A cross-connection is any temporary or permanent connection between a public water system or consumer's potable (i.e., drinking) water system and any source or system containing nonpotable water or other substances. An example is the piping between a public water system or consumer's potable water system and an auxiliary water system, cooling system, or irrigation system.
Backflow is the undesirable reversal of flow of nonpotable water or other substances through a cross-connection and into the piping of a public water system or consumer's potable water system. There are two types of backflow: backpressure backflow and backsiphonage.
Backpressure backflow is backflow caused by a downstream pressure that is greater than the upstream or supply pressure in a public water system or consumer's potable water system. Backpressure can result from an increase in downstream pressure, a reduction in the potable water supply pressure, or a combination of both. Increases in downstream pressure can be created by pumps, temperature increases in boilers, etc. Reductions in potable water supply pressure occur whenever the amount of water being used exceeds the amount being supplied, such as during water line flushing, fire fighting, or breaks in water mains.
Backsiphonage is backflow caused by a negative pressure (i.e., a vacuum or partial vacuum) in a public water system or consumer's potable water system. The effect is similar to drinking water through a straw. Backsiphonage can occur when there is a stoppage of water supply due to nearby fire fighting, a break in a water main, etc.
Water suppliers usually do not have the authority or capability to repeatedly inspect every consumer's premises for cross-connections and backflow protection. Instead, each water supplier should ensure that a proper backflow preventer is installed and maintained at the water service connection to each system or premises that poses a significant hazard. This generally includes dedicated fire protection or irrigation systems and premises with auxiliary or reclaimed water systems; industrial, medical, laboratory, marine, or other facilities where objectionable substances are handled; premises exempt from the State Plumbing Code; classified or restricted facilities; and tall buildings.
A backflow preventer is a means or mechanism to prevent backflow. The basic means is an air gap, which either eliminates a cross-connection or provides a barrier to backflow. The basic mechanism is a mechanical backflow preventer, which provides a physical barrier to backflow. The principal types are the reduced-pressure principle assembly, the pressure vacuum breaker assembly, and the double check valve assembly. A secondary type is the residential dual check valve.
An air gap is a vertical, physical separation between the end of a water supply outlet and the flood-level rim of a receiving vessel. This separation must be at least twice the diameter of the water supply outlet and never less than one inch. An air gap is considered the maximum protection available against backpressure backflow or backsiphonage but is not always practical and can easily be bypassed.
An RP is a mechanical backflow preventer that consists of two independently acting, spring-loaded check valves with a hydraulically operating, mechanically independent, spring-loaded pressure differential relief valve between the check valves and below the first check valve. It includes shutoff valves at each end and is equipped with test cocks. An RP is effective against backpressure backflow and backsiphonage and may be used to isolate health or nonhealth hazards.
A PVB is a mechanical backflow preventer that consists of an independently acting, spring-loaded check valve and an independently acting, spring-loaded air inlet valve on the discharge side of the check valve. It includes shutoff valves at each end and is equipped with test cocks. A PVB may be used to isolate health or nonhealth hazards but is effective against backsiphonage only.
A DC is a mechanical backflow preventer that consists of two independently acting, spring-loaded check valves. It includes shutoff valves at each end and is equipped with test cocks. A DC is effective against backpressure backflow and backsiphonage but should be used to isolate only nonhealth hazards.
A RDC is similar to a DC in that it consists of two independently acting, spring-loaded check valves. However, it usually does not include shutoff valves, may or may not be equipped with test cocks or ports, and is generally less reliable than a DC. A RDC is effective against backpressure backflow and backsiphonage but should be used to isolate only nonhealth hazards and is intended for use only in water service connections to single-family homes.
Mechanical backflow preventers have internal seals, springs, and moving parts that are subject to fouling, wear, or fatigue. They can also be bypassed. Therefore, all backflow preventers must be tested periodically to ensure they are functioning properly. A visual check of air gaps is sufficient, but mechanical backflow preventers must be tested with properly calibrated gauge equipment.
One excellent reference is the AWWA Manual M14, Recommended Practice for Backflow Prevention and Cross-Connection Control, available from the AWWA Bookstore at 6666 West Quincy Avenue, Denver, Colorado 80235 or at www.awwa.org.
Another excellent reference is the USC Manual of Cross-Connection Control, available from the Foundation for Cross-Connection Control and Hydraulic Research at fccchr.usc.edu.