There are three options for backflow installation. Inside a vault, inside a building, outside and above ground in an enclosure. Let's talk about all three.
would have to be considered the legacy method still widely practiced among designers today, but as you probably know, an RPZ can never be installed below grade. Beyond the issue of being unsuitable for RPZs, there are compelling reasons to discontinue the use of vaults altogether.
Here are the two most important reasons why a vault should never be specified again
Backflow preventers are designed to stop cross connections from happening. When they are installed in a vault, that can defeat the entire purpose of this device. Vaults flood. We've heard this from water jurisdictions and backflow testers alike for years in different areas of the country. They flood and most people are aware of that. In fact, we polled a group of engineers and water jurisdictions during a webinar on backflow preventer installation. 70 percent said they knew vaults flood.
The key problem with a flooded vault is that this causes a cross connection through submerged test cocks - whether they are open or closed. Read much deeper into this topic in our blog post on utility vaults. There you'll find quotes from the USC Foundation for Cross Connection Control and Hydraulic Research's Cross Talk. Their opinion on the matter is pretty clear now that they've written about the possible contamination from a flooded vault three separate times in their Cross Talk magazine. Here are links to the first, second, and third editions discussing this topic.
The language from USC FCCCHR says vaults may fill with water so you may be wondering how often this really happens. Watch the video below to see a compilation of photos and videos sent in to us by testers in the field.
According to OSHA, water utility vaults or pits are considered a confined space. When a backflow in a vault is tested each year, the tester must climb down into this confined space. According to the national institute for occupational safety and health, an average of 92 fatal injuries occur each year in confined spaces. You might be thinking, "these testers are trained professionals, surely they know how to manage a confined space." That’s a good observation. But what about the building's maintenance person, property owner, or the adventurous kids that can get in the vault by merely opening the lid? We have never seen confined space danger signs posted at or on the vault. Testers agree that there is little stopping curious people from opening them up.
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. What describes the ‘perfect storm? 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. If this is a four story building, that’s a lot of water!
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.
This creates the conditions for the “perfect storm” scenario. 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.
This is the scenario that must be avoided: the perfect storm.
Is this an unnecessary liability for the designing engineer?
Here is an example of a catastrophic flood caused by an RPZ dumping water. Remember, this is exactly what it was designed to do. This flood in the images to the right occurred in a hospital mechanical room causing over $1M in damage. 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. To the right, you'll see the discharge rates for Wilkins RPZs from 1/4" to 10". 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. It's the most common way designers plan for this water discharge. The problem is that the drains are just too small to evacuate as much water as is necessary, especially for the larger devices. In order to properly dispel all that water, you'd need either many drains, or very large ones. This comes with it's own problems - cost. A member of the Chicago ASPE Chapter, David DeBord, said in an article in 2013 that “The floor drain capacity required for RPZs 3” diameter and larger are likely to be cost-prohibitive due to necessary pipe diameter and fall rates.” You can read the full article here.
Installing backflow preventers in below ground vaults poses an unnecessary risk to the water supply because of vault flooding. On the other hand, installing reduced pressure zone backflow assemblies indoors creates anunnecessary risk to the building as well as the designers through litigation. How do you avoid all these risks? Install backflow preventers above ground and outside. This is safest way to protect the property, maintenance personnel and testers, and yourself.
To maintain proper protection of the water supply, backflow prevention valves can be installed outside and above grade. To protect the devices themselves, use protective enclosures. If you are ready to make the change to an above ground enclosure, here are your next steps:
Here's what you need to know about choosing an enclosure. It is important to make sure your enclosure manufacturer complies with the ASSE 1060 standard. This will ensure the enclosures are strong and rugged, have a locking mechanism and drainage, and will maintain the proper temperature for your climate. There are three classes of ASSE 1060 approved enclosures, and they provide different levels of protection from the elements. Make sure you get a class 1 enclosure with the proper heater if you need freeze protection. You can read more about ASSE 1060 here.
There are different materials available and you should choose the one that will perform best for your needs. The most common materials used for backflow enclosures are aluminum, fiberglass, and cages. You may also see brick and block enclosures in the market. Read about each of these to find the right material for your job. Each has it's own strengths.
Specifying an ASSE 1060 enclosure in the material of your choice will get you most of the protection your project requires. You should also consider details such as color and size. Most manufacturers offer their enclosures in different colors. For example, Safe-T-Cover enclosures come in four standard colors at no extra charge including a green and tan for most landscaping environments. Other colors are available for an additional fee.
Also, the size of the enclosure can vary not only by the size of the device, but also the type. N-type backflow preventers have a significantly smaller footprint and therefore the enclosure may be up to 70% smaller than the enclosure for an inline backflow preventer. Both color and size aid in blending an enclosure into the landscaping of your project. You can also place the enclosure near the property line but not at the front entrance to avoid an eyesore.