Backflow is the undesirable reversal of flow of a liquid, gas, or other substance in a potable water distribution piping system as a result of a cross-connection due to back-siphonage or back pressure.
Back-siphonage is the reversal of normal flow in a system caused by the negative pressure (vacuum or partial vacuum) in the supply piping.
Back-siphonage can be created when there is a stoppage of the water supply due to nearby fire-fighting, repairs or breaks in the city water mains, etc. The effect is similar to the sipping of an ice cream soda by inhaling through a straw which induces the flow in the opposite direction.
Back pressure backflow is the reversal of normal flow in a system due to an increase in the downstream pressure above that of the supply pressure.
Back pressure-backflow is created whenever the downstream pressure exceeds the supply pressure which is possible in installations such as heating systems, elevated tanks, and pressure producing systems. An example would be a hot water space-heating boiler operating at 15- 20 lbs pressure coincidental with the reduction of the city supply below such pressure (or higher in commercial boilers). As water tends to flow in the direction of least resistance, a back pressure- backflow condition would be created and the contaminated boiler water would flow back into the public drinking water supply.
A cross connection is a direct arrangement of piping line which allows the potable water supply to be connected to a line which contains a contaminant and/ or pollutant. An example is the common garden hose attached to a sill cock with the end of the hose lying in a cesspool. Other examples are a garden hose attached to a service sink with the end of the hose submerged in a tub full of detergent, supply lines connected to bottom-fed tanks, supply lines to boilers, supply lines to automatic irrigation systems.
The garden hose is the most common form of a cross connection. It can be easily connected to the potable water supply and used for a variety of potentially dangerous applications.
The purpose of a sill cock is to permit easy attachment of a hose for outside watering purposes. However, a garden hose can be extremely hazardous because they are left submerged in swimming pools, lay in elevated locations (above the sill cock) watering shrubs, chemical sprayers are attached to hoses for weed killing, etc.; and hoses are often left laying on the ground which may be contaminated with fertilizer, cesspools, and garden chemicals.
The purpose of a sill cock is to permit easy attachment of a hose for outside watering purposes. However a garden hose can be extremely hazardous because they are left submerged in swimming pools, lay in elevated locations (above the sill cock) watering shrubs, chemical sprayers are attached to hoses for weed killing, etc.; and hoses are often left laying on the ground which may be contaminated with fertilizer, cesspools, and garden chemicals.
A hose bibb vacuum breaker should be installed on every sill cock to isolate garden hose applications thus protecting the potable water supply from contamination.