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Local
Cross Connection
Control Programs
Apex
Cary
Durham
Holly Springs
OWASA
Orange Cty, Chapel Hill and surrounding areas.
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REDUCED PRESSURE PRINCIPLE BACKFLOW DEVICE
| 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.
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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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