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You are here: Home > Sprinkler School™ > Parts and Components > Backflow Devices > Backflow Causes and Preventative Measures
Atmospheric Vacuum Breaker(AVB) Or Anti-Siphon Valves For Irrigation Systems

Backflow: Causes and Preventative Measures

Stopping contamination

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Atmospheric Vacuum Breakers: Installation


For a neighborhood in Cedar Hills, Utah the residents got more contamination than they ever thought was possible. Thirteen households were affected last September when it was discovered that their water was making them sick. It turned out that these thirteen homes had all been recently switched to the city’s water supply after the former water company went bankrupt. The city was notified of the suspicious yellow looking water and they soon discovered an unknown pipe was allowing in secondary water. This pipe however, was never identified in the construction plan and was never brought to the city’s attention when the switch was made.

When relying on a water company to provide you with clean water, most residents assume this is easily accomplished. Could you imagine finding out you did not just have the usual flu that was running around, but had been poisoned by the very water you pay for? This would most likely be a case of backflow, which simply put is when water in an irrigation system, instead of flowing out, flows back in, carrying with it contamination from whatever was around the output source. This doesn’t have to be sewage; it could also be worms, debris, etc. Backflow devices keep instances like this from happening. When it comes to irrigation many toxins could be getting into the system; animal waste, pesticides, and fertilizer are all common examples of why backflow devices are required.

Major Offenders


The two major offenders of backflow are backsiphonage and backpressure. During this course we will familiarize you with these two major causes. Backpressure materializes when a system’s pressure is superior to that of the city’s supply pressure. Typically occurring because of changes in piping elevation caused by pumps, or thermal expansion caused by a water heater, backpressure can be easily avoided by following a few simple steps.

A water heater’s thermal expansion is the most frequent cause of backpressure. The water heater is directly connected to the cold water line, and as we all know when something heats it also expands. If the pressure is even a small amount greater than the city’s supply pressure, you will have backpressure.

Backsiphonage is caused by a below average atmospheric pressure inside of a water system. Simply put, backflow happens when the pressure in a city’s water main becomes negative. Almost like the water supply is being sucked out of your house, backsiphonage is cause by high water withdrawal rates. One example of backsiphonage is if a construction crew is working and directly hits the city’s water main. The hose that is attached and submerged in toxic water at a local home will want to suck up any liquid causing the water to move to a lower pressure area, thus poisoning the city’s water.

Different Backflow Devices

Depending on the circumstances, installing a backflow device will require knowing whether the situation is considered a high-hazard or a low-hazard situation. High-hazard means that any person consuming that fluid could be poisoned. Irrigation is always considered a high hazard because of the chemicals and wastes on the ground. Low-hazard would be a liquid that has a strange taste, color or smell. Low-hazard is drinkable but will not be pleasant to the senses. Because not all backflow devices can handle both high and low-hazard situations, four different kinds of mechanisms are available. AVBs (atmospheric vacuum breakers), PVBs (pressure vacuum breakers), DCs (double check valves), and RPS (reduced pressure principle assemblies) are the four main backflow prevention devices.

Both AVBs and PVBs prevent only backflow due to backsiphonage. AVBs, however, have the capability to bare the pressure of 12 hours out of a 24 hour period and do not have a shutoff valve downstream to them. AVBs are also much more economically priced than PVBs. PVBs can withstand a much higher pressure at 24 hours a day, 365 days a years and do allow downstream shutoff of valves. Both breakers do literally the same thing, but PVBs do not have the limitation of time. If you are wondering why you would even bother putting in an AVB, a great example would be if you were installing a small irrigation system composed of let’s say two zones. This would be a case where it would be much cheaper to use AVBs. However, installing more than five zones might warrant PVBs since you would only need to install one, not five or more AVBs.

The next style of backflow devices to consider is DCs and RPs. DCs protect against backpressure or backsiphonage, but are designed for low-hazard applications. RP also protects against both backpressure and backsiphonage, but are designed for high-hazard applications. RPs are essentially an upgrade from DCs in that both have two spring-loaded check valves, except RPs have a relief valve between the two check valves.

When it comes down to it, inquiring from your local AHJ (Authority Having Jurisdiction) about mandates in the area will be the best advice you can get. They will inform you on which type of device to use for any project you might have, as well as where to place it. Always check with the local authorities before installing any type of backflow device.

Common Mistakes

Common mistakes have been seen by many of the most skilled professionals. Backflow devices have a tendency to be overlooked when installed; some customers will put them in without considering elevation, or even not reading the instructions properly and installing them backwards. Owner of Sprinkler Warehouse, Steve Okelberry suggested inquiring to the local AHJ before installing since they will have a recommendation as to where to place your backflow.

Since the AHJ needs to inspect your backflow device annually, it is important that the AHJ have easy access. Having it placed ten feet off the ground because you need to compensate for elevation is not going to work when adjustments or repairs need to be made.

Backflow Device Winterization

Winterization in very cold areas is probably the most important thing to remember when installing backflow devices. In cold climates it is essential to remember what you should incorporate into your pipes; plastic is a great alternative for cold weather. This will prevent damage to your backflow device and shouldn’t cause any breakage, if something does break it will be easier to fix. Another alternative would be to add quick-releases into a backflow device, that way you can unscrew them right there, pull the assembly out, and take it somewhere warm. Always remembering during blowouts to put the air compressor hose in after the backflow device is essential as well. Locking up the springs can cause failure and cross-contamination.

It is important to note that recently backflow devices have been the target of thefts, stealing the copper fittings off of backflow preventers. This has been taking place all over the nation, both residentially and commercially. When pieces are stolen it can cause backflow to occur, exposing people to contamination. Engineering cages are available to prevent anyone from stealing fittings. Don’t become a victim of theft or backflow by securing your device, properly installing the products and having all questions answered by your local AHJ.

You must also remember that if you are ever unsure of a procedure or installation step, always ask a professional, and re-read the instructions. Our irrigation school is here to help familiarize you with different kinds of backflow devices and teach you how to manage an at home system.

 
   
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