You can read about these and another five reasons to keep backflow preventers out of below grade vaults here. These days it is almost equally as common to see a backflow preventer installed inside the building as it is to see one installed in a vault, especially since RPZs can't be installed below grade.
However, most plumbing engineers, building owners, and property insurers are unaware of the potential flood risk of the RPZ. You can click here to watch a video of a 3" RPZ dumping water.
It may surprise you. The key difference between a reduced pressure zone backflow assembly and a double check backflow assembly is that RPZs are designed to dump water to protect the water system.
We have written an in depth blog that has proved to be very helpful on figuring out what the differences between DC and RPZ backflow preventers. The most important thing a designer must understand is the worst case scenario. What can happen. We all know that with an RPZ, when water demand stops the water between the valves often evacuates into the relief valve.
Some many think that that event defines the limit of what water can ever flow into a drain. But that simply isn't true. Watch the video below to watch an RPZ dumping water. We just happened to be driving by and stopped to investigate the water flowing out of this enclosure. First let's look at a flow-stop situation , one that might naturally occur at the end of the day.
It's possible, we'll look at a real example further down, that a small pebble can lodge in the 2 check valve.
Now what if there's something nearby in the system that causes back siphon? Because the 2 check valve is not closing, all the water that has been delivered to the building will continue to flow out the relief valve until the private lines are cleared. Now we'll consider a failure of the 1 check valve. Under normal operating conditions, this failure would go unnoticed.
After all, water is being called for by the user through the opening of taps. The water flows in undeterred. But with this imbalance in the system, changes in demand tend to rock the remaining valves open and closed sporadically. The imbalance created by the 1 failure makes the relief valve more prone to opening momentarily, allowing debris to block the closure of that valve.
Under such conditions, a constant flow of delivered water will begin to flow directly out the relief valve. This reduces water pressure for the user, but delivery will continue.
The real damage begins when the user stops using water such as at the end of a work day. With the relief valve blocked open and the 1 valve inoperative, all the water that the purveyor can provide will flow unabated out the relief valve wherever it might be, and continue until the water source is interrupted.
Here is an example of a catastrophic flood caused by an RPZ dumping water. Remember, this is exactly what it was designed to do. You are looking at 2 sides of one wall. On the left, we see that the sudden water flow and volume moved the wall into the next room right photo , which happened to be a telephone and low-voltage wiring room.
The insurer sought recovery from all the risk holders including the engineer, architect, contractor, subcontractor, and even the most recent recorded tester; While the details of who paid what were not made public, we do know that the property insurer was made whole by one or more of the listed defendants. This flood risk is still not common knowledge, but it really should be.
Backflow preventer manufacturers also make a flood control valve that is designed to be used in conjunction with reduced pressure zone devices. We've looked into what it does and wrote about the Watts model of flood control valve here. Some manufacturers have materials which show where they expect backflow valves to be installed - outside and above ground.
You can see an example of this on page five of this brochure from Wilkins. Additionally, backflow manufacturers have made the relief valve discharge rates available to the public for all their RPZ models and sizes.
Backflow manufacturers are making an effort to inform and prepare designers and property owners about the amount of water a relief valve can discharge. It's then up to engineers to design the backflow prevention solution the best way they can. Some try to do this using drains, sinks, or spouts. Experts at the University of Southern California write about the dangers of flooded utility vaults in their publication, Cross Talk.
They recommend that all backflow prevention assemblies be installed above-grade. When comparing the costs of installing your backflow preventer above ground or below ground, there really is no comparison.
The above-ground model will save you money on initial installation, routine maintenance, and long-term life of your project. Their contributions help provide long-term safe drinking water for client communities. Water districts must stay up to date on regulations and enforce them. That includes annual testing of backflow preventers and working closely with plumbers and engineers.
This combined effort between water districts and staff, as well as the local government, will ensure everyone is doing what they can to keep our drinking water safe. It contains a wide range of enclosure options and considerations that are available. Topics: Backflow Preventers. Call The Safe-T-Cover Blog.
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