Understanding Back Flow
|By MARY ELIZABETH WILLIAMS-VILLANO|
When you turn on the tap, the last thing you expect to see is worms. Yet that’s what one Michigan homeowner got when a malfunctioning lawn sprinkler coupled with a water main break sucked nematodes into his water system. there they were, alive and swimming around in the bathtub he’d just filled for his child. Needless to say, that child went to bed without a bath that night.
This was a relatively benign incident. There are much worse backflow stories, such as the family in Colorado that was sickened by, ironically—raw sewage sucked into their drinking water from an improperly installed water purification system. Backflow prevention devices keep things like this from happening.
Backflow is probably the most complex and confusing aspect of the entire irrigation field. As a contractor, chances are you will eventually be called upon to service or install backflow devices, if you haven’t already.
What is backflow?
Very simply, backflow is what happens when water in an irrigation system, instead of flowing out, flows back in, carrying with it contamination from whatever was around the output source.
In the case of an irrigation system, fertilizer, animal waste, pesticides, and, yes, insects can be sucked back into a system through a cross-connection.
A cross-connection is any physical link between a source of potable water and a source of non-potable or contaminated water. These sources include sewage lines, cooling and fire suppression systems, and they happen on both commercial and residential properties.
What causes backflow?
Two major culprits cause backflow: backsiphonage and backpressure.
Backsiphonage occurs when the pressure in a city’s water main becomes negative. “The best analogy is that it is trying to literally draw or suck the water out of your building,” said Jon Everett, technical services manager for the Zurn/Wilkins operation of Zurn Industries, LLC, based in Erie, Pennsylvania.
It’s caused by high water withdrawal rates, such as a pipeline break. Or, when there’s a large fire down the street, and the fire department hooks up several pumper trucks. Water, normally under pressure, seeks to move to a lower pressure area. When it erupts out of a pipeline break, all the other water in the system seeks to go out, too.
Backpressure occurs when a system’s pressure is greater than the city’s supply pressure. This is typically caused by pumps, changes in piping elevation, or thermal expansion caused by a water heater. In fact, the most common cause of backpressure is thermal expansion due to a water heater.
Since a cold water line is directly connected to the water heater itself, when that water is heated, it begins to expand. “At the very moment we create a pressure that is one scintilla greater than the city’s supply pressure, we’ve got backpressure,” said Everett. “That means we’re pushing water back into the public supply.”
High-hazard and low-hazard
The type of backflow device that should be installed will depend on whether the situation is considered ‘high hazard’ or ‘low hazard.’ “High hazard means that if you consume that fluid, it could be toxic to you,” said Bruce Parrott, vice president of sales administration at Febco, a division of Watts Water Technologies in North Andover, Massachusetts. Irrigation is always considered high hazard, because of the chemicals and wastes on the ground.
‘Low hazard’ means that there might be something in the water that makes it aesthetically unappealing, such as a strange color, taste or smell. But you could drink it without suffering harm. It’s important to understand the difference between high and low hazards, because different backflow devices are used for each type.
Kinds of backflow devices There are four main kinds of backflow prevention devices: AVBs (atmospheric vacuum breakers), PVBs (pressure vacuum breakers), DCs (double check valves), and RPs (reduced pressure principle assemblies).
AVBs and PVBs have similar names, but are very different. Both AVBs and PVBs prevent backflow due to backsiphonage only. AVBs can only be exposed to pressure 12 hours out of a 24 hour period and don’t have a shutoff valve downstream of them.
By contrast, PVBs can withstand pressure 24 hours a day, 365 days a year, and do allow downstream shutoff valves.
Everett explains the difference between AVBs and PVBs further.
“When a zone valve opens up, it flows water out to the individual sprinkler heads. An AVB would be placed downstream of that zone valve. Therefore, it’s only going to be pressurized when water is flowing through it. A PVB, on the other hand, would be installed before your irrigation zone valves, and would be sitting there under pressure all the time. Both do exactly the same job, but the AVB has the limitation of time.”
So why would someone put in an AVB? “AVBs are very inexpensive,” explains Everett. “Let’s say we have an irrigation system with only two zones. It’s going to be far less expensive for me to put in a single AVB.
