Backflow can be prevented by an air gap or a backflow prevention device.

There are three types of backflow prevention devices:

• vacuum breaker
• double check valve assembly
• reduced pressure backflow assembly.

Find out about:

• typical applications
• installation requirements
• testing.

Air gap

An air gap is the most effective and least expensive form of backflow protection. Acceptable Solution G12/AS1 requires that an unobstructed air gap must be maintained between the lowest part of the supply pipe inlet and the highest level of the overflow outlet. The air gap separation must be the greater of 25 mm or twice the supply pipe diameter.

Backflow prevention devices

If the system is a high pressure system and a pipe is directly connected to an appliance or sanitary fixture, an air gap may not be able to be used. In this case, a backflow prevention device must be installed.

The appropriate device for a particular installation will depend on the:

• hazard level of any potential contaminant
• potential for cross-connection
• type of backflow expected
• physical limitation of the device and the environment.

Cross connections are rated according to Building Code Acceptable Solution G12/AS1 to three hazard levels:

• High – this has the potential to cause death.
• Medium – this would endanger health.
• Low – this is a nuisance but does not endanger health.

Generally, the higher the hazard, the higher the risk, so the safer the device must be.

Vacuum breaker

A vacuum breaker contains a float disc and an air inlet port. Under normal water flow, the float disc closes off the air inlet port, but if the normal water flow is interrupted, the float drops, closing off the system against backflow and, at the same time, opening the air inlet port.

A variety of vacuum breakers are available:

• An atmospheric vacuum breaker (AVB) is one of the simplest and least expensive backflow prevention devices and can provide excellent protection against backsiphonage. It consists of a gravity plunger or float disc that is forced upwards when the supply pressure is turned on, thus sealing off the atmospheric vent overhead. As soon as the supply is interrupted or terminated, the float drops down and opens the downstream pipework to atmosphere. There must be sufficient pressure to fully lift and seal the float on the vent, so it is not suitable for use on very low pressure systems.

• A hose connection vacuum breaker (HCVB) is a specialised type of atmospheric vacuum breaker designed to attach directly to the hose tap. It has a spring-loaded check valve that seals against an atmospheric outlet when the water supply is turned on. When the supply is turned off, the device vents to atmosphere, thus protecting against backsiphonage conditions. It is non-testable and should not be used as protection against backpressure or be subject to continuous pressure (2 hours maximum is permitted), i.e. no control valves should be located downstream of the device.

• A pressure vacuum breaker (PVB) evolved due to a need to have a vacuum breaker that that can be subject to constant pressure and is able to be tested in line. It is similar to the atmospheric vacuum breaker except that there is a spring to hold the disc float in the open position during normal operation. They have two isolating valves and two cocks for testing, one for each chamber. These devices can be used under constant pressure but do not protect against backpressure. They must be at least 300 mm higher than any downstream piping.

Double check valve assembly (DCVA)

Essentially, this backflow protection device consists of two independently operated check valves within one body. One check valve simply acts as a back-up. Because there is a risk that both checks will fail at the same time, regular testing is imperative, and the device is limited to use in medium and low hazard situations. This valve will protect against backpressure and backsiphonage but is not fail-safe. Because of the spring pressures, there can be a significant reduction in pressure (up to 40 kPa) across this device.

Reduced pressure backflow assembly (RPBA)

This backflow protection device incorporates two independently-acting, spring-loaded check valves separated by a differential pressure relief valve. Pressure between the two valves is lower than the supply pressure during normal operation. If either check valve leaks, the pressure relief valve will open, discharging water out of the system.

This device provides the maximum protection of any valve and can be used in high hazard situations.

Backflow prevention device	Typical applications
Air gap	Taps, sinks.
Vacuum breakers (VB)	Industrial plants, cooling towers, laboratories, laundries, swimming pools, lawn sprinkler systems, fire sprinkler systems.
Double check valve assembly (DCVA)	In-house pumps, elevated tanks, non-toxic boilers.
Reduced pressure backflow assembly (RPBA)	Industrial plants, hospitals, morgues, chemical plants, irrigation systems, pumps, elevated tanks, boilers, fire sprinkler systems.

Installation requirements

All backflow prevention devices require a Building Consent for installation and must be:

• installed as near as practicable to the potential point of contamination
• protected from physical and frost damage
• isolated from corrosive or toxic environments
• installed above surrounding ground level so that leakage from air ports and discharge ports is readily visible
• installed in a position and manner to be accessible for maintenance and testing
• fitted with a line strainer upstream to prevent particles in the pipework from rendering the device ineffective
• attached only after the pipework has been flushed
• installed without the application of heat.

Testing

Backflow prevention devices may be testable or non-testable. Their use in a particular situation depends on the degree of hazard. Non-testable devices may only be used on low-hazard rated systems.

Testable devices must be tested on installation and at regular intervals to the standard set down by Acceptable Solution G12/AS1: 3.7 Testing. Non-testable devices should be checked every 2 years maximum.

More information

• www.watercorporation.com.au/B/backflow.cfm
• cfpub.epa.gov/safewater/watersecurity/guide/productguide.cfm?page=backflowpreventiondevices