How to prevent Backflow Contamination Backflow is the reversal of normal flow process when unintended water, liquids and other harmful substances are forced in the reverse direction and into potable water distribution systems. The plumbing and water supply system is devised in a way that water flows out of tap under pressure. However under certain conditions or low pressure due to a main burst, the water can flow in the opposite direction. This is called back siphonage and the reverse flow is referred to as backflow. Water backflow may also occur if a pump outlet is inadvertently connected to the water supply. This is called back pressure. It is very important to prevent backflow as contamination of drinking water can cause serious health problems. Backflow protection should be ensured while installing and/or repairing plumbing systems. The choice of system may be determined by the size of the pipe, its location and the need to have it inspected. Several methods are available to reduce the chance of cross-connection. Some of these include: Air Gap: The easiest way to prevent backflow of water is to install an air gap. The air gaps must be twice the diameter of the pipe and not less than 1 inch. Air gap is an unobstructed, vertical space between the water supply and the flood rim of the receptacle. A physical segregation occurs between the potable water system and the possible source of contamination. Air gaps should be used on direct and inlet connections. They provide protection when toxic material is apparent in water. Atmospheric Vacuum Breaker: Atmospheric vacuum breakers are the simplest and least expensive back-siphonage preventers. However, they do not protect against backflow backpressure situations. Atmospheric-type vacuum breakers are similar to pressure-type vacuum breakers but are not spring-loaded and the vented chamber remains open. Atmospheric-type vacuum breakers can be installed only where the contained water is at atmospheric pressure. Reduced Pressure Principle Backflow Preventer: This system includes a check valve in the inlet and outlet with a vented chamber in the middle. The check valves remain in a closed position via a spring-loaded pin. A test port is also available with this system to determine the operating conditions for the preventer. Double Check Valve Assembly: A double-check valve assembly is a single device with two check valves in a series flow alignment. The valves are spring-loaded and remain in a closed state. The valves need approximately a pound of pressure to open. The spring loading allows small debris to be captured, yet the valve will seal. Test apparatuses are included in the system. Double-check valve assemblies may not be permitted in all areas. These are used for non- hazard risk installations. The valve can be used where the contaminated material is gas, steam, food, air or any substance that is offensive but not hazardous. They can also be tested independently, in place, to determine if they are functioning or clogged. Pressure Vacuum Breaker: Pressure-type vacuum breakers are spring-loaded check devices installed in the flow path with a chamber vented to the atmosphere. The unit has to be installed 12 inches above the usage point. The valve remains open and the vented chamber is closed during normal operation. When a vacuum occurs, the check closes and the vented chamber opens. If a check valve leakage occurs, only air flows into the potable water line. A pressure-type vacuum breaker requires installation at the highest part of the fixture or system being isolated. Barometric Loop: When a continuous section of pipe is installed 35 feet high and returns back to the originating level, a barometric loop is created. This section of piping prevents back-siphonage from occurring. At sea level, a column of water can only rise 33.9 feet. Barometric loops cannot protect against back-pressure. They are typically not an option in normal installations. It should be remembered that cross connection control devices should not be installed in pits or submerged locations. They must be readily available for testing and maintenance. For more information on the prevention of backflow contamination contact Mr. Mike's Plumbing for assistance and advice.
The water supply system must be designed and used to prevent contamination from backflow. On this page:
Backflow is the unplanned reversal of flow of water (or water and contaminants) into the water supply system. For backflow to occur, there must be a physical connection, or cross-connection, between the water supply and any delivered water or contaminant. A common situation is the end of a garden hose submerged in a bucket or other container of liquid. Backflow can also arise from appliances, pools, and water storage tanks such as header tanks and rainwater tanks. As well as using the methods described below to minimise the risk of backflow, advise building owners to take simple precautions such as not submerging garden hoses or spray heads from showers and sinks, and always turning off the water supply at the tap when it is not being used. Causes of backflowBackflow is caused by a difference in pressure and may occur due to:
Backflow can only occur where there is a connection or cross-connection. Cross-connections can occur in any situation where fixtures are connected directly to the main supply such as:
Code requirementsBuilding Code Clause G12 Water supplies requires that potable water supply must be protected from contamination and installed in a manner that avoids the likelihood of contamination within the system. Acceptable Solution G12/AS1 requires backflow prevention to be provided where it is possible for water or contaminants to backflow into a piped potable water supply. Backflow can be prevented either through an air gap or a backflow prevention device (see below). The Acceptable Solution also provides that there must be no likelihood of a cross-connection between a private water supply (such as a rainwater tank) and mains water supply. Responsibility for preventing backflow may rest with:
Using an air gap to prevent backflowIn most situations, an air gap is the most cost-effective and reliable form of backflow prevention. An air gap should be used to prevent backflow from rainwater tanks and other water supply tanks into the mains-supplied water system. Air gaps should also be used to prevent backflow of contaminants from all appliances and fixtures that are connected to the water supply. For swimming and spa pools, provide a dedicated water supply with an approved air gap. Acceptable Solution G12/AS1 requires that the air gap must be the greater of 25 mm, or twice the diameter of the supply pipe.  Principle of air gap used for backflow prevention If mains supply is used to top up a private water supply, backflow can be prevented by using a floating weight can be used to operate a valve, ensuring that the maximum water level always remains at least 25 mm below the mains inlet. Alternatively, a double non-return valve can be used. Mains water supply top-up valve and air gap backflow prevention If a piped supply is used to top-up the rainwater storage tank, a simple commercially available floating switch will ensure that top-up water is added only when the level in the tank is low. A float valve should not be used because it will add piped water whenever there is any draw-off. If the system is a high pressure system and a pipe is directly connected to an appliance or sanitary fixture, it may not be possible to use an air gap. In this case, a backflow prevention device must be installed. The appropriate device for a particular installation will depend on the:
Cross-connections are rated according to Building Code Acceptable Solution G12/AS1 to three hazard levels:
Generally, the higher the hazard, the higher the risk, so the safer the device must be. Vacuum breakerA 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: Atmospheric vacuum breaker 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. Hose connection vacuum breaker 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. Pressure vacuum breaker 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 non-return valve assembly Double non-return valve assembly Essentially, a double non-return valve assembly (also known as a double check valve assembly or DCVA) consists of two independently operated non-return valves within one body. One non-return valve simply acts as a back-up. Because there is a risk that both valves 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 zone device Reduced pressure zone device 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.
All backflow prevention devices require a building consent for installation and must be:
TestingBackflow 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. Updated: 12 September 2016 |
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