Back-siphonage backflow Is the reversal of normal flow in a system caused by negative pressure (a vacuum or partial vacuum) in the supply piping. 

Back-siphonage backflow can be created when a stoppage occurs in the water supply. This may happen in a public water system because of nearby fire fighting, repairs or breaks in the main water supply line. The effect is similar to the sipping of an ice cream soda through a straw. The liquid is drawn into the low pressure (suction) area. 

Back-pressure backflow is the reversal of normal flow in a system because downstream pressure is greater than the supply pressure. 

Back-pressure backflow occurs when the higher pressure in the user system pushes undesirable substances back into the water supply system. Causes can be booster pumps, the backflow from an elevated irrigation system back to the well head when the pump is shut down, or the siphonage from a spray tank back through the fill hose to the supply line when the water supply pressure is lost or the pump is turned off. 

A cross-connection is a direct or indirect arrangement of piping that allows the potable water supply to be connected to piping that contains a contaminant. An example is the common garden hose attached to a water spigot with the end of the hose lying in a puddle of water. Other examples are a hose attached to a service sink with the end of the hose submerged in a tub full of detergent, supply lines connected to bottom-fed tanks (such as pesticide spray tanks or fermentation tanks) and supply lines to boilers. 

The ordinary garden hose is the most common offender. It can easily be connected to potable water supplies and used for a variety of potentially dangerous applications, including mixing, diluting, and spraying pesticides and fertilizers.

Common devices are: 

1. Air gap. 
2. Atmospheric vacuum breakers (AVB), including hose connection vacuum breakers. 
3. Pressure-type vacuum breakers (PVB). 
4. Double-check valves (various arrangements). 

Reduced pressure zone assembly.

An air gap is the physical separation of the potable and non- potable water supply systems by an air space. This is usually the easiest and least expensive backflow prevention method. The vertical distance between the supply pipe and the flood rim should be two times the diameter of the supply pipe, but never less than 1 inch. The air gap can be used on a direct pipe connection or a tank inlet connection, and for all toxic substances. 

The most commonly used atmospheric -type vacuum breaker incorporates an atmospheric vent in combination with a check valve. Its operation depends on the water supply pressure to open the check valve and seal off the atmospheric vent. This permits the water to flow to downstream equipment. A loss of pressure in the supply line permits the check valve to drop and seals the orifice while, at the same time, the vent opens, admitting air into the system to break the vacuum. 

An atmospheric vacuum breaker cannot be subject to continuous pressure. That is, there can be no shut-off valves downstream of its location. Nor can it be subjected to more than 12 hours of continuous pressure. 

Both AVBs and PVBs must be properly elevated and sized for the application. An AVB must be 6 inches above the highest use point; a PVB, 12 inches above the highest use point (per state plumbing code). 

Atmospheric-type vacuum breakers can be used on most inlet- type water connections that are not subject to back-pressure, such as low inlet feeds to receptacles containing toxic and non-toxic substances, valve outlets or fixtures with hose attachments, single- zone lawn sprinkler systems and commercial dishwashers. 

Hose bibb vacuum breakers are small, inexpensive devices with hose connections that simply attach to water spigots, threaded faucets or other fixtures where a hose could be attached that could be introduced to a contaminant. Like the atmospheric-type vacuum breaker, they should not be used under continuous pressure (i.e., with a downstream valve) nor be subject to back-pressure backflow conditions.

A pressure-type vacuum breaker (PVB) is a backflow prevention device designed to operate under continuous supply pressure, so a shut-off valve may be located downstream of the device.

"Continuous" is defined as 12 hours or longer. A device not designed for continuous supply pressure may "freeze" in one position and become inoperative. Like the AVB, the PVB cannot be subject to back-pressure backflow conditions.

These devices are also pressure-type vacuum breakers, but they are designed with an intermediate atmospheric vent to ensure a break in the vacuum. They are intended for low health hazard crossconnections and are made for 1/2-inch and 3/4- inch lines. They are used on cattle drinking fountains, trailer park water 

A double-check backflow prevention assembly consists of two independently operating check valves with inlet and outlet shut-off valves and four test cocks. It is designed to prevent reverse flow in water lines. It is used in a variety of low health hazard situations. It will prevent backpressure backflow, will function under continuous pressure and is available in sizes from 1/2 inch to 10 inches. Typical uses are main supply lines, food cookers, lawn sprinklers and fire sprinklers.

This device is an assembly of two independently operating check valves with an intermediate hydraulically dependent differential relief valve. These units are intended to protect the potable water supply from reverse (back) flow when subjected to emergency conditions of combined back-siphonage and backpressure, even if both check valves fail. conditions under continuous pressure. RPZ units have shut-off valves on the inlet and outlet pipes and test cocks. They are available in sizes from 1/2 inch to 10 inches. 

Typical uses are in main supply lines, boilers, cooling towers, sewage treatment systems, and pesticide loading and handling facilities such as chemical feed lines to submerged inlet tanks.

How It Operates:
The reduced-pressure zone assembly automatically reduces the pressure in the zone between the two check valves (I & 2) to at least 5 psi below the inlet pressure. The diaphragm spring on the relief valve (3) is set so that if a pressure differential of 2 psi or less occurs across the diaphragm (between inlet and zone pressure), the relief valve will open and discharge zone water to the drain. If either or both check valves fail, the differential relief valve will open, letting water from the zone out to the drain, preventing backflow from back-pressure or back- siphonage.