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A backflow prevention device is used to protect potable water es from contamination or pollution due to back flow. In water supply systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower etc. When pressure fails or is reduced, as may happen if a water main bursts, pipes freeze or there is unexpectedly high demand on the water system, then such reduced pressure in the pipe may allow contaminated water from the ground, from storage or from other sources to be drawn up into the system.
A double check valve or double check assembly (DCA) is a backflow prevention device designed to protect water supplies from contamination. It is also a valve used in air brake systems on heavy trucks. It consists of two check valve assembled in series. This employs two operating principles: firstly one check valve will still act, even if the other is jammed wide open. Secondly the closure of one valve reduces the pressure differential across the other, allowing a more reliable seal and avoiding even minor leakage.
Technicly they're both do the samething which is to prevent water going back to the City water line. Back flow works like a boarder custom checks once you closed it, it will not alow any water to go through. If you compare it to the double check valve it alows some water to back in the system while you're closing the valve.
