If you are a certified tester, you learned to field test the second check of a reduced pressure principle assembly (RP) one of two ways. The procedure directed you to either test it with a reverse differential pressure (higher outlet pressure) or a direction of flow test with a greater inlet pressure.

Let’s take a step back in history and consider how double check valves were tested in the early 1900’s. Note the image of an assembly from circa 1912. The tester closed both shut-off valves and simply opened a drain valve at the inlet of the check. If the outlet gauge maintained pressure, the check was considered tight with backpressure.

With time, the industry concluded that given enough backpressure on the check valve disc, different failures may be missed. For example, with 60 psi on the outlet and 0 psi on the inlet, debris may be pushed away from the seating surface in effect performing a repair. More important was the discovery that ‘rubber’ discs were cut by the seats, and the elevated backpressure wedged the check onto the seat forcing a seal and providing a false result for a damaged component. This brought the evolution of the sight tube test method, as the one pound per square inch differential (PSID) direction of flow test was a more telling or a more difficult test to pass than line pressure applied as backpressure.

Now with that information as background, let us consider the field test of an RP. Nearly all procedures in North America today look for a pass/fail minimum 5 PSID closing differential for the first check. Note that there is no reverse differential test, as the relief valve would be forced open. We have all learned that the minimum opening differential for the relief valve is 2 PSID. And then there is the second check valve.

We learned in the tester certification class that the minimum closing differential for the second check is 1 PSID. There is a lot of emphasis on this design specification, to the point where it’s an element of written exam questions. However, not all test procedures look to verify this value, let along it being pass/fail criteria.

Several years ago, the procedure used in Southern California determined an apparent pressure drop across check valve 1, and then an actual value was recorded when the outlet shutoff valve was determined to be tight. This process required the tester to bypass supply pressure through the gauge manifold and apply it at the outlet of the second check. This is simply a backpressure test using supply pressure.

A tester then could record the inlet check as ‘tight’ if the apparent and actuals values were the same. However, the procedure provided for an ‘optional’ test. The terminology was a little misleading, as the tester was to perform the optional test, if the outlet shutoff valve was also determined a tight. What was the optional test – located in an appendix of the manual? It is direction of flow test with the high- and low-pressure gauge hoses positioned at test cocks three and test cock four, respectively. Okay, this procedure evaluated the second check in both the direction of flow and reverse flow conditions.

The procedure presently published in the USC 10^th Edition Manual of Cross-Connection Control only addresses the reverse direction of flow test on the second check. Once this tightness has been determined, the technician proceeds to the tightness of the inlet check valve.  There is no longer an ‘optional’ reference to a second check direction of flow test. However, that procedure may still be located in Appendix A.2.2.

Students of the USC manual product performance standard located found in the 10^th edition (Section 10.1.2.2.3.6) will advise us that the direction of flow test of the second check meeting the 1.0 psid (27 ¾” specifically) requirement is performed in the laboratory before the backpressure test (Section 10.1.2.2.3.9). Certainly, there must be a reason for that sequence.

The 10^th edition was published in 2009 with little explanation why the optional provision was removed from the procedure. Indeed, now that the manual has been in print for a decade few schools provide that instruction during the training. When asked about the direction of flow test, many newly minted testers will respond with ‘what?’

Since the reason for the procedure de-emphasis is unknown, let’s speculate for a moment. Suppose an argument was made suggesting that the heart and soul of an RP are the inlet check valve and the relief valve. Then what is the purpose of the second check? Well, to protect the zone pressure between the two check of course. Something must protect the relief valve from hydraulic shock or thermal expansion pressure increases, for example. Therefore, the second check has little relevance to the concept of preventing backflow through the assembly. The check simply needs to hold ‘tight’ to protect the zone.

It’s an interesting notion. That being the case, consider the loading of the second check valve being less, such as 0.5 PSID? Keep track of this: We all know that check valve disc elastomer parts soften with time. Over a couple of years, the direction of flow closing differential for a lower spring loading may approach zero. Now you see, that’s not a concern if the field test pushes the check on to the seat with backpressure.

Many recertification students – particularly those that work in the landscape irrigation industry, have dual tester certifications. They have performed the 10^th edition and 1-hose field test procedures on RP’s in locations where quick-closing zone valves are installed at the RP’s outlet. There have been dozens of reports where the second check disc has been cut like a cookie cutter due to the hydraulic shock pounding the disc on to the sharp seat.

The 10^th edition backpressure field test wedges the cut disc on to the seat, providing a ‘tight with backpressure’ result. The component damage or failure is determined with a direction of flow test, which is used in the 1-hose field test procedure. Remember the historical sight tube perspective? The low differential direction of flow test is a more stringent evaluation.

If you are currently using the 10^th edition procedures and are not aware of the second check’s direction of flow test, please research it and try it. Especially in areas of downstream water hammer, where the disc may look like a baby’s teething ring.

And finally, if the test report form asks for a second check differential reading, that is not where one records the apparent or actual drop across the first check. But if you do perform a direction of flow test for the second check and get a reading of 6.4 PSID for example, what might that tell you? Maybe a failed second check spring that was replaced by a technician using a first check or inlet check spring? That problem would have never been identified with a backpressure field test.

Author: Stu Asay, PE, PhD, IAPMO Sr Director of Research Programs