When we consume water from our tap, we automatically assume that it is safe for consumn. We expect the water to be clean and free of any pollutants or toxins. Backflow preventers play a considerable part in ensuring that is the case every time. To explain what a backflow preventer is, we first need to discuss what backflow is, when it comes to water.
Water in Buildings
When water enters any building, it is pressurized by the water purveyor (usually the city or county). This pressure ensures that when you turn on the tap, the water actually starts flowing. Without pressure, the water would never flow. Because we use water for significantly more than to drink it, there are many risks of pollutants or toxins contaminating the water. Manufacturing processes or additions of chemicals (fertilizer or pool chemicals, for example) can render the water harmful for human consumption.
As buildings use water differently, there could be a time when more water is requested than can be supplied initially. A famous example for this is a fire hydrant. The hydrant, when opened will create such a high flow rate, that it pulls water back out of buildings to service its consumption. That is where the danger comes in, as the potentially contaminated water is now pulled back into the water supply and could be distributed across the entire neighborhood. This event is called a cross-connection. As the city’s water supply is cross-connected with the potentially contaminated water within the building.
One of the first-ever documented cross connections, that caused a significant amount of harm happened in Chicago at the World’s Fair in 1933. This article discusses it in more detail: Chicago Backflow Incident. Another more recent incident happened in Pittsburgh, where two fire truck pumps overwhelmed the water supply and caused fire suppression foam to leak into the water system. More information here.
Where are Backflow Preventers usually installed?
Generally, backflow preventers can be found on the main incoming water line into the building. If the building has a sprinkler system, the very first component in the sprinkler system will be a backflow preventer. Backflow assemblies can be found in several locations throughout a building, Depending on the hazards that are present, individual water consumers may be protected by their own assembly. Some examples would be pools, ice machines, dishwashers, and other applications where water is mixed with chemicals. Backflow hazards range from minor to severe, depending on the effect the contaminated water can have on human health.
How does a backflow preventer work?
Backflow preventer assemblies are comprised of two one-way valves, referred to as check valves. These valves close when no water flow is present and thus restrict the reverse flow of the water. As there is two valves, there is a level of redundancy built into these devices. This redundancy ensures that the public water is still protected, even if one of the check valves has failed.
There are several other components to a backflow preventer, such as the shut-off valves and test cocks, to provide a means of testing the assembly. There are two main backflow preventer types in use today, though there are about 15 different types altogether. The two most used are:
DCVA – Double Check Valve Assembly
- The most common type of backflow prevention aseembly.
- Suitable for minor-hazards. The American Water Works Association (AWWA) defines this hazard as a situation in which a cross-connection might create a nuisance or be aesthetically unpleasant but would NOT create a health hazard.
- The main parts of a DCVA are an inlet shutoff valve, two independently operating spring-loaded check valves (usually inside a single valve body), four test cocks, and an outlet shutoff valve.
RPZ – Reduced Pressure Zones
- More reliable than a DCVA, but also more elaborate.
- Generally required for severe-hazard settings, defined by the AWWA as those in which an unprotected cross-connection could introduce substances capable of causing illness, death, or disease spread if introduced into the public water supply.
- Equipped with a relief valve in addition to two spring-loaded check valves. The relief valve will dump water in the event of a check valve failure, to ensure the water supply is protected.
Testing of backflow assemblies is generally required by the water authority. The American Water Works Association (AWWA) otulines the certification requirements for qualified technicians. Testing of an assembly is generally required every year, though some municipalities may require it more frequently. All testing needs to be completed by a qualified professional.
A backflow test involves a simulation of a backflow event with a specialized gauge under protected conditions. This will yield a test result indicating the functionality of the unit. If there is backflow, meaning water flowed back through the unit, it must be repaired or replaced immediately. Most backflow can be repaired, which involves gaskets and the check valves themselves. In some cases the entire backflow preventer may need to be replaced.
It is important to work with a qualified professional when it comes to backflow preventers. Incorrect installations or repairs can cause premature failures. Incorrect testing may also damage the units and without the necessary certifications, your water authority may reject the tests altogether. Regular testing of the backflow assemblies is crucial, as the liability for contaminating the water supply is ultimately your responsibility.
Nexus Fire & Safety can help you meet all your backflow testing requirements and offer the most cost-effective approach should one or more of your assemblies fail testing. Our technicians are fully certified and can assist with the design, installation, inspection and maintenance of all your fire safety systems, including backflow preventers. Get in touch today to learn more