American Backflow Prevention Association
Southern California Chapter

How Does the DC Work?
Part Six

Submitted by Jim Purzycki, Orange County Director

In our first five articles we described how an RP operates in a working and non-working condition. Before you can properly diagnose any condition, it is important to know how the assembly is supposed to operate. In our test procedures we must be able to diagnose a working assembly and also know when it is not working properly. This next article we will talk about how the Double Check Assembly (DC) and the Double Check Detector Assembly (DCDA) are supposed to operate.

A DC is simply two approved independently operating check valves that can hold a minimum of 1 PSI in the direction of flow with the outlet of the check valve open to atmosphere. These checks must be located between an inlet and outlet shut-off, and have 4 properly located test cocks. The check valves in a DC must hold a minimum pressure (1.0 PSI minimum) in the direction of flow. If a check valve is holding less than 1.0 PSI let us use .5 PSI as an example, testers will incorrectly say, “The check valve is leaking”. This leads some people to believe this check valve would not stop a backflow condition because it is leaking. The check valve is not leaking at .5 PSI because it is still sealing off the area upstream and downstream of the check valve with a .5 PSI loading. The check valve is performing below the minimum criteria as established in the test procedure (1.0). The minimum criteria in a test procedure are set at a point that will trigger a repair before the assembly can degrade to the point where it cannot prevent backflow (0.0). As long as our check valve has a positive loading it can prevent backflow but only when it is above 1.0 does it meet the minimum criteria as established in the test procedure. So once the test procedure generates data that the check is maintaining less than 1.0 PSI we must repair the check to its original working specifications.

Conditions that can cause a check to perform below its optimum level are many. The cause of check failure is due to the failure of the disc to seal with adequate pressure against the check seat. Many times the check spring is blamed for this lack of pressure but this is usually not true. The more common cause of failure is dirt and debris between the disc and seat. Another common problem is disc degradation where the disc will not seal against the check seat. The third common cause is a restriction in the travel of the guide limiting the movement of the check valve prohibiting it from sealing properly.

There is a variation of a DC called a (DCDA). This is a double check created for fire sprinkler applications. A DCDA consists of an approved DC with a bypass arrangement that consists of a by-pass water meter and an approved by-pass DC. This by-pass is piped around the mainline DC. The purpose of this by-pass arrangement is to detect and register the first 3 gallons per minute (GPM) of flow across the backflow preventer into the fire system. Many testers think a DCDA is simply any small by-pass DC piped around any main line DC with a meter attached and because the by-pass DC is smaller the first flow will go through it. This is not true. In order for the by-pass to detect and register this 3 GPM the two DC’s and the water meter must be engineered so that the larger assembly will have a slightly higher differential at the low flow condition (0-3 GPM). This will assure the first 3 GPMs travels through and is registered by the water meter in the by-pass. Then if the fire system demands more than 3 GPM, the main line assembly will open up and flow to the designed flow requirements of the system. An unknowing installer may install a mainline DC and pipe in a by-pass that looks similar to the DCDA. These unapproved DCDAs cannot guarantee that they will detect this first 3 GPM because they are not factory-engineered assemblies but rather a collection of 2 DC’s and a water meter assembled to look like a DCDA.

The reason it is important to detect this low flow of water is that most fire protection systems do not have a mainline water meter at their point of service from the water purveyor. Because fire systems are an emergency connection, water purveyors do not want the expense or extra flow loss of going through a water meter. Because fire systems are considered an emergency connection there should be no flow to detect across a mainline water meter anyway. Water purveyors use this by-pass to assure that water users with fire systems do not flow water through this emergency connection and also with the ability of the by-pass to detect small flows, they can also detect if there are any small leaks that may be under ground and out of sight.

The check valves of a DCDA are similar to the DC and the repair process will be similar. In many cases the spring loading of the mainline assembly or the by-pass may be different from the standard DC, but the repair procedures and the test procedures to diagnose its workings are the same. Before we can repair any assembly, it is important to have correct data on the workings of the assembly to be sure we know what we are fixing and just as importantly, that it really does need to be repaired.



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Revised 06/19/2005 6:51 PM