Backflow and Cross Connections - What they are and how they happen
- Sections
Backflow, what is it?
Backflow is a flow of solid, liquid or gas from any source opposite to the normal direction of flow, back into the potable water supply/system.
There are 2 types of backflow, backsiphonage and backpressure.
Backsiphonage is caused by negative pressure in the supply piping. Some common causes of backsiphonage are:
- High velocity in pipelines
- Line repair or a break that is lower than a service point
- Lowered main pressure due to high water withdrawal rate such as fire fighting or water main flushing
- Reduced supply pressure on the suction side of a booster pump
Backpressure is caused whenever a potable system is connected to a non potable supply operating under a higher pressure by means of a pump, boiler, etc.There is a high risk the non potable water may be forced into the potable system whenever these interconnections are not properly protected
Below is an example of how water typically flows under normal conditions.
Below are examples of Backsiphonage and Backpressure
In this first picture a fire hydrant is opened, allowing water out of the water mains at a high rate of flow reducing the pressure in the mains to a point that there will be a reversal of flow elsewhere in the system. If a connection to a contaminated source exists, backsiphonage will occur from this source, and contaminate the system
This illustration shows that even in a residential application Backsiphonage can occur. Here we have a common garden chemical sprayer connected to a garden hose. If the faucet in the house is turned on it can create a venturi effect at the tee where the hose pipe and the faucet pipe meet and pull the water and chemicals from the garden hose.
In this illustration we have a boiler that is used in both residential and commercial applications. A boiler system is connected to the potable water source, as water in the system evaporates and must be replaced. If the boiler system pressure exceeds the potable water system, the contaminated water from the boiler system will flow into the potable water.
This illustration shows a typical irrigation system with fertilizer injection system commonly used on golf courses. If the pump creates higher pressure than the water main pressure or if the water main is shut off or breaks, then the chemicals can flow into the potable water system.
This illustration shows a similar example to the above situation but in a chemical processing plant. This shows a chemical injection pump feeding the system at higher pressures than the incoming potable water forcing chemical back into the potable system
