Water distribution systems are designed to flow water in a specific direction: from the source to the consumer and then from the consumer to the sewer system. However, some operating conditions can cause a pressure difference opposite to the intended flow direction, causing backflow when the system lacks protective measures. The main risk associated with backflow is contamination of the drinking water supply with harmful substances.
What causes reflux?
For backflow to occur, there must be a pressure change that pushes the water in the opposite direction to the normal direction of flow. These pressure changes can be induced by gravity, vacuum formation and other physical effects. Based on how the back pressure difference is established, the causes of backflow can be divided into back siphoning and back pressure.
|
CAUSE OF REFLUX |
DESCRIPTION |
|
Rear siphoning |
May occur when pressure is reduced upstream of the point of use. Some common locations for back siphoning are tanks and drinking fountains, especially when water pressure drops below atmospheric pressure. Back siphoning can be caused by undersized or broken pipes and sudden spikes in water demand. |
|
Counterpressure |
It can occur when pressure is increased downstream by a system that uses water . Backpressure is common in the piping of multi-story buildings due to the effect of gravity, and can also be induced by high static pressure in fire protection systems. Other causes of back pressure include pumping and expanding water at high temperatures. |
Although the physical process is different for backsiphonage and backpressure, the end result is the same: water tends to flow backwards due to reverse pressure and can carry pollutants upstream.
Make sure your water supply is protected from backflow.
The role of cross-connections
A cross-connection can be defined as any existing or potential connection between a potable water supply and a pipe or device that retains a non-potable fluid, including the water itself after use. The definition applies even when the non-potable fluid is present only occasionally. Note that fluids may be considered undrinkable due to their own composition or due to the solid particles and gases mixed with the fluid. The following are some examples of systems that create cross-connections:
- Automatic fire extinguishing systems
- Irrigation systems
- Beverage production equipment
- dialysis equipment
There are also cases where a cross-connection is created accidentally. Suppose a hose is used to fill a swimming pool: when the hose is suspended above the pool, there is no cross-connection; however, one is created if the hose is submerged, since water can flow out when the pressure is gone.
Eliminating cross-connections is not possible because they are essential for equipment that requires pressurized water. Therefore, protective measures must be implemented to prevent backflow and potential contamination of drinking water supplies.
Classification of backflow preventers
Backflow preventers can be classified based on physical construction, application, or installation type. The main types of backflow prevention assemblies are listed below, and each type has a dedicated American Society of Safety Engineers (ASSE) standard.
|
DEVICE |
APPLICABLE STANDARD |
|
Reduced Pressure Reflux Assembly |
ASSE 1013 |
|
Double Check Valve Assembly |
BAKE 1015 |
|
Pressure Vacuum Circuit Breaker |
BAKE 1020 |
|
Reduced Pressure Detector Assembly |
ASSE 1047 |
|
Type II Reduced Pressure Detector Set |
ASSE 1047 |
|
Recheck the detector assembly |
ASSE 1048 |
|
Type II Double Check Detector Assembly |
ASSE 1048 |
|
Spill Resistant Vacuum Circuit Breaker |
ASS 1056 |
Each type of backflow prevention assembly has different operating characteristics, which means not all may be suitable for a specific application. Before specifying a device, it is important to make sure it meets the application requirements.
Application requirements
Backflow prevention devices are used in many applications and industries, each with different performance requirements: fire protection systems, agriculture, water supply systems, etc. Backflow prevention devices in these systems must be suitable for the flow conditions encountered:
- Water flow is constant in hydraulic and plumbing applications.
- Irrigation systems only work about 2% of the time and are static the remaining 98%.
- Fire protection systems are permanently on standby, but must sustain a specific static pressure to respond effectively when needed.
Installation requirements
In addition to being classified by device and application, backflow prevention systems also differ in how they are installed. Its location can be external, internal or underground, and its orientation can be horizontal or vertical. Each configuration comes with specific challenges:
|
LOCATION |
KEY ISSUES TO CONSIDER |
|
Open air |
Soil erosion and freezing temperatures, and human factors like vandalism and theft. Requires a protective wrap in most cases. |
|
Interior |
Avoid discharging water. |
|
Below level |
Not all backflow prevention devices are suitable and the installation must be accessible for testing, maintenance and repair. |
|
Vertical |
Not all backflow preventers are suitable. For this reason, horizontal installations are more common. |
In applications that require an uninterrupted supply of water, multiple connections or sets of collectors may be required – this includes some industrial applications as well as hospitals and resorts.
Federal Considerations for Backflow Prevention Devices
The Safe Drinking Water Act (SDWA) was passed by the US Congress in 1974. The SDWA holds the water supplier responsible for quality at the source, meeting the standard established by the US Environmental Protection Agency. However, once the water has been delivered, the customer is responsible for preventing contamination through backflow.
The SDWA was updated in 2014, requiring all plumbing equipment to be manufactured with less than 0.25% leaded brass – this includes backflow prevention devices.
Conclusion
While SDWA ensures water suppliers provide quality, backflow from customers' equipment can cause contamination if ignored. Make sure your installation is equipped with backflow prevention devices, appropriately selected for the applications for which they are intended. Working with qualified plumbing design professionals you can ensure this requirement is met while meeting all applicable building codes.
