibb vacuum breaker would have been perfect!
In order to prevent contamination and polution flowing into their distribution system, a water supplier may require installation of backflow prevention assemblies. The style most commonly required to protect the distribution system is called a "Reduced Pressure Principal" (RPP) backflow preventer (also known as "Reduced Pressure Zone" or RPZ). This type is preferred because it employs 3 elements of protection: two spring loaded check valves to prevent reversal of flow; and a relief valve to discharge used water.
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, downstream to fill the user's need. The flow of water pushes the spring-loaded check valves open. The tension of the #1 check spring maintainins the pressure difference in 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, continuously flowing water 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 increase in the water user's system (pump, elevation, heat, etc.) is overcoming the pressure delivered by the water supplier. Because of the debris fouling the #2 check, the pressure in the Zone falls to the Relief Valve Opening Point. The Relief Valve will discharge until the supply pressure again exceeds the Zone pressure.
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.