Editing Backflow prevention device (section)

==Terminology==
'''Backflow''' means the undesirable reversal of flow of a liquid, gas, or suspended solid into the potable water supply; a '''backflow preventer''' is designed to keep this from happening. Points at which a potable water system connects with a non-potable water system are called '''cross connections'''. Such connections occur naturally in appliances such as [[clothes washer]]s and [[dishwasher]]s, but they must be carefully designed and installed to prevent backflow. Another common location for a backflow preventer is the connection of a [[fire sprinkler]] system to a water main, to prevent pressurized water from flowing from the fire suppression system into the public water supply.

'''Back-siphonage''' occurs when higher pressure fluids, gases, or suspended solids move to an area of lower pressure fluids. For example, when a [[drinking straw]] is used to consume a beverage, suction reduces the pressure of fluid inside the straw, causing liquid to move from the cup to inside the straw and then into the drinker's mouth. A significant drop of pressure in a water delivery system creates a similar suction, ''pulling'' possibly undesirable material into the system.  This is an example of an '''indirect cross-connection'''.

'''Back-pressure''' occurs for example when air is blown through the straw and bubbles begin to erupt at the submerged end. If, instead of air, [[natural gas]] had been forced into a potable water tank, the gas in turn could be carried to a kitchen faucet. This is an example of a '''direct cross-connection''', with undesirable material being ''pushed'' into the system.

Back pressure can force an undesirable contaminant to enter potable water piping. Sources of back pressure may be boilers, heat exchanging equipment, power washing equipment, fire sprinklers, or pumps in the water distribution system. In some cases there may be an almost continuous risk of overcoming the static water pressure in the piping.  To reduce the risk of contamination, a backflow preventer can be fitted.  A backflow preventer is also important when potentially toxic chemicals are used, for instance for commercial/industrial [[descaling]] of boilers, or when chemical [[bleach]]es are used for residential power washing.

A closely related device is the '''[[backwater valve]]''', which is designed to prevent [[sewage]] from backing up into a building and causing basement flooding.<ref name=Backwater>{{cite web|title=Backwater Valve Installation|url=http://www.absolutedp.com/backwater-valve-installation|publisher=Absolute Draining & Plumbing|accessdate=9 October 2014}}</ref><!-- NOTE: The forgoing ref is far from ideal, but until something better is found, it does contain useful information -->