Backflow Preventers Frequently Asked Questions
- What is backsiphonage?
- What factors can cause backsiphonage?
- What is pressure backflow?
- What factors can cause a backpressure backflow condition?
- What is a cross-connection?
- What is the most common form of a cross-connection?
- What is an atmospheric vacuum breaker?
- Will an anti-siphon vacuum breaker protect against a backpressure backflow condition?
- What is continuous pressure?
- Can an atmospheric vacuum breaker be used under continuous pressure?
- Where is an atmospheric vacuum breaker used?
- Where is a hose bibb vacuum breaker used?
- Where is a pressure vacuum breaker used?
backsiphonage is the reversal of normal flow in a system caused by a negative pressure (vacuum or partial vacuum) in the supply piping.
backsiphonage can be created when there is a stoppage of the water supply due to nearby fire-fighting, repairs or breaks in city main, etc. The effect is similar to the sipping of an ice cream soda by inhaling through a straw, which induces a flow in the opposite direction.
Backpressure backflow is the reversal of normal flow in a system due to an increase in the downstream pressure above that of the supply pressure.
Backpressure backflow is created whenever the downstream pressure exceeds the supply pressure which is possible in installations such as heating systems, elevated tanks, and pressure producing systems. An example would be a hot water space-heating boiler operating under 15-20 lbs. pressure coincidental with a reduction of the city water supply below such pressure (or higher in most commercial boilers). As water tends to flow in the direction of least resistance, a backpressure backflow condition would be created and the contaminated boiler water would flow into the potable water supply.
A cross-connection is a direct arrangement of a piping line which allows the potable water supply to be connected to a line which contains a contaminant. An example is the common garden hose attached to a sill cock with the end of the hose lying in a cesspool. Other examples are a garden 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, and supply lines to boilers.
Ironically, the ordinary garden hose is the most common offender as it can be easily connected to the potable water supply and used for a variety of potentially dangerous applications.
The most commonly used atmospheric anti-siphon vacuum breakers incorporate an atmospheric vent in combination with a check valve. Its operation depends on a supply of potable water to seal off the atmospheric vent, admitting the water to downstream equipment. If a negative pressure develops in the supply line, the loss of pressure permits the check valve to drop sealing the orifice while at the same time the vent opens admitting air to the system to break the vacuum.
Absolutely not! If there is an increase in the downstream pressure over that of the supply pressure, the check valve would tend to "modulate" thus permitting the backflow of contaminated water to pass through the orifice into the potable water supply line.
This term is applied to an installation in which the pressure is being supplied continuously to a backflow preventer for periods of over 12 hours at a time. Laboratory faucet equipment, for example, is entirely suitable for a non-pressure, atmospheric anti-siphon vacuum breaker because the supply is periodically being turned on and shut off. A vacuum breaker should never be subjected to continuous pressure unless it is of the continuous pressure type and clearly identified for this service.
No! Codes do not permit this as the device could become "frozen", and not function under an emergency condition.
Atmospheric vacuum breakers may be used only on connections to a non-potable system where the vacuum breaker is never subjected to backpressure and is installed on the discharge side of the last control valve. It must be installed above the usage point. It cannot be used under continuous pressure. (Also see #7)
Hose bibb vacuum breakers are small inexpensive devices with hose connections which are simply attached to sill cocks and threaded faucets or wherever there is a possibility of a hose being attached which could be introduced to a contaminant. However, like the atmospheric vacuum breaker they should not be used under continuous pressure.
Pressure vacuum breakers may be used as protection for connections to all types of non-potable systems where the vacuum breakers are not subject to backpressure. these units may be used under continuous supply pressure. They must be installed above the usage point. (Spill resistant models for indoor use are also available.)