There are few things as important as access to safe, clean drinking water. Water is necessary to sustain all life on our planet. It is a need that exists everywhere — in rural areas, in cities, and everywhere in between. As the world’s population continues to grow, the importance of systems to ensure water is available where and when it is needed cannot be overstated. It is estimated that by 2050 we will have 10 billion people on Earth. Those 10 billion people will require more resources than ever before, including clean, safe drinking water. The world is also experiencing changes in weather, including the power of natural events like droughts, fires, floods, tropical storms, and tornadoes.
America, in many areas, has an obsolete and failing infrastructure. There are serious problems with our public water and sanitary systems, the power grid, and our roads and bridges. These issues are real and will require upgrades and wholesale changes and improvements in the next few years. Changes are already taking place in many areas. When we look at the topic of water, we are working to address shortages through the expanded use of recycled water. As with all things, there is a cause and effect with every step we take. The use of recycled water is a solution to the shortages in our drinking water systems that allows us to greatly reduce the use of potable water for irrigation, processed systems, toilet flushing, and other sanitary uses. The use of recycled water can and does make systems more complex, which must be taken into consideration when we look at cross-connection and backflow prevention programs. That being said, it’s time to really go back to the basics and look at this topic from a beginner’s perspective.
To get back to the beginning, we need to understand what backflow is. We define backflow as the undesirable reversal of flow in a water system from its normal direction. Once water enters the service line into a facility, the piping must become a one-way street and flow from upstream to downstream into the building and throughout its water system. Water is a lazy fluid that always seeks the path of least resistance. High pressure goes to low pressure, always. The laws of physics will control how and where water will go. The water in piping systems can’t see the flow arrows on the outside of the piping — it simply searches for a lower pressure zone to move into.
There are two distinct types of backflow. The first type is backpressure backflow. This type of backflow occurs when the downstream pressure becomes higher than the upstream pressure. It can be caused by either a drop in supply pressure, or an increase in the downstream or building pressure. Pumps, thermal expansion, elevation, and water hammer all create backpressure backflow. The second type of backflow is backsiphonage backflow. This is caused by a sub-atmospheric pressure occurring in the water system. A number of factors can cause this phenomenon, including high velocity flow or large demands on a distribution system, such as hydrant flushing. Simply draining a water system can cause a backsiphonage event.
Another term we need to understand is cross-connection. This refers to a point in a water system where we can no longer be sure of the quality of the downstream liquid. Our job is to take those points of connection and to make the protected cross-connections where the proper protection has been installed to prevent the possibility of backflow from taking place. In a modern plumbing system, it is impossible to eliminate all cross-connections. That is why we establish cross-connection control programs, which we define as the use of the proper methods, devices, or assemblies to prevent any type of backflow from occurring.
There are two types of cross-connections. The first is a direct connection, where the possibility of both backpressure and backsiphonage backflow is present. Some examples of this would be the water supply to a fire protection system or the make-up supply line to a hydronic boiler or chiller. The second type is an indirect connection where only backsiphonage is possible. Dropping a hose into a swimming pool to add water, or most lawn irrigation systems, are examples of indirect cross-connections.
There are several factors to consider when selecting the proper protection required and we install protection in several ways. Containment protection, or point of service protection, is installed at the water meter or service connection to a building or facility. It is also sometimes referred to as premise isolation. It protects the public distribution system from the user’s facility. This protection program does nothing to protect individuals in the facility or building. The type and location of the protection is normally mandated by the water supplier or water purveyor. The other type of protection is isolation, or point of use protection. This is backflow protection at each individual fixture or appliance. Isolation protection requirements are, in most areas, mandated in the jurisdiction’s adopted plumbing code.
What type of backflow protection is needed in a specific situation? This is an often-asked question from both consumers and trades people alike. The answer is based on a number of factors which must be considered. Every installation should be looked at as unique. There is no one-size-fits-all solution when it comes to cross-connection control. While we attempt to eliminate as many actual or potential cross-connections as we can with the use of a physical air gap, there are times when that is impossible and a mechanical device or assembly must be used to protect the potable water supply.
This is why a survey of the system is needed to ensure that protection is in place and that it is the correct protection. We look at water systems both for containment protection, which is a fairly simple process, and for the more complicated insolation protection requirements. In a containment survey, we look at how the facility itself is using water to assign the entire system a degree of hazard. The building supply is always a direct connection, so we know our containment protection device or assembly must protect against both backpressure and backsiphonage backflow. We are simply deciding on the service connection protection. In most commercial water services, it will be a double check valve or a reduced pressure principle assembly. On residential services, the most popular protection is an ASSE 1024 dual check valve, although some water supplies may require a testable assembly. Some jurisdictions require no protection at all on residential properties.
Isolation protection surveys are much more complicated. We need to look at the entire system and work to ensure that the proper backflow protection is installed everywhere it is needed. Starting at the service connection, we need to trace every foot of piping and examine every fixture, appliance, or connection. We will be looking at both direct and indirect connections. Isolation surveys take time to do accurately. The surveyor is looking at whatever existing protection is in place, what is missing, and where changes should be made. They are also documenting this information and making recommendations on how to bring the system into compliance with industry standards, local regulations, and codes requirements.