On the other hand, if I have multiple zones, say, six or eight, now I have to buy six or eight AVBs.” Instead, you would buy one PVB and install it before the zone valves, and it will prevent backflow from all of those zones.
DCs are also good for protecting against backpressure or backsiphonage. However, these are designed mainly for low-hazard applications. An RP (sometimes also called an RPP) protects against both backsiphonage and backpressure and against a high level of hazard. “If any kind of chemical is being mixed downstream of an RP, it will prevent that chemical from getting back into the public water supply via either backpressure or backsiphonage,” said Everett.
You could think of an RP as a kind of upgrade to a DC. Both DCs and RPs have two spring-loaded check valves. In an RP, however, there is a relief valve between those two check valves that constantly looks at the difference in pressure across the first check valve. As long as that pressure differential is adequate, the relief valve remains closed. But if the pressure differential across the first check ever narrows, the relief valve will open up and discharge some water.
In reality, what type of backflow device you’ll use will probably be mandated by the local authority having jurisdiction (AHJ). Always check with the AHJ before you install any backflow device.
Common and not-so-common mistakes
Obviously, there’s no one-size-fitsall in the backflow game. Because it’s complicated and filled with subtleties, mistakes are common. There are certain errors that people with years of experience see time and time again.
Bernard Clarke is the owner of Backflow Prevention Device Tester, a testing and installation firm, and Clarke Sales, a backflow parts distribution firm that also does equipment research, design and training, both based in Valencia, California. He began concentrating on backflow and cross-connection work in 1976, and has seen just about every mistake in the book during his long career.
“They’ll put em in backwards,” says Clarke, “Or they’ll put them in without asking the question, ‘Is there any elevation?’” Clarke elaborates further. “The contractor will ask, ‘Where do you want the backflow device? Right here at the meter?
Okay, we’ll put it in 12 inches above the ground.’ Well, by the way, the pipes go around a corner and up a mountain that’s higher than the backflow device.”
“The backflow device has to be higher than the mountain, by 12 inches if it’s a vacuum breaker,” Clarke continues. Vacuum breakers are put in many times to backpressure, which is illegal, or incorrect.”
Sometimes a contractor takes things too literally, as in one case Parrott documented. “Somebody put an AVB 20 feet up in the air, on top of a pole. He wanted to get it up high (to compensate for the elevation), but how are you ever going to test or repair it?” That example seems comical, but the local AHJ probably didn’t laugh. Backflow devices are usually required to be installed “belt-high,” without needing any permanent scaffolding or ladders to reach them, since they must be tested once a year.
“If I were to give one bit of advice, before you install, check with the AHJ and make sure the backflow device is placed where they want it, and is of the type that they demand,” says Everett. “A lot of municipalities are very, very specific about how far they want them to be from the meter, what type of pipes you should use to install them, and what variety of backflow preventer you use.”
“In freezing areas, you don’t want to put an RP below ground, in a pit, because once it expels water, it could suck back in what it just expelled. So double-checks are installed instead, because they have no ports to the atmosphere. They can put those down in a meter box,” said Parrott.
Incorporating plastic has another purpose, too, and that’s winterization. “One company went with a more plastic body in the middle,” said Troy Butolph, co-owner of Butch and Troy Landscaping in Bend, Oregon. “That way, if it freezes and breaks, that’s what’ll break, and it’s easy to fix. Another company went a different route, they built unions (quick-releases) into their backflow devices, so you can simply unscrew them right there, pull the assembly out, and take it inside where it’s warm.”
Sometimes, a contractor will unintentionally damage a backflow device doing a “blowout” in an attempt to winterize. “Some guys will put the air compressor hose in before the backflow device and not after,” says Butolph. “But the springs are loaded to a diaphragm flange system, and if you put in too much air pressure, say, 60 to 100 psi through the pipe, you can push those springs back and they can end up locking. That’ll cause failure and cross-contamination.”
When it comes to backflow, nothing is simple or easy. Fortunately, there’s lots of training available out there from manufacturers and distributors, should you need to know more. It’s a tricky subject, so make sure you do your homework first.