Part 2 of a 3-part series
Certain backflow preventer installations should never be installed indoors. The flange-size reduced pressure zone backflow preventers, those assemblies with a pipe size of 2½” and larger, is one of them. The risk of personal and/or property damage is simply too great because a flood will happen, even with conventional drain systems in the mechanical room floor.
In our last blog we discussed the various backflow preventer assembly failure scenarios and which failure combination represents the perfect storm, that single highest damage risk. Today we’re discussing the flood itself.
A few years ago, Safe-T-Cover sent me on a mission to witness a failing RPZ backflow preventer. But as you might expect, such an event is difficult to capture live. All it does is react. It has no idea – or opinion about – what will happen to the surroundings after the fact. It simply reacts.
So without a way to predict when an RPZ will be forced into its reactive mode, I did what any normal person would do: I called the fire department. I told them what I wanted to see and they said they would like to see it too. Delighted at the idea of creating a bonafide catastrophe, we decided to create the event ourselves at the Nashville Fire Department Training Facility.
In setting up this failure, we removed the closing mechanism of the #1 check valve simulating a mechanical failure to close. Next, we jammed a small piece of solder debris into the hinge area of the relief valve where it is likely to get caught, and finally, we shut off the flow beyond the #2 check valve simulating a stop in user demand.
The fire department fed the assembly a flow of water at 88 PSI, a flow rate that would be consistent with overnight flow rates in well-served cities. While we were happy with the simulation, there are a few under-reported aspects of the test that should be kept in perspective. First, this is only a 3” reduced pressure zone backflow assembly. It is important to think about how much more damage a full grown – 8” or 10” – backflow assembly would do. It’s a difference in orders of magnitude rather than degrees.
Secondly, the flow rate reached only about half of what the manufacturer specified was “possible.” This is precisely because of the mere 88 pounds of flow rate. If this assembly was in the basement of a high rise, and the failure had been the #2 check valve, the initial head pressure would have caused the flow rate to be much higher. On the other hand, without a constant flow from the public side, the head pressure would fall reasonably quickly and water release would eventually cease. Therefore, such an event, a failure of the #2 check valve, is not considered the worst case in terms of property damage. It is still clearly a large head volume from a large building which could be devastating and should not be underestimated.
In Part 3 of our series, we'll find out what's considered the safest place to install a backflow preventer. For more information, you can watch our webinar on best practices, or check out our guide “Trends in Backflow Preventer Installation.” It can show you some of the more common installation locations and the risks you take when you install a backflow preventer in a basement or a vault.