It is important that everyone in the industry, including surveyors, system designers, installers, inspectors, and backflow assembly testers, understand the plumbing code requirements in their jurisdictions, and state and federal regulation concerning cross-connection control. In many cases, certified testers may be called to test assemblies that are installed incorrectly or may not provide the proper protection to the system they are installed to protect. There is more to being a good backflow tester than simply knowing the steps contained in the test procedure. It is also important to the consumer, or end user, that the proper protection be installed. If a simple, non-testable device or method can be installed to protect the water supply, then the installation of a more expensive, testable backflow prevention assembly should be avoided. The additional cost, pressure loss, and maintenance requirements of a backflow prevention assembly should not be passed on to the public unless it is necessary. One size does not fit all when it comes to backflow prevention and cross-connection control.
As we look at the required protection needed, there are several factors we need to consider in order to look at the adopted plumbing code for guidance concerning what type of device, assembly, or method to use to protect the potable supply. The first and most important consideration is the actual or potential danger that may be posed by the cross-connection. The definition of potable water may vary slightly in different reference books, but simply put, it is water that is suitable for drinking, culinary, and personal purposes. It is also water that is free from impurities in amounts sufficient to cause disease or other harmful effects. More simply put, it is water that is safe to drink and is aesthetically pleasing. In looking at the actual or potential danger, better known in the industry as the “Degree of Hazard,” we must decide if the danger has the potential to only affect the aesthetic quality of the water as a non-health or low hazard pollutant, or if it could cause illness or death, which should be considered a health hazard or high hazard and would be considered a contaminant.
Once the decision is made regarding the degree of hazard, we can then move on to the other important considerations. These include the type of backflow that is or may be present. Is backsiphonage a possibility? Backpressure? Are both a possibility? We also need to know if continuous pressure is required or if the system will only be pressurized on a limited basis. What are the pressure and volume requirements? What type of climate and environmental conditions exist? Can we provide proper access to the equipment? All of these factors must be considered, although the degree of hazard outweighs any of the other considerations.
Once we know the answers to the question above, we can look at the protection itself. Looking at backflow protection methods we define as non-mechanical protection, we have the physical air gap and the barometric loop. Both of these methods will protect against both high and low hazard backsiphonage. Installed correctly, they can provide critical protection, but each has its limitations. The air gap is used extensively on many plumbing fixtures and appliances. An air gap must be twice the effective opening of the supply piping and always a minimum of one inch. It provides a physical separation between the water supply and any contaminants. The installation of an air gap, however, eliminates all system pressure, and once water passes through an air gap, it may no longer be considered potable water. The barometric loop uses vertical distance to prevent backsiphonage. Since water cannot rise more the 33.9 feet in a perfect vacuum at sea level, the barometric loop stretches 35 feet above the highest fixture level using height and the weight of water to prevent backflow. Neither of these methods will prevent backpressure backflow, but if conditions permit, it makes sense to install protection that has no moving parts or requires maintenance or field testing to ensure its continued protection.
When we look at mechanical devices and assemblies, we find a number of options in our backflow toolbox. In low and high hazard backsiphonage situations, we can use vacuum breakers for these cross-connections. Vacuum breakers include several different types.
Protection available for low hazard backpressure and backsiphonage situations include double check valve assemblies, dual check valves, and backflow preventers with an intermediate atmospheric vent. This protection can be subjected to continuous pressure and is not required to be installed above
the downstream piping.
If we look at high hazard backpressure and backsiphonage protection, we have limited choices. The reduced pressure principle assembly is the necessary protection in this hydraulic condition. The assembly contains two spring loaded closed check valves and includes a hydraulically activated, independently operating differential pressure relief valve, spring loaded to an open position, located in the reduced pressure zone between the check valves. This assembly allows us the highest level of protection, but it does come at a price. The assembly has a rated pressure loss during flow of between 14-26psi. Its differential pressure relief valve will discharge water in certain situations. At times, this discharge volume can be significant, which, if the assembly is not installed with the correct provisions for drainage, may lead to flooding and water damage in a high discharge volume event. The assembly’s differential relief valve is the component that ensures protection of the water system in the event of a failure of the assembly’s check valves. It alerts the building personnel that the valve requires service while at the same time maintaining a lower pressure in the reduced pressure zone that is upstream of the first check valve, making a reversal of flow impossible.
For more information about vacuum breakers, double check valve assemblies, and reduced pressure principle assemblies, I recommend reading “Mechanical Cross-Connection Control 101” by Dr. Stu Asay.
The secret to cross-connection control can be simple if we follow the correct steps. Look at each connection as a unique installation. Evaluate the degree of hazard and the type of backflow that could occur based on the laws of physics. Select the proper protection and have it correctly installed. Understand that the installation of a backflow preventer will change the existing hydraulic conditions in a water system and may create a closed water system. The resulting conditions may require the installation of thermal expansion protection and may also lead to a significant loss of pressure or volume within the water system.
Once protection is in place, have it tested and maintained using trained and certified individuals to ensure its continued reliability. Reevaluate the water system on a regular basis and after any new construction or significant change in use to ensure continued compliance with existing codes and regulation. That way, our potable water supply will be protected.
Article by Sean Cleary first appeared in Working Pressure magazine