Backflow can be described as “a reversal of the normal direction of flow within a piping system” or as “the flow of water or other liquids, mixtures or substances into the distribution pipes of a potable water supply from any source other than the intended source of the potable water supply.”
The potential for a backflow condition occurring in a water system is all too likely in many of our homes, factories and public buildings. The existence of improper plumbing connections presents cross-connections that may, under backflow conditions, permit the water to flow the wrong way within the piping.
The probability of backflow taking place at any given outlet may actually be very small. But in view of a large number of service connections, the multiple cross-connections at each service connection and the potential for cross-connections to be created, then the probability becomes very significant and must be dealt with in a positive manner.
Backsiphonage can be described as “a reversal of the normal direction of flow in a piping system due to a drop in the supply pressure to the point where a vacuum, partial vacuum or negative pressure occurs in the upstream piping”.
Backsiphonage can be caused in the water user’s system by insufficient internal piping hydraulic capacity, by a drop in pressure in the user’s upstream piping, or by a drop in pressure in the public water system. Generally, backsiphonage occurs more frequently in the water user’s system inside the building and on the upper floors, than to the public water system since the volume or duration of a backsiphonage condition is usually not of sufficient quantity or duration to reach the public water system.
Backsiphonage can be caused
In the public water system by a water main break
By a break in the consumer’s piping
If the water is turned off for maintenance or repair
If a fire hydrant is struck
If the fire department is drawing water to fight a fire
By any abnormally heavy water use from the water main
Backpressure can be described as “a reversal of the normal direction of flow in a piping system due to a downstream pressure that is greater than the normal supply pressure”. When the pressure is greater in a water user’s water system than the pressure in the public water system, then the water will reverse its normal direction of flow and move towards the public water system.
Backpressure can be created by:
Internal pumping systems
Any other system that can create a water pressure that is greater than the normal supply pressure
So if a fixture, such as a boiler, creates a pressure greater than the supply pressure then there will be backflow unless the appropriate backflow prevention device is installed. Pumps on secondary or auxiliary water system installations are a primary cause of backpressure and can be found at a variety of premises.
A typical backflow situation involving pumps is illustrated by visualizing a pump supplying well water to a plumbing system that is also connected to the public water supply. If the pump is capable of producing a higher pressure than the public water system or if the public water system pressure should drop, then the pump can discharge its water through the internal plumbing system into the public water main.
A cross-connection can be described as “any arrangement whereby backflow can occur”, or as “any arrangement of pipes, fittings, fixtures, or devices that connects a non-potable water system to a potable water system”. It is the point at which a water-using fixture is connected to the water user’s potable water system.
An Unprotected Cross-Connection
An unprotected cross-connection provides the path or route through which backflow can occur, and it can be:
Actual or potential
Direct (subject to backpressure and backsiphonage) such as a boiler or indirect (subject to backsiphonage only) such as a toilet tank fill valve
Permanent or temporary
Boiler make-up lines
Chiller make-up lines
Commercial grade dishwashers
Commercial grade garbage disposals or grinders
Fire protection systems
Hand-held lawn sprayers
Hose bibs with a hose attached
A janitor’s sink with a hose attached
Soap, sanitizer or wax induction systems
Suction tees or aspirators
Tanks or vats with a submerged inlet
X-rays and photo developing equipment