Why Backflow Prevention Is So Important
Apr 17 2014, 5:48pm
The Secret (and Hysterical) History of Backflow Prevention
Aptly enough, the modern word “Backflow” is derived of two words of ancient Latin origin: “Bacchus” (for the Roman God of too much wine) and “Flowous” (for what typically happens to people who drink too much wine). Indeed, Julius Caesar was history’s first great Backflow Specialist, due to his famous lines “Veni, vino, vettchie,” (I came, I drank, I threw up).
But for purposes of this article we’ll talk strictly about the modern usage of the word backflow, which simply means the reversal of the normal flow of water in a plumbing system. Backflow becomes a problem only when nonpotable fluids or substances can be drawn back into the potable or drinking water lines from an unprotected cross-connection between a potable and a nonpotable line. The two ways backflow can occur are through backsiphonage or backpressure backflow. [Please excuse the italics now being thrown around in this article with wild abandon.]
An excellent example of a cross-connection relevant to the irrigation industry would be the potable water line that supplies an irrigation sprinkler system, (definitely nonpotable). As for demonstrating backsiphonage and backpressure, let’s use several examples.
Suppose some neighborhood kids are playing with matches and accidentally start a fire at a nearby stable where you board your local prize winning donkey. A crew of enthusiastic fire fighters soon arrive and proceed to hook up fire hoses to every nearby fire hydrant they can find in a valiant effort to save your ass. Water pressure in the main lines consequently goes to zilch or less. At this point, all the fluids in all of the potable and nonpotable lines connected to the main line will be effectively pulled or backsiphoned out of those lines and back into the mainline, unless effective backflow preventative measures have been taken on those lines. Some examples of the potentially hazardous fluids that could be drawn back into the mains would include bacterial contaminated irrigation sprinkler water, heavy metal laden fire sprinkler system water, rust inhibitor chemical laced boiler water and even gold fish contaminated fish pond water. And the odds are that some of these contaminants will remain in the drinking water (and will therefore be consumed by water users), once water pressure is restored to normal.
Once upon a time, in a smallArizonatown which will remain nameless, because I still do some work there, there was a potable water line that was used on occasion as a backup supply line for a golf course lake, whenever the well that normally supplied the lake was out of order. This incoming potable water line was plumbed directly down into the lake itself, unfortunately. One day an overly ambitious golf course employee managed to reverse a pump installed by the golf course people on the incoming potable
water line and then proceeded to pump most of the lake into a one square mile area of the town’s water mains. [That’s my backpressure example- but I can’t resist telling the rest of the story: The water department official in charge of explaining why the town’s water wasn’t suitable for drinking responded by merely saying that the town’s periodic water quality samples “had gone wacky!” And the witless newspaper reporter who interviewed this wacky official promptly missed his once in a lifetime chance for a Pulitzer prize by failing to ask any kind of penetrating follow up questions such as, for example: “Huh?” And the town official therefore never had to explain, that what he really meant was that the mayor was actually humbly beseeching the voters of the town to not drink the water until such time as they could get most of the duck poop out of the lines!]
Backflow Specialists typically distinguish between cross-connections that are High Hazard in nature versus Low Hazard. High hazard situations are ones where backflow could cause sickness or death (death by duck poop, for example). Low hazard situations refer to issues of water quality such as odor or taste only, where no injury due to pollution is possible. A nearby chart categorizes the hazard ratings and some other important characteristics of the various kinds of units.
As mentioned earlier, irrigation sprinkler systems are designated as high hazard cross-connections due to the common presence of bacteria. Further health-endangering contaminants often found in these lines include pesticides, fertilizer and herbicides. Indeed, in chemigation, such chemicals are directly fed into these systems. And this highlights a serious problem in the irrigation industry. The use of a low hazard Double Check assembly (DCA) on irrigation lines, though common in some parts of the country, is a dangerous and inappropriate use of this unit. DCA has no means of preventing backflow should both spring loaded check assemblies in the unit be held open with rocks or debris. In fact, DCAs in an irrigation system application would probably prove legally indefensible, should a backflow incident occur.
Having agued for the exclusion of DCAs from irrigation systems, let’s now examine some more appropriate assemblies, starting with the Atmospheric Vacuum Breaker (AVB) and the Pressure Vacuum Breaker (PVB). The AVB is a high hazard device but is seldom suitable for irrigation systems on commercial properties, since it lacks test cocks and cannot be tested annually by a backflow tester (as is required by most states and local codes now). Both AVBs and PVBs have an air inlet float designed to open the unit to the atmosphere when line pressure is lost. This opening of the valve to the air breaks any vacuum that could allow backsiphonage to occur. The PVB is a decidedly superior assembly over the ABV because: a) it’s a testable assembly; b) its air inlet float is spring-assisted; and c) it has a small spring loaded check assembly, which is a nice little touch. As the chart clearly shows, however, neither assembly can be used where the irrigation system can be backpressured, (that’s because all the important air inlet float assembly is just not going to open, if it’s pressurized from either direction). So here is an important tip: Backpressure on an irrigation system commonly occurs in three principle ways: a) from a pump on the system; b) from gravity, as when part or all of the system is at a higher elevation than that of the Vacuum Breaker itself, (beware hills, raised planter beds, etc.), or c) from the system being supplied by two or more incoming water lines- (in such a looped system, each supply line backpressures all of the other backflow assemblies). If any of your customers have AVB or PVB “protected” existing sprinkler systems with characteristics like these, an immediate conversion of vacuum breakers to RP assemblies, (see below), is needed to safeguard the safety of the drinking water. (Remember the lake water backflow incident described earlier- I didn’t mention it before, but there was a PVB on that line going into the lake. But, like, so what? Its poor little air inlet valve never had a chance to open!)
Back to the chart: The Air Gap method of backflow protection, (to dispose of it quickly since there’s no way for anybody to make any money off of it) is basically a physical gap in the plumbing system, so that there is no actual contact between the potable and nonpotable fluids. This is a great idea in theory, but of only limited actual application, since all line pressure is lost at the Air Gap, and the last thing your customers want added to the job is a pump!
For this reason, the last item on the chart- the Reduced Pressure Principle Assembly (RPA) is the recognized work horse of the industry for both domestic and irrigation lines. The RPA is a testable unit consisting of two spring loaded check valves, (similar to the Double Check Assembly), but also containing a relief valve assembly to vent water whenever it senses either backsiphonage or backpressure. The RPA should be used whenever backpressure backflow is possible.
Of course, like the economist that predicted nine of the last five recessions, RPs tend to vent water for reasons other than just backflow prevention. Rocks or dirt in the first check valve, or rapidly fluctuating line pressures are the most common reasons for such venting of water. But properly installing, testing, repairing and or troubleshooting assemblies would be a great subject for another article. In the immortal words of Julius Caesar as he gazed across the gently flowing waters of the Rubicon- “We’ll just have to cross-connect that river when we come to it!”
Jeff Keim is a Certified Backflow Specialist and President of BPDI. No animals were injured in the writing of this article. Originally published (and heavily censored) in the July 1999 issue of “Landscape & Irrigation” Magazine.