A backflow preventer is designed to keep your water clean. It keeps substances such as liquid, gas, or suspended solids from being put back into your potable water supply, better known as your drinking water. Cross connections are points in which your potable water system connects to the non-potable water system, and where backflow prevention occurs. Cross connections occur naturally in your household appliances like your washing machine, or dishwasher and are carefully designed to prevent any backflow.
Causes of Backflow
Backflow occurs for one of two reasons. One of those reasons is called back pressure. Back pressure is the result of the pressure in the system being higher than the pressure of the supply. For example, if the system pressure has been increased. This can occur where thermal expansion increases the pressure. Another cause of backflow is called back siphonage. Back siphonage is the result of supply pressure being below the pressure of the system. This can occur when a supply is interrupted, or drained down.
Back-siphonage occurs when higher pressure fluids, gases, or suspended solids move to an area of lower pressure fluids. For example, think of drinking from a straw. You use it to consume a beverage by suction, this suction reduces the pressure of fluid inside the straw, causing liquid to move from the cup to inside the straw, and then into the drinker’s mouth. An example of indirect cross-connection would be a significant drop of pressure in a water delivery system. This creates a similar suction as the straw, pulling possible undesirable materials into the system.
Now to explain back pressure you’re going to have to think of the straw again. When you blow into a straw and create bubbles in the bottom of the glass, this is back pressure occuring. So if, instead of air, natural gas has been forced into a potable water tank, the gas, in turn, could be carried into your kitchen faucet. This is an example of a direct cross-connection, with an undesirable material being pushed into the water line.
Back pressure can force an undesirable contaminant to enter potable water piping. Some examples of sources that contain back pressure may be boilers, heat exchanging equipment, power washing equipment, fire sprinklers, or pumps in the water distribution system. A backflow preventer is important when potentially toxic chemicals are used. To reduce the risk of contamination, a backflow preventer can be fitted. In some cases there may be an almost continuous risk of overcoming the static water pressure in the piping. For instance for commercial/industrial descaling of boilers, or when chemical bleaches are used for residential power washing.
Another device that is closely related to backflow prevention is the backwater valve. This valve is designed to prevent sewage from backing up into a building and causing flooding.
Backflow Prevention Devices
The simplest, most reliable way, to provide backflow prevention is to create an air gap. An air gap is simply an open vertical space between any device that connects to a plumbing system for example a valve or faucet. Anything that creates a place where contaminated water can collect or pool. A simple air gap has no moving parts, other than flowing water. Many plumbing codes specify a minimum air gap distance that is required for different circumstances. For example a drain connection for a dishwasher
A specialized backflow preventer valve may be installed strategically in the plumbing system as an alternative wherever there is a risk of contaminated fluids entering the water supply pipes. These valves are used whenever there is not a sufficient vertical clearance, or physical space to install an air gap. They may also be used when pressurized operation, or other factors, rule out the use of an air gap. Because these valves use moving parts they are often required to be inspected, or tested periodically.
Many health regulatory regimes require an air gap, or mechanical backflow prevention assembly that goes between the delivery point of mains water, and local storage in order to prevent contamination, due to back pressure. Where submerged mains inflow is permitted, a backflow prevention assembly is required. This protects the potable water system from any contamination hazards. A check valve is a basic form of backflow prevention. There are often times where a more complex device is required, because check valves are not considered to be reliable.
In the US, the Environmental Protection Agency (EPA) holds local water suppliers responsible for maintaining a certain amount of purity in potable water systems. In many countries, approved backflow prevention assemblies are required by law, and many states or local municipalities require annual testing of backflow prevention. They must be installed in accordance with plumbing, or building codes. A typical backflow assembly has test cocks and shut-off valves, and it must be tested, and by law in most cases, double checked when it is installed. It must be tested if relocated, or repaired, and also on a periodic basis. Most of the time the law also requires the double check (DC), reduced pressure principle device (RP) device, or an air gap when backflow prevention is mandated.
The precise measures required to prevent backflow depend on the risk of contamination. The risk of contamination is the condition of the water in the connected system. The contamination risk is categorized into different risk levels:
- Category 1: No risk. Potable water
- Category 2: Aesthetic quality affected, e.g. water which may have been heated
- Category 3: Slight hazard from substances of low toxicity, e.g. cold water storage tanks
- Category 4: Significant hazard, e.g. pesticides
- Category 5: Serious health risk, e.g. human waste
How to Avoid Backflow
Backflow prevention must be automatic. Manually-operated valves are not usually acceptable.
Automatic check valves are required to prevent back pressure. Regulations for these check valves specify the design capabilities of the valve used, in accordance to the hazard. Category 2 contamination may be prevented by a single check valve, however, a category 3 requires a double check valve. A double check valve is manufactured as a convenient single unit, or even integrated into tap (faucet) fittings. There has recently been an introduction of the reduced pressure zone valve in the UK. These valves are complex, requiring certified installation and annual checks. They are used for category 4 systems, such as fire sprinklers where the system has an antifreeze additive This valve is a form of double check valve where the intervening zone is drained, and normally kept empty. If the downstream valve leaks and permits backflow, this will drain out through the vent rather than building up pressure against the upstream valve. Category 5 requires an air gap, not merely a valve.
Back siphonage may be prevented by the use of a vertical air gap. This may be a small gap, or a large gap. An air gap is the unobstructed vertical space between the water outlet and flood level of the fixture. Standards for these air gaps are grouped by the amount of separation they provide, and their acceptability for the various risk categories. The size of the acceptable gap also depends on the capacity of the incoming supply. Air gaps also protect against back pressure. However, most air gaps also limit the system pressure that can be transmitted.
Common examples of an air gap in domestic plumbing are:
- Faucet above the bathtub
- A hand held shower must have its hose fastened so that the shower head cannot rest below the water level in a bath, or basin.
Sanitary Sewer Backflow
Backwater sanitary valves, also known as check valves, are often referred to as “backwater valves”, or “backflow preventers”. They are intended to prevent backflow of sewage on the sanitary sewer line during a flood, or sewer blockage. They have no connection with potable water. Sewage lifting stations provide comprehensive protection against sewer backflow. They pump the water above the back pressure level and into the sewer. Even when the sewer is completely full.