August 4, 2009
Some of the most significant advances in water safety are due to the many mechanical devices that prevent “backflow”. Backflow exists when water is siphoned back into the water supply due to negative water pressure.
How does siphonage occur in my water system?
Siphonage occurs for many reasons. To understand siphonage, one must understand the basic principles of water pressure. The basic principle we will focus upon is as follows:
Water runs down hill or, pressure seeks the path of least resistance.
In simple plumbing systems, water pressure is created by water towers. Water towers hold thousands of gallons of water and discharge water when any demand for water is placed upon them. Municipal water systems consist of a vast grid of pipes that all share a common water source. It is one giant system connecting your home to every other home, place of business, swimming pool, or any other place where water is supplied by that municipality.
The height of the tower and the weight of the water within creates water pressure. Below are several examples:
Example 1: A break in the municipal water main will affect the pressure to your home. Due to gravity, any water upstream from that break will empty or flow backwards to the point at which the break exists. As the water flows backwards, a vacuum forms and physically draws the water creating a siphon.
Example 2: A municipality experiences several fires simultaneously throughout the city and dispatches multiple fire trucks. A single fire hydrant turned on to maximum capacity will negatively affect water pressure. Depending upon the demand, it is possible to sustain negative pressure to those parts of the system that are disproportionate to the demand.
Why is back flow siphonage dangerous?
The best way to explain the dangers of back siphonage are to give some genuine examples that have led to sickness or death.
Example 1: A homeowner was working in his garden. He had spread fertilizer in his flower bed and was watering some plants. He dropped his running hose onto the ground of the flower bed to answer a phone call. Upon his return, he took a sip of the water.
Several blocks away, a fire was extinguished by a fire truck. While he was away, negative pressure caused the fertilizer to be siphoned into the water supply. Upon his return, the water was flowing normally. Unfortunately for him, he was poisoned by the fertilizer.
Example 2: A lady was washing her car at her home and left the garden hose submerged in the wash bucket. A water main break several miles away caused the soapy and contaminated water to siphon into the water supply. She and her family became very ill from the contaminated water.
Example 3: A gentleman was changing the coolant in his truck. He drained the coolant from his radiator into a bucket. As in example 2, he left his hose submerged in the bucket. Due to an unusual event of circumstances, the coolant was siphoned into the water supply. Several people to include some of his neighbors were lethally poisoned.
There are many documented situations where backflow siphonage has caused major illness and death. What can be done to prevent backflow siphonage?
Although education is the best way for prevention, plumbing codes have become very stringent to prevent cross contamination and backflow siphonage. Technological advances in the plumbing industry have yielded mechanical devices that when installed and maintained properly, have significantly decreased illnesses sustained from backflow.
Although vacuum breakers exist in many different shapes and sizes, the most common example can be seen on outside hose bibs (outdoor faucets). The vacuum breaker simply assures that water cannot flow backward into the system beyond the breaker itself. Inside a vacuum breaker is a poppet valve that is held up by the water pressure within the system which prevents air to enter the device. When the pressure drops, the poppet valve drops and allows air to enter the system which breaks the siphon.
Backflow preventers also come in a variety of shapes and sizes and are used in many different applications. The most common application is found in residential irrigation systems. In this application, the backflow preventer is used to assure that contaminated water that may gather around sprinkler heads isn’t back siphoned into the potable water supply.
Many municipalities require that backflow prevention devices be installed just beyond the water meter. This requirement helps protect the municipal water source as a whole. The backflow preventer installed at the water meter doesn’t protect the homeowner, but rather all of his neighbors. When everyone has a backflow preventer, it is comforting to know that our chances of becoming ill due to the actions of our neighbors are greatly reduced.
Most manufacturers specify that the device be tested annually by a properly trained and licensed tester. Please see Backflow Testing in our Services section.
Note: When a backflow preventer is installed between the meter and the main water supply of the dwelling, the system becomes a “closed” system. Please see the Thermal Expansion section for more information.