Backflow prevention: Protecting our water supply
March 6, 2018 by Robert Waters
In today’s modern world, maintaining the supply of safe, clean drinking water can be challenging with so many potential chemical contaminants. In addition, there are many different ways that potable water systems can be cross connected with other systems. Regulated water suppliers have a responsibility to provide water that is usable and safe to drink under all foreseeable circumstances.
Backflow into a public water system can pollute or contaminate the water in that system and make it unsafe to drink. For this reason backflow prevention devices, and municipal cross connection programs that govern them, are essential to our modern potable water supply systems. The need for cross-connection control exists in all types of buildings: industrial, commercial and residential.
Backflow preventers help prevent potable water contamination in such critical areas as municipal water systems, food processing plants, medical and dental water supplies and many industrial applications.
A backflow prevention device is a mechanical device which provides a physical solution to backflow that can occur in water piping. Backflow can also be prevented using an air gap to eliminate a cross-connection, or to provide a physical break to backflow conditions.
There are many different types of mechanical backflow preventers such as reduced pressure principle assemblies, atmospheric and pressure vacuum breakers, and dual check valves with and without venting ports. The type of backflow preventer used in any given application depends on the type of backflow that can be expected in that application, whether the device will be under constant pressure or not, and the degree of hazard of the fluid that could backflow into the potable water system.
Before explaining these different backflow prevention devices further, let’s take a closer look at the different types of backflow that can be seen in plumbing systems, and the differing degrees of hazard. So what is backflow and how does it happen in a plumbing system?
Backflow is the unwanted flow of water in a reverse direction back into the supply pipe of the potable water system. This reverse flow is very serious when it contains water that has undesirable chemicals, pollutants or bacteria. There have been many cases over the years in water supply systems in North America that have resulted in serious contaminated potable water, resulting in people becoming seriously ill and even causing death.
There are two types of backflow conditions: backpressure and backsiphonage.
Backpressure occurs when the user’s system is at a higher pressure than the potable supply water system. This can cause undesirable water to be “pushed” back into the potable water system piping. Several situations can cause this type of pressure imbalance, with the most common being potable water systems in high-rise buildings, systems with booster pumps and potable water system connections for boiler systems.
Backsiphonage is caused when a negative or reduced pressure exists in the potable supply water system. This can cause undesirable water to be “sucked” into the potable water supply. There are several common causes of backsiphonage, including water main repairs or breaks that are lower than the service point, and lowered main pressure due to high water withdrawal rate, such as fire-fighting or water main flushing. Backsiphonage can also be caused by undersized supply piping causing high velocity in pipelines and a reduced supply pressure on the suction side of a booster pump.
For backflow of contaminated water to occur a “cross-connection” must exist. A cross connection is defined by the Canadian Standards Association (CSA) as any actual or potential connection between a potable water system and any source of pollution or contamination, such as non-potable water, industrial fluid or gas. A cross-connection is any temporary or permanent piping connection between a potable water system and a heating or cooling system, irrigation system or an auxiliary water system. Examples are by-pass lines, change over devices, or make-up water lines.
Fresh water make-up lines are used in most boiler and chiller applications, which may contain boiler treatment chemicals or glycols. Make up water lines are also often used for industrial chemically-treated baths.
Fire sprinkler systems will often incorporate a hookup connection for the fire truck pumper to boost the pressure and flow to the sprinkler system. Irrigation pumping systems on farms will pump water from irrigation channels into the sprinkler system. Many are also connected to the potable water system for times when natural sources water levels are low.
There are also potentially dangerous cross-connections that occur in residential applications. These include outside hose bibs and lawn sprinkler systems.
All of these different types of cross-connections do not pose the same levels of threat to the general public. So when using backflow preventers it is important to also know about different degrees of hazards that a given application poses. The degree of hazard is based on the type of fluid that could flow back into the potable piping system.
If the fluid is considered toxic, it could create a health hazard, causing sickness or death. If the fluid is non-toxic it may be considered more of a nuisance, which could cause the taste, colour or appearance of the water to be affected, but will not be hazardous to health.
Based on the application and the toxicity of the fluid, the degree of hazard will be considered to be minor, moderate or severe. The more severe the risk, the more elaborate and more expensive the backflow preventer requirements become. Figure 1 shows the backflow preventer selection criteria from the City of Guelph backflow prevention bylaw.
