10 Considerations for Specifying Backflow
August 3, 2021 | By Courtesy of Watts
With a backflow preventer that is properly specified, installed, and maintained, the risk of events due to backsiphonage and backpressure can be avoided.
Photo credit: Lost_in_the_Midwest/Adobe Stock
Backflow continues to represent a significant threat to our fresh water supply. Caused by a cross-connection between a potable water system and a suspected source containing used water, industrial fluid, gas, or any other contaminating material, backflow events are difficult to detect until irregularities are present and potentially serious symptoms arise.
With a backflow preventer that is properly specified, installed, and maintained, the risk of events due to backsiphonage and backpressure can be avoided.
Specifying a correct backflow solution is dependent on the application. Awareness of the potential pitfalls presented by the existing plumbing system’s design is critical. Following are 10 considerations when specifying.
1. Flow Performance
Flow performance is important when it comes to minimizing system pressure loss as the available pressure throughout the system needs to be calculated during the design process. If the pressure loss is too high for the system to provide adequate pressure to its components, it could mean having to add a booster pump or use larger diameter piping, both of which could add significant expense. Though pressure loss is an area of concern in all systems, it is especially critical within a fire prevention system.
All backflow manufacturers publish pressure/flow curves for their devices. When comparing flow curves, make sure they are both indicating the same situation—as increasing flow curves and decreasing flow curves are often very different.
2. Calculated Building Flow Demand
The total theoretical demand for a water supply system is calculated by adding the known maximum demand for all fixtures in the system. The flow characteristics of a backflow device can directly impact the overall flow performance of buildings potable water or fire sprinkler system. When specifying, be aware of the building flow demand requirements.
At the highest rate of typical use, you don’t want to exceed the rated flow, the flow rate above which you’ll experience greatly increased wear and tear. For fire systems, full fire flow should not exceed the UL rated flow of the valve.
3. Serviceability
Despite best efforts, wear and tear to a backflow preventer will occur from normal operation and unavoidable external sources – such as excessive debris in the water. The backflow preventer will inevitably need to be serviced, which should be done by a professional plumber.
There are a number of factors to consider when selecting a backflow preventer: How easy is it to access the internals? Can the parts be repaired individually? Are they readily available? Can components like the relief or shutoff valves be removed for service or replacement?
The harder to service, the more hours required, the more costly the repair.
4. Application Conditions
The application conditions will have a significant impact on the design of a system, the install, as well as budget. Local authorities have jurisdiction over the type of valves that are used in each type of application. The overall length of the valve can typically be found on the technical data sheet provided by the manufacturer and is critical when replacing an existing application. It can also impact a new installation if space is at a premium.
What type of environment will this device be located in? For instance, a reduced pressure zone assembly may not be used in a sub-ground level box or pit as they may become flooded. Also consider what kind of weather the valve will be exposed to, and how it will affect the exposed components.
5. Valve Size/Weight
Weight factors into the device selection mostly in regards to the labour costs associated with handling and installation. Also, consider what material it is made from. Composites and stainless steel are typically lighter than cast or ductile iron. Smaller is better when it can allow for extra space for critically needed valves in the system and take up less space in the mechanical room.
6. Installation Options
Backflow preventers are installed in a variety of locations, from large open spaces to small cramped mechanical rooms. They are designed to allow for multiple installation configurations to enable the design engineer to fit the device into the space. Installation options are horizontal [most common], vertical, ‘N’ Pattern, and ‘Z’ Pattern.
In general there is little to no change in valve performance based on its installation configuration; however you should always read the manufacturer’s technical information and consult with local plumbing code officials before making a buying decision.
7. Troubleshooting Ability
The way to test a valve will depend on the local water authority’s requirements, however the valve itself can affect how easy the valve will be to test and troubleshoot. Testers will need access to test cocks in order to effectively test the device’s function. If the valve has plenty of clearance around all sides this typically won’t be an issue, however when this is not possible (against manufacturer recommendations) or when retrofitting a valve, walls or existing piping may make access to test cocks difficult or impossible—which will lead to re-piping or replacing the valve entirely.
One use for test cocks, other than testing the valve, is flushing a valve to try and clear debris that may be fouling the checks. Larger test cocks will allow more flow, increasing the chance that you can clear the debris.
In order to test and troubleshoot a valve, it’s important that at least the downstream shutoff valve shuts tight and is leak free. For this reason, high quality shutoffs are an important part of the backflow assembly, and the ability to repair them in order to make the valve testable again is also key.
8. Shutoff Options
There are many shutoff valve options available when specifying a backflow preventer including: ball valves—typical for assemblies 2-in. and allow for easy ¼-turn on or off; butterfly valves—when a shorter lay length is desirable, can be gear operated for slow close operation to avoid water hammer (a requirement in fire protection systems), or controlled electronically; NRS [non-rising stem] gate valves—desirable when you don’t have clearance above the hand wheel and don’t need a visual indication of position (open/closed); OSY [outside stem and yoke] gate valves—standard for large diameter fire valves because the rising stem gives a visual indication of position and can be fitted with a tamper switch; PIV [post indicator valves]—typically an option on NRS gate valve, these valves are meant to attach to a post that will visually indicate “open” or “shut”, and are typically used in fire systems when the valve will be inaccessible, such as underground or behind a wall.
9. Code Compliance
It is important to understand your local codes and requirements, as a device may be allowed in an application in one area and considered insufficient in the next. It is advisable to maintain a relationship with local plumbing inspectors and water authorities so you can avoid either failing an inspection, or installing a more expensive device when a more economical one would have satisfied the requirements.
10. Importance of Agency Approvals
There are resources available providing information, education, product testing and certification, building product evaluations, and code development assistance. These organizations can help: IAPMO – the International Association of Plumbing and Mechanical Officials (IAPMO) iapmo; USC Foundation for Cross Connection Control and Hydrualic Research, fccchr.usc.edu; and The American Society of Sanitary Engineers (ASSE), asse-plumbing.org.
Plumbers, contractors, and engineers have more options to recognize, plan for and prevent potential backflow threats before they occur, but ever-changing plumbing codes and complex plumbing systems have made it a difficult landscape to navigate. Specifying the correct backflow preventer is the first step to a safe and secure water supply.
