American Backflow Prevention Association
Southern California Chapter



How Does the RP Work?
Part Three

Submitted by Jim Purzycki, Orange County Director

 
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.

 

 
 
   

home | newsletters | chapter activities | chapter contacts | tester information | calendar | links

Comments to denise@socalabpa.org
Revised 06/19/2005 6:48 PM