The Safe-T-Cover Blog

Backflow FAQ - Flooded Vaults, Standard Details, and RPZ Flooding

Posted by Randy Holland on Apr 13, 2017 4:12:00 PM

 

A Backflow Installation Q&A

We recently held an online webinar on backflow installation and waterworks best practices. There were both water purveyors and design engineers present. After the presentation, we opened it up to questions and there were so many good ones that we decided to gather the questions and answers together into a single post. Here is what cross connection control managers and civil engineers across the country are asking.

How often does backflow really happen?backflow-preventer-vault-flooded.jpg

If everything is going well, not often. However, each individual water municipality has to follow and enforce regulations as well as stay up to date on best practices. There are still backflow events happening all the time. It's not something that makes headlines often, but that is mostly because it's not the most pleasant or commonly understood topic. For proof of backflow, look no further than Corpus Christi.

What percentage of vaults flood?

Probably a higher percentage than you think, which is frightening because of the potential cross connection a flooded vault can cause. Repeatedly, water purveyors, cross connection control managers, and backflow testers tell us that flooded vaults are a completely normal occurence. However, we at Safe-T-Cover don't have the direct experience to give a specific number. The best thing you can do to get an answer is to read this article from respected backflow tester and instructor Chris Mayhew.

How can I make sure my choice in backflow placement is flexible and able to be used long into the future? backflow_preventer_enclosure_on_top_of_vault.jpg

It's great to be keeping the future of water system safety in mind. You'll pay three times over if you start with a vault, and then find out later that you've got to retrofit this to an above ground solution because the hazard level changed and now requires an RPZ. Then you're going to have a wasted vault sitting there. Safe-T-Cover sold something like 50 above ground enclosures to the state of North Carolina because they changed their code, and they had to change all their double checks at state buildings into RPZs at a tremendous cost. If they had simply started by putting a double check in an enclosure it would have simply been a matter of retrofitting the new device on the old pipe set, but because they began with a vault, they had to start over. That's the huge implication.

How does indoor design of an RPZ create risk for me as a designer?

Even the smallest RPZ can discharge a dangerous amount of water at system pressure. The way most engineers work around these dangers is to install a floor drain or floor sink. However, this is almost NEVER sufficient. Take the flow rate of the RPZ and the drain's capacity into consideration to find out how big of a drain is needed. Without having to look into the math, you can watch our video of one of these devices drumping water - just a small 3" device - you'll know that the drain will not carry all the water from this backflow preventer.


This is just part of the discussion that took place after our free webinar. Hear it all and more for free now.


failing rpz backflow preventer.jpg

How about fire vs domestic supply - is there a difference to consider for indoor placement? How about irrigation?

Some fire systems have to have RPZs. Namely, systems that carry fire retardant chemicals within them, or even antifreeze. Such contaminates render the line a high hazard because they are sharing the wastewater line. There are two good reasons to stick with a double check assembly for fire lines without these additives. First, head loss. An RPZ will deplete system pressure. In most cases, the loss is negligiable, say, 4 to 9 lbs of pressure. But some cities are running on fumes when it comes to ambient pressure anyway, and any loss is costly. Second, remember that fire fighting is about mitigating a clear and present risk of life. What happens if the Perfect Storm scenario occurs just as the fire fighting starts? Do you want the RPZ to dump all it's available water on the ground at the moment you need it most? A #1 check valve failure and a blocked relief vavle will cause all the water needed to fight the fire to be evacuated out the relief valve.

Regarding irrigation, every irrigation system, whether commercial or residential, should have an RPZ. Without doubt, the irrigation system interacts with the highest concentration of contamination on a daily basis. For commercial users the risk is lawn chemicals. For the residential customer, it's lawn chemicals and fecal waste from pets and other animals. The EPA's Cross Connection Control Manual explicitly recommends RPZs for all irrigation lines. Many states are beginning to confront this risk with explicit code.

Why not just provide guidance? Why is a standard detail required?Watts 6 inch 957N RP NRS.png

As you know, when these RPZs dump water, they dump a lot of water, so they really need to stay outside and in the civil engineer's domain. However, we know that if we're going to get the civil engineer to pick this up and handle this properly, a standard detail is going to help tremendously. Providing a standard detail gives the guidance so that this relatively small part of their job can be done efficiently and is more likely to pass through plans reveiw quickly. That's why we're advocating standard details and most primarily because yes, you could figure it out, but unless you provide enough details to ask an engineer who's never specified it before to start doing so, they're just not going to pick it up and start doing it.

The water district in my area doesn't have an above ground standard detail. This makes it difficult to design anything other than a vault - any advice?

What I would suggest is that you contact Safe-T-Cover here, and say, "Here's my situation. I'd like to put this above ground. Do you have a standard detail that I could submit to my city and see it'd be approved?" We have done so many of these standard details over the years, that there's a likelihood that we already have a drawing that would support what you're trying to do. Then you just submit it and see what the issues are at the city level, at the plans review place, and go from there.

Can Safe-T-Cover provide a standard detail template that we can work from to make our own?

Yes. Safe-T-Cover funds the development of standard details to get placed into city districts and they pay for the cost to get that done for us because they know they'll be enriched by the higher level of best practices in the area. As long as you're a metro area, they'll pay us to get that done. It's essential that you understand that this doesn't have to cost you anything. By providing you with a set of templates that aren't just a set of blank or generic documents, we can show you several cities full of results that you can redline and tear up and say, "This won't work, but that would. I'd like to change that to this." We'll take care of it for you. Number one, yes we can provide you with generic templates, but number two, we can actually go further and are happy to do so, to give you actual completed solutions, and stay on the project in the interest of helping you with language and verbiage that's consistent with best practices as well.

Want more Guidance?

These are just the broad questions we received during our recent online webinar. We didn't have time to address all of them live, so this FAQ page is meant to be a resource for design engineers and water purveyors that want to follow best practices and protect our drinking water. To hear the presenter speak about these topics in greater detail, you can watch the recorded version of this backflow preventer installation webinar. You'll learn the specifics on why everyone in the industry needs to work together to keep double check valves out of vaults, and RPZ valves out of buildings.

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Topics: Backflow Prevention, Civil Engineering, RPZ Flooding, vaults