Cross connection control hardware is available from very simple to very complex and it falls in three broad categories: vacuum breaker types, check valve types, and reduced pressure principal types.
Vacuum breakers are among the simplest and least expensive mechanical types of backflow preventers. Vacuum breakers can be either atmospheric (AVB) or pressure type (PVB). They provide excellent protection against backsiphonage, but they should not be utilized to protect against backpressure conditions.
They will prevent backflow when the pressure in the water system upstream of the device falls below atmospheric pressure. Atmospheric vacuum breakers can be very simple such as those used for hose bib connection (HCVB) or laboratory sinks. They are not designed to be under constant pressure and should not have any shut off device downstream.
Pressure vacuum breakers are a more elaborate version designed to be utilized under constant pressure. They would be used in sizes up to 10-inches for applications in agriculture, irrigation and industrial applications.
Check valve type backflow preventers also come in many different versions such as single (SCVAF), double (DCVA), dual (DuC), dual with atmospheric port (DCAP) or intermediate vent (DuCV). All of the dual and double check valves consist of two independently acting, internally loaded check valves in series.
They are intended to prevent backflow from both backpressure and backsiphonage and are used in systems where minor to moderate hazard levels exist. Check valve type backflow prevention devices are used in applications ranging from individual outlets of appliances or faucets, residential water supply lines, boiler water feed lines and fire protection systems.
Reduced pressure principal backflow preventers are used for the most severe hazard applications with toxic fluids. These devices are modified double check valves with an atmospheric vent capability placed between the two checks, and designed such that the zone between the two checks is always kept at least two pounds less than the supply pressure. This design can still provide protection against backsiphonage and backpressure even if both checks become fouled. They are used in severe hazard installations such as industrial plants, car washes, funeral homes and hospital autopsy rooms.
Backflow preventers cannot be simply installed and forgotten. Mechanical backflow preventers have internal seals, springs and moving parts that are subject to fouling, wear, and/or fatigue. Also, mechanical backflow preventers and air gaps can be bypassed. For this reason, qualified personnel with properly calibrated gauge equipment must test backflow prevention devices annually to ensure they are functioning properly.
CSA Standard B64 is the standard that covers vacuum breakers and backflow preventers that are used in potable water supply systems to prevent cross-connections. CSA B64 has been part of the National Plumbing Code for many years and has been adopted by all of the provinces. But according to Kevin Wong of the Canadian Water Quality Association (CWQA), supplying potable water is a municipal responsibility and not all municipalities have a cross connection program in place.
“There are some good programs in place such as in Ontario in Guelph and Peterborough, but it is not consistent across the country,” says Wong. He estimates that only about 10 per cent of the municipalities in Canada have an active and well-run cross connection program.
“The real power of a cross connection program is when a municipality has a bylaw in place and inspectors that are enforcing it,” notes Wong.
A municipal cross connection control program will monitor the installation, maintenance and field testing of backflow prevention devices in accordance with local bylaws and other codes and standards. According to Wong, training and education is also a critical part of these programs as well.
“Having trained and certified tradespeople who can inspect and repair units is very important,” says Wong. Editor’s note: Wong has since left CIPH and CWQA.
Tradespeople must be knowledgeable about the cross connection control and backflow prevention policies within the jurisdictions they are working in. Whatever type of system you are installing–hydronic heating systems, water treatment equipment, fire sprinkler systems, irrigation systems, and so on–it is essential that you know what backflow prevention equipment is required.
Protection of our potable water supply is everyone’s responsibility. It is safe to say that no one wants to be the cause of anyone getting sick or worse due to backflow issues.
Robert Waters is president of Solar Water Services Inc., which provides training, education and support services to the hydronic industry. He is a mechanical engineering technologist graduate of Humber College and has over 30 years experience in hydronic and solar water heating. He can be reached at firstname.lastname@example.org.
There is a big difference between backwater valves and backflow preventers:
A backwater valve prevents raw sewage or storm water from backing up into the home through floor drains, toilets, showers, etc.
A backflow preventer deals with protecting potable water sources from being contaminated by a reverse flow of foul water.In Canada we are spoiled with abundant fresh drinking water and many of us tend to take this clean and safe water for granted. Most Canadians have absolute faith that the water delivered to them through the public water system is always safe to drink.