American Backflow Prevention Association
Southern California Chapter

How Does the RP Work?
Part Four

Submitted by Jim Purzycki, Orange County Director

In our first three articles we discussed how a properly working RP reacts when subjected to backpressure and backsiphonage. In the next few articles, we will concentrate on how an RP reacts when components of the assembly are not working properly and how we can diagnose the condition. In this article we will discuss the relief valve.

Lets assume we perform a field test on an RP and generate a 1.5 PSID (Pounds Per Square Inch Differential) relief valve opening point. Does this mean the assembly is “leaking” or will not prevent backflow? The answer is probably no. We know from our tester training that 2.0 PSID is the minimum acceptable relief valve opening point. If the relief valve opens at 1.5 PSID the relief valve will open and keep the pressure in the area between the two checks lower than the inlet pressure if subjected to a backflow condition. For the assembly to perform optimally, it must operate at or above this minimum standard in this case 2.0 PSID.

The cause of a relief valves opening below the 2.0 PSID minimum can vary greatly between different models. The assumption that a spring has worn out and that is why the relief valve will not open is usually incorrect. The most common cause of low relief valve opening points is a restriction on the travel of the relief valve stem mechanism. Either the guide of the relief valve becomes damaged or a scale or corrosion will cause the guide to not travel optimally leading to a low relief valve opening point.

What happens when our field test data presents an excessively high relief valve opening point, something above 5.0 PSID? High relief valve opening points can happen for different reasons depending on the make, model and size. The most common cause for a high relief valve opening point is when the relief valve disc does completely embed into the relief valve seat. The usual cause for this is when the relief valve disc is not traveling its full length. An example of this scenario can happen if a rolling diaphragm in the relief valve is pinched or twisted not letting the relief valve travel its full designed length. If the stem does not travel its full length, the relief valve disc cannot fully embed into the relief valve seat. If this disc is just touching the relief valve seat, and not fully embedded, then the relief valve opening point will be excessively high.

The proper relief valve opening point is important for the proper working of an RP. In our next article, we will be looking at non-working check valves in our RP.



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Revised 06/19/2005 6:49 PM