American Backflow Prevention Association
Southern California Chapter



How Does the RP Work?
Part Five

Submitted by Jim Purzycki, Orange County Director

 
In our last article we discussed what happens to an RP when the relief valve is not working properly. In this article we will see what happens when the check valves are not working properly.

Let us talk a little about the first check. Look below to our diagram of an RP. In this example we show an inlet pressure of 100 PSI. The pressure after the first check shows us 90 PSI which means we have a 10 PSID. This is the load the first check is generating on a properly working first check. If the first check was completely fouled and there was no differential produced that means we would have 100 PSI before and after the first check (0 PSID), then the relief valve spring would cause the relief valve to stay open. The first check rarely fails where there is no differential. The usual case is that instead of a 10 PSID as shown in our example the differential begins to fall as the first check begins to wear out. Let us assume we know our relief valve has a 2.1 PSID opening point. Let’s add further that our first check is starting to degrade and it can only generate a 2 PSID. In other words our inlet pressure is still 100 PSI and the pressure after the first check is 98 PSI and we know we have a 2.1 relief valve opening point, what would happen to our relief valve? The answer is that the relief valve would open up and begin to discharge. If we have a 100 PSI inlet pressure and a pressure of 98 PSI after the first check you can see where the 98 PSI along with the 2.1 PSI from the relief valve spring loading would cause the diaphragm to move causing the relief valve to open because there is a greater pressure on the downstream side of the relief valve diaphragm (98 +2.1 =100.1 PSI) than on the upstream side (100 PSI).

Some administrative authorities require the loading on the first check to have a minimum of 3.0 PSID higher value than the relief valve opening. By having a buffer greater than 3.0 PSID, this would help minimize relief valve discharge from a small pressure fluctuation in a static condition. This would mean that if our relief valve opening point is 2.1 PSID than we would have to have a first check loading of at least 5.1 PSID to pass the field test. If a 3.0 PSID buffer was not required in your area, then any first check value greater (above 2.1 PSID) than the relief valve opening point would keep the relief valve closed and would be a passing check value.

The cause of check failure tends to be due to the failure of the disc to seal against the check seat easily. Many times the check spring is blamed for a check failure but this is usually not true. The more common causes are dirt and debris on the disc, disc degradation where the disc will not seal, or a check guide restricting the travel of the check component.

The criteria for the workings of the second check, like the first check, must maintain a higher pressure upstream of the check than the downstream pressure. This differential is established by the spring loading of the second check spring which is designed to be a minimum of 1.0 PSID. Our test procedure for the second check is different than the first check because it is a backpressure test. In our field test of the 2nd check, we take the higher inlet pressure form test cock #2 upstream of the first check (100 PSI) and with needle valves and hoses place it into our number four test cock (85 PSI) causing the pressure on the downstream side of the second check to rise until it is higher than the upstream side of the second check. When the second check fails, the higher pressure would go past the leaking second check into the area between the two checks. As the pressure in this area increases, the relief valve senses the differential. When the pressure in the area between the two checks increased to 98.0 PSI (relief valve opening point 2.1 PSID) then the diaphragm would move causing the relief valve to open. The causes of failure on a second check are similar to the first check.

In conclusion, the field test is the way we generate the data needed to determine which part of the assembly is performing below the accepted minimal standard. When the numbers fall below the minimum standards established by the accepted test procedure, a repair must be facilitated to bring the working condition of the assembly above the minimum standards. The generation of accurate data is very important and this means using an accurate test kit and proper test procedures and techniques to assure the data we generate properly reflects the working condition of the assembly.


 

 
 
   

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Comments to denise@socalabpa.org
Revised 06/19/2005 6:50 PM