In
Part 1 of this series of articles we discussed how a proper working
RP operates. In the second article we discussed what happens to
an RP when backpressure is applied on the downside of the RP.
Backpressure is a backflow condition that is concerned with a
pressure increase at the outlet of our assembly. The operation
of the second check is very important in controlling backpressure.
In this article we will apply the other backflow condition of
backsiphonage to our RP and see how it acts.
Backsiphonage
is a condition which causes a sub-atmospheric pressure to be
applied to the inlet of the assembly. Backsiphonage can happen
for several conditions one of the more common conditions is
excessive water demand in the distribution system. When the
inlet pressure to our assembly (100 PSI in our illustration)
goes down to sub-atmospheric, or negative, the 100 PSI is reduced
to a negative pressure. The pressure at the inlet of the RP
is what keeps the relief valve closed. When the pressure at
the high pressure side of the elastic element in the relief
valve is reduced to a negative, the relief valve will open because
of the relief valve spring load and any pressure remaining in
the area between the 2 check valves exerting on the low pressure
side of the relief valve elastic element.
The
relief valve of an RP can only open for the two backflow conditions
of backpressure or backsiphonage or the simulation of these
two conditions. When there is a sufficiently higher pressure
on the high side of the elastic element of the relief valve
than on the low pressure side, the relief valve will stay closed.
If the pressure on the low pressure side of the relief valve
plus the relief valve spring loading is sufficiently greater
than the high pressure side, the relief valve will open.
A
simulated backpressure condition can also cause the relief valve
to open. One of the more common simulations of backpressure
happens when there is a pressure fluctuation at the inlet of
the RP. In an RP where there is no flow going through the assembly
and then the inlet pressure goes from 100 PSI down to 80 PSI
this can cause the relief valve to open. This would happen because
there would be higher pressure on the low pressure side of the
relief valve elastic element versus the high pressure side (high
pressure side would be 80 PSI and the low pressure side would
be 90 PSI plus the relief valve spring loading).
If
a water hammer condition happens on the downside of the RP because
of a quick closing solenoid, this in crease in pressure would
cause a backpressure condition which could cause the relief
valve to open as we explained in article number 2.
Just
because a relief valve discharges water, does not always mean
the RP is not working. Pressure fluctuations can simulate conditions
that can lead a person to assume the assembly is not working
properly. To be sure whether the assembly is working or not
a test kit must be attached and proper test procedures applied
to determine the working condition of the assembly. In our next
article we will discuss what happens inside an RP that is not
working properly.
