Is Your PVB Under Back Pressure?
Aug 13 2013, 6:12pm
A Simple Test to Determine if a PVB is Under Back Pressure
The overwhelming majority of Pressure Vacuum Breakers, in my experience, are installed on commercial irrigation systems. And these PVBs need to be tested annually by a certified backflow tester. One of the advantages of the following procedure is that it doesn’t interfere with the annual test, it merely augments it. And if testers know this process, the local water will be safer, it will eliminate water purveyor and property owner liabilities, and you can make money replacing improperly installed PVBs with RPAs.
The tester conducts the air inlet test normally, then the check valve test, also in the normal way. Both shut off valves are shut at this point. The tester disconnects his hose from the PVB. Ordinarily, the tester would now close both the test cocks and open the #1 shut off valve, then the #2 shut off valve, to restore service. The following procedure differs:
Leaving both test cocks open and leaving the #1 shut off valve closed, you open the #2 shut off valve. This enables you to get a snap shot of what is going on downstream of the PVB, all the while isolating the PVB from the normal line pressure. (Remember: the #1 shut off valve is still closed.) [The function of the open #1 test cock is to demonstrate that the #1 shut off valve is still shut, leak tight, and not interfering with this test.]
The #2 test cock might spurt out water for a few seconds from the trapped line pressure down stream of the PVB. A really huge system might take a minute or so. At some point, if the water continues to flow, even weakly, you should inspect the irrigation system for hills or elevated planters to determine if you also have an elevation problem contributing to the flow.
Much worse (and more commonly found), is a situation where you have 2 or more PVBs on this system that are looped together and actively back pressuring each other. In this case, the flow of water out of the #2 test cock is obviously under line pressure. How to determine which backflow assemblies are looped together? While leaving the #2 test cock open on the first PVB, turn off, one by one, all of the other PVBs on the irrigation system. After turning off the looped PVB, the flow will immediately stop. If you have 3 or more backflow assemblies tied together, you’ll have to leave all the other backflow assemblies turned off, opening them one at a time to solve this. When you find another looped backflow assembly by this method, the water will start to flow again from the #2 test cock. Turn it back off and hunt for another.
The older the system, the more likely you will find this condition. In the good old days, irrigation contractors weren’t too familiar with all the niceties of the plumbing code. For example, I was rudely introduced to my first set of looped together PVBs in 1986, when I was a newly minted backflow tester. I was testing some PVBs in an apartment complex built in the 1970’s inGlendale,Arizona. (And just for the record, I remember 3 looped-together PVBs in this complex.)
For the alert and industrious tester today, all this brings up business opportunities. In thePhoenixmetro area, our firm’s tests have found a great many looped together PVBs. So many, that most of the local water authorities have modified their test forms so that the tester needs to check a box indicating that he tested for back pressure, in his test of the PVB.