If you are involved in the design, installation or maintenance of irrigation systems, you need to become familiar with backflow preventers and the role they play in keeping our drinking water safe. They are more than just fittings on a system of piping to deliver water. Backflow preventers are installed in the irrigation system to protect our drinking water, save lives and prevent illness. If you understand what they do and why they are needed, you will be able to ensure that the projects you are involved with comply with local codes. A good understanding of backflow preventers will also help to reduce the risk of litigation against you from creating a cross connection and putting public health at risk.
You may think that our water supply is not threatened by water used in irrigation systems. But, as reported in the October 31, 1997, issue of the Los Angeles Times, backflow incidents involving irrigation systems do happen. This article reported, "Drinking water in the system serving two schools and as many as 1,600 homes in Calabasas hillside neighborhoods was contaminated with treated sewage water intended for irrigation after a plumber working on a landscaping job mistakenly crossed two pipes." Several people became ill due to this cross connection and residents were warned not to drink from their faucets or from local water vending machines until the fresh water lines were flushed and tested. The plumber involved was exposed to potential litigation including the cost of the clean-up.
This article will provide you with an introduction to cross connection and backflow prevention to help keep you within the law and outside the courtroom.
The Safe Drinking Water Act (SWDA) of 1974 established a national program that would ensure the quality of America's drinking water. It included a Federal mandate that the local water district provide safe drinking water and authorized the Environmental Protection Agency (EPA) to set standards for contaminants in public water systems.
State and local codes have now been developed to identify the requirements for ensuring that the water distribution system provides safe water. Local codes vary, but most cities have established guidelines on the type of backflow preventers to install to help protect the potable water supply. You should become familiar with your local codes as they relate to irrigation systems.
First, let's review some basic terms. Backflow preventers are specially-designed valves used to protect our potable (drinking) water supply from pollutants or contaminants due to backflow from cross connections.
Backflow can be defined as the unwanted reverse flow of any liquid, solid or gas in a piping system. In an irrigation application, this means that water within the irrigation system may find its way back into the potable water system during a backflow incident.
Cross connections are the link or channel connecting a source of pollution with a potable water supply. Therefore, the irrigation piping system may create a cross connection between the potential contaminants and the potable water supply.
Pollutant is a substance that would affect the color or odor of the water, but not pose a health hazard. Applications where pollutants could enter the potable water supply are considered non-health hazard applications.
Contaminant is a substance that would cause illness or death if ingested, and therefore, is considered a health hazard. For example, water in the irrigation system could come in contact with fertilizers used on the lawn, waste from animals, pesticides used in the surrounding landscape, or muddy water. Contaminated water, which collects around a submerged sprinkler head or chemicals in a chemigation system, could be drawn into the irrigation system. Therefore, most codes consider irrigation systems health hazard applications. The appropriate type of backflow preventer should be installed to ensure these potential contaminants do not find their way back into the potable water supply.
There are two basic forms of backflow: backsiphonage and backpressure. In backsiphonage, the water from the irrigation system is siphoned or "sucked" into the potable water supply. This can be caused by negative pressure or loss of pressure. An example of this might be a hand-held pesticide sprayer connected to a garden hose. If negative pressure occurs while the sprayer is connected to the garden hose, such as a break in the main line water supply, the contents of the pesticide sprayer could be siphoned out of the container. The hazardous chemicals could then be carried down the line into the potable water supply and cause serious illness or death. If the garden hose had a backflow preventer (hose bib type of vacuum breaker in this application) installed at the point of use, the contaminant would be prevented from entering the main water supply.
Backpressure is another common way contaminants enter the potable water supply. This occurs when the pressure in the system is greater than the supply pressure. In an irrigation system this can occur when an auxiliary pump, which is intended to boost pressure in the irrigation system, has greater pressure than the city water supply. Then the booster pump can "push" the potentially contaminated water back into the potable water supply. However, if the proper backflow preventer is installed at the point where the irrigation system and the city water supply meet, our drinking water will be protected.
