Backflow Devices

A backflow device is located very near the water source to prevent an irrigation system from polluting domestic drinking water. Some agencies require the backflow device to be located within 10 feet (3 m) of the water meter to minimize the amount of unprotected piping. Once a backflow is installed, all piping downstream from it is considered protected from pollution to the domestic water supply.

Types of Backflow Devices

The two basic types of backflow devices are:

• Reduced pressure (RP)
• Pressure vacuum breakers (PVBs)

Reduced pressure types can be at any height in relation to the irrigation system, while PVBs need to be a minimum of 12 inches (300 mm) above the highest head. This placement can present a true challenge if the grade slopes upward from the water source in the street. As a result, RP backflow devices are often required for any commercial projects, and PVB types are only sometimes allowed for single-residence projects.

Therefore, try to use RP backflow devices for your work.

Backflow Device Size/Flow

Febco 825Y and 860

Backflow devices are usually smaller than the mainline pipe attached to them. A review of the flow through a certain meter size, its related mainline size, and the suggested backflow size clearly indicates this sizing.

Assume you have a 1-inch (25 mm) water meter that allows a maximum flow of 37.5 gpm (142 l/m). The mainline pipe table indicates that at the velocity of 5 fps (1.52 m/s) a mainline of 1 1/2 inches (40mm) would allow a flow of 32 gpm (121 l/m), while a 2-inch (50mm) mainline would allow 50 gpm (189 l/m). Therefore, if you wish to use the maximum flow from your source of the full 37.5 gpm (142 l/m), anticipate having a 2-inch” (50 mm) mainline size for most of the mainline.

The backflow table indicates that a 1-inch (25 mm) backflow would allow 48 gpm (182 l/m), so anticipate a backflow of that size. Remember: In this example, you may very well have a 2-inch (50 mm) mainline connected to a 1-inch (25 mm) backflow device. This configuration is extremely  common and efficient. In fact, a 1-inch (25 mm) backflow with the flow of 37.5 gpm (142 l/m) will have a pressure loss of about 11 psi (0,76 bars), and a 2-inch (50 mm) backflow with the same flow will have a greater loss – about 13 psi (0,90 bars). Therefore, we recommend making an efficient match between backflow size and flow amount.