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The reduced pressure principle back-flow prevention device (RP) uses check valves to prevent water from flowing backwards through the pipes.  A check valve is a disc or flap that can be pushed open when water flows in the normal direction, allowing water to flow through the pipe.  But the check valve is loaded with a spring and closes when water attempts to flow in the opposite direction through the pipe.

The RP is very similar to a double check valve assembly.  The RP has shut-off valves on either end of the assembly and two spring-loaded check valves in the middle like the double check valve assembly.  In addition, the RP contains a hydraulically operating, spring-loaded pressure differential relief valve between the two check valves.  This addition makes the RP suitable for cross-connection control when the back-flow could create a danger to public health.

The pressure differential relief valve is a a valve which opens and closes in response to differences in pressure on either side of the valve.  There are two pipes leading to the relief valve.  If the water pressure is equal in both pipes, then the valve remains closed. However, if the water pressure is greater in one pipe than in the other, the relief valve opens and allows water to flow out into an outflow pipe.  This is a way of channeling water away from the cross-connection during high pressure back-flow so that the high pressure does not break through the check valves and allow contaminated water into the potable water system.

When the water flows through the RP in the normal direction, as shown above, water forces the check valves open just as it does in a double check valve assembly.  In addition, some water flows down two small pipes which lead to either side of the pressure differential relief valve. Since pressure is being applied equally to both sides of the valve, the relief valve remains closed.

However, when water flows in the reverse direction through the RP., the check valves close, as shown above.  If the back-flow pressure is great, some water will break through the first check valve and flow toward the second.  Some of this water will also flow toward the relief valve from above.  Since no water is flowing toward the other side of the relief valve, the pressure on the valve will be uneven.  This will force the relief valve to open, allowing water to run out into a backup system.



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Last modified: Wednesday January 23, 2013.

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