There are several different types of backflow preventers available on the market today. Some of the new designs have superior flow characteristics, shorter lay lengths, come in multiple body styles and are easier to service. Based on the application, system layout and type of hazard, local codes may vary on which type of backflow preventer should be installed for a particular application. Consult your local codes or authority having jurisdiction to determine which devices are approved in your area for specific applications.
There are several different types of backflow preventers on the market today
Several organizations and agencies have established guidelines for the design, manufacture, and performance of backflow preventers. Some organizations and agencies involved with the approval and/or certification of devices include the Foundation for Cross Connection Control and Hydraulic Research at the University of Southern California, the American Society of Sanitary Engineers, the American Water Works Association and the Canadian Standards Association.
The following is an overview of the types of backflow preventers that are commonly used in irrigation systems.
Atmospheric Vacuum Breaker: The Atmospheric Vacuum Breaker (AVB) is a simple device consisting of a float, which is free to travel on a shaft and seal against an air inlet. The poppet seals the air inlet when the unit is pressurized. When backsiphonage occurs, the poppet drops to allow air to enter the downstream piping and break the siphon. An AVB protects against backsiphonage but not backpressure. There are many restrictions to the installation of the AVB. The AVB must be installed on the discharge side of the last shut-off valve. Code requirements vary as to the height the AVB must be installed, but a minimum of six inches is usually required. The AVB must be installed with the air inlet in a level position. No valve of any type may be installed on the discharge side of an AVB. Because of the installation restrictions, most codes consider the AVB for use in non-health hazard applications. It is not testable but may be serviced in-line if installed with shut-offs to isolate the valve.
Pressure Vacuum Breaker: The Pressure Vacuum Breaker (PVB) consists of a single spring-loaded check valve and a single air relief valve that opens to break the siphon when pressure drops to one psi. Because the assembly has a spring on top of the float, it can
be used under continuous pressure. However, it cannot be used under backpressure applications, since the valve will open under flow in either direction. To prevent backpressure on the valve, it must be installed at least 12 inches above the highest piping or outlet downstream of the assembly. Some local codes allow the installation of the PVB in health-hazard applications. The PVB is testable and serviceable in line.
Double Check: A double check valve incorporates two spring-loaded check assemblies. In a non-flow condition, the check valves hold one psi minimum in the direction of flow. In a flow condition, the check valves open but will close should reverse flow occur. Double checks protect against backsiphonage and backpressure. However, if both checks were to foul at the same time, backflow through the valve could occur. Therefore, a double check is considered a non-health hazard device and may not be approved for irrigation applications under some local codes. The double check is testable and serviceable in-line.
Reduced Pressure Assembly: A Reduced Pressure Assemly (RP) contains two spring-loaded check assemblies with a hydraulically balabnced differential relief valve. Should either of the checks become fouled, the hydraulic balance will be disturbed and the relief valve will open, discharging the potentially contaminated water to the atmosphere. RPs are used in health-hazard applications where backpressure and backsiphonage may occur. They are testable and serviceable in-line.
Other types of backflow prevention assemblies are available, but are not typically used in irrigation systems. In some states you must be a certified plumber to install, service and/or test backflow preventers. Other states allow installation by an irrigation contractor, but must be tested by a certified tester. Consult local codes or the authority having jurisdiction in your area to determine what is required. If you are interested in becoming a certified tester or trained in the maintenance of these valves, contact your local junior college and local plumbers training programs to find out what programs are offered in your area.
The quality of our potable water supply is of ever growing concern. Regulations to protect our water are becoming stronger. It is important to understand that backflow preventers are designed to protect our potable water supply from pollutants and contaminants. To reduce your exposure to liability and potential damage to our potable water supply, you should become aware of the requirements for backflow prevention protection in your area. Local codes identify which device is appropriate for specific installations. u
Editor's note: Terry Nelson is marketing associate of CMB Industries, Inc., manufacturers of Febco backflow preventers.