Back-flow preventers in irrigation - what are they?

One of the major responsibilities of a water supplier is to ensure the safety of those relying on their supply.  This includes keeping the water safe for human consumption, and that includes keeping it contaminant-free.  When a water supplier like a municipality or private supplier sends water outward, it's under very high pressure so that it can reach all the required destinations AND while many users may be opening flows simultaneously.  Each home or office that a water supply line services will have what's called a pressure-reducing valve that drops the pressure to a safe level for the plumbing and fixtures of that building.  Because there's always positive pressure on the water, there's little chance that contaminants are going to flow backwards into the water supply pipes and into the supplier's water supply.  But... what if all of a sudden there was a loss of pressure?  Maybe a water main broke, or in some cases supply pumps ceased operation?  The pressure could drop and allow backward flow of water which could bring in contaminants into the water supply.

Back flow preventers

Although there are other examples, today we'll take the one of irrigation systems for lawns.  In normal operation, these push water out on to lawns... lawns that may have contaminants such as chemical fertilizers, but it could be anything.  An irrigation head may be sitting in a pool of naturally tainted water as well.  If during operation, the water supplier lost pressure, it could be possible that water near an irrigation head would flow backwards (and even be pulled backward by vacuum) and into the water supply line that supplies everyone water for consumption.  In comes the purpose of the back-flow preventer.  The back-flow preventer contains diaphragms and a series of valves that allow water to flow through it when there is positive pressure from the side of the water supply line.  When this pressure fails to exist, the back-flow preventer opens valves that cause any water pulled backwards to be discharged rather than sent backwards into the water supply line.  It's a safety that keeps contaminants out of the water supply lines.

Back-flow preventers are usually required to be inspected on a regular schedule to ensure the safety of the water supply.  It's often done by the water supplier or a third party they hire, and property owners are usually billed for this service.  You probably see back-flow preventers every day.  They're above ground and have one pipe on each end going into the ground.  They come in different sizes depending on which purpose they are serving.  Irrigation systems use some of the smaller versions, but you'll see industrial sized units for commercial applications.

Posted by Shaun Larson on


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