Backflow Preventer

A plumbing device installed in the supply line to prevent water from flowing backwards into the water supply when pressure is lost.  This is done by a series of valves that open with a loss of pressure causing a break in the flow thereby preventing a vacuum effect pulling water and potential contaminants back into the supply line.  They are commonly used for irrigation systems and easily recognized above ground and resemble an upside-down "U" shape which includes the supply line coming out of the ground, the backflow unit itself, and the supply line going back into the ground to continue on its route.  These are sometimes removed during cold seasons to prevent freezing and resulting damage.

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