Tracking the Flow of Backflow Prevention
In a California suburb, a man sprayed his lawn with a commercial weed killer that contained an arsenic compound. His applicator was an aspirator device connected to the garden hose, to which was attached a bottle of the arsenic poison. When the man finished spraying, he turned off the hose and disconnected the applicator. Shortly thereafter, he turned the hose on again to get a drink of water and died from arsenic poisoning.
Irrigation consultant Bob Healey with Irrigation & Management Services in Natick, Massachusetts, relates a case involving Holy Cross football team, which was stricken with infectious hepatitis owing to subsurface hose bibs and a nearby fire. Healey, a member of the American Society of Irrigation Consultants (ASIC), explained that as water was being pumped to fight the fire, the water main pressure was reduced enough to cause backsiphonage from the hose bibs.
Backflow prevention devices are mandatory in most municipalities, cities and states for the principle example that is cited above. They play a key role in keeping our drinking water safe. You need to understand what they do and why they are necessary. If you understand that, you will be able to ensure that the projects youre involved with comply with local codes.
But lets start at the beginning. And the beginning for using and installing backflow devices is to ensure that the examples cited above do not occur. Simply put, backflow devices will prevent water that is on your clients property from moving back into the main water supply.
Backflow is defined as the flow of non-potable water or other liquid, solid or gas into the distributing pipes of a potable water supply because of an unwanted reverse flow, backsiphonage or backpressure. This could allow contaminants that could be in the water to flow back into the potable water putting the public at risk.
Any connection between the potable water supply and a source of contamination is called a cross- connection. A cross-connection is the point at which an irrigation or other line is tapped into the main water supply.
There are two kinds of cross-connections; a direct cross-connection and an indirect cross-connection. The difference between the two is that a direct cross-connection is subject to back-pressure, and an indirect cross-connection is not.
There are two basic forms of backflow: backsiphonage and backpressure. Backsiphonage can be caused by leaks in the distribution system or other high withdrawals of water from the system that lower the pressure to the point that a vacuum is created. This vacuum can draw used water or other contaminants back into the public water supply system through open outlets, like sprinkler heads or hoses that might be submerged at the tie.
The other cause of backflow is backpressure, which occurs when a water users piping system creates a higher pressure than that in the public water system. This can be caused by pumps, thermal expansion or elevated piping.
To select the proper backflow device, you must determine the degree of hazard that exists within the system. High-hazard is commonly applied to systems containing contaminants, and low-hazard applies to those with pollutants.
Most regulatory bodies provide these guidelines and should be consulted before selecting a backflow prevention device for a project, so the only thing left to decide is what type of backflow youre protecting your system from and what would be the most appropriate form of backflow protection, he asserts.
Although commercial backflow prevention regulations seem pretty consistent, at least in their guidelines and requirements, residential backflow regulations vary widely from jurisdiction to jurisdiction.
Commercial backflow prevention regulations are pretty solid and enforced, offers Brad Noll, engineer manager for Wilkins/Zurn in Paso Robles, California, But regulations for residential properties really need some work. We regularly see poor installations and little to no testing and inspections.
However, the landscape industry is gradually moving toward more protection at the residential level, Noll continues, from atmospheric vacuum breakers to pressurized double check valves and RPs.
Although jurisdictions are increasingly codifying backflow prevention for residential applications, it appears that for now, responsible specifications are up to the industry. A mandate for requiring specific backflow prevention devices at the residential level is one thing, but enforcement is quite another, Noll asserts.
1.) The Air Gap is a physical separation between the free-flowing discharge end of a potable water supply pipeline and an open, non-pressurized receiving vessel. The separation between pipe and tank is twice the diameter of the piping and no less than one inch. This method is approved for high hazard.
2.) A Reduced Pressure Principle Backflow Prevention Assembly (RP) contains two independently acting check valves with a pressure differential relief valve between them. The units also have two shutoff devices one upstream and one downstream of the check valves. This assembly is approved for high hazard situations, backsiphonage and backpressure.
3.) The Double Check Valve Assembly consists of two independently acting check valves. These units also have two tightly closing shutoff valves located at each end of the devices. This assembly is approved for low-hazard, backsiphonage and backpressure.
4.) A Pressure Vacuum Breaker (PVB) employs an independently acting, spring-loaded check valve and an air-inlet valve that opens to admit air when the pressure within the body of the device approaches atmospheric pressure. This assembly is approved for high-hazard backsiphonage.
5.) An Atmospheric Vacuum Breaker (AVB) consists of a check valve and an air-inlet valve. This assembly is approved for high-hazard backsiphonage.
Evolving Codes and Regulations
Local and state regulations regarding cross connections must be followed regarding the selection, installation and testing of the approved backflow prevention device. Each state and many municipalities have varying regulations and codes, so not all states approve the same devices for the same conditions.
How do you know which backflow prevention device is appropriate for your project? Some jurisdictions follow all or parts of the Universal Plumbing Code and the International Plumbing Code, but these are just guidelines, offers Dale Strong, president of the American Backflow Prevention Association. The safest way to ensure you are selecting the proper backflow prevention device is to first contact the water supplier to that project, Strong adds. Codes and regulations vary widely across the country and even from municipality to municipality.
Vinchesi agrees: Backflow prevention devices cant really be over- or under-specified; its all codified. You specify what the codes say according to your site parameters. The rules are pretty much cast in stone, he adds. The problem is when the local inspectors dont know the law. Perhaps a second opinion is in order if you have questions after a meeting with the backflow prevention inspector.
For example, these devices should not be placed in locations that could become subjected to localized flooding. Few jurisdictions allow the installation in a pit, unless the pit is waterproof. Installation of these devices should also require clearance for testing and servicing.
RPs and double checks require a minimum 12-inch clearance above the grade to ensure an air gap between the relief valve and any water that could pool beneath the devices. The PVB cannot be installed where there could be backpressure only backsiphonage and must be installed at least 12 inches above the highest sprinkler system outlet.
Just like the PVB, an AVB cannot be installed where there could be backpressure, only where there could be backsiphonage. The AVB must be installed at least six inches above the highest system outlet and cannot be under continuous pressure.
It is important to understand that backflow prevention devices are designed to protect our potable water supply from pollutants and contaminants. Become aware of the requirements and local codes for backflow prevention protection.
Backflow prevention is a public safety issue that is not always understood or addressed in irrigation designs and installations, adds Dan Benner, president of the American Society of Irrigation Consultants. An important part of an irrigation consultants role on a project is that the appropriate equipment is functioning properly at the finish of an installation. Were the clients safety net for compliance.