Each backflow device is designed to handle a specific group of system characteristics – including hydraulic requirements of the specific backflow prevention design. Factors such as need for continuous pressure versus non-continuous pressure, backpressure versus back-siphonage, and flow-requirements versus velocity must be specified to determine the type of device to be used.
Existing conditions of pressure loss must accounted for prior to installation of a backflow device. All plumbing systems experience pressure loss. Pressure loss begins with the friction of the water against the walls of the pipe and increases with each additional fitting added to the system.
Many of the fittings and components in a plumbing system can be identified as fixed orifice items such as pipes. fittings, gate valves and ball valves. The pressure loss for fixed orifice items can be identified as a constant value and is characteristic of each type and size of valve. Once this value is established for a particular valve, the pressure loss through that valve or fitting can be calculated by simply knowing the specific gravity of the fluid running through the pipe and the flow rate (in gallons per minute).
Elevation in the plumbing system is also a very large factor. Approximately 1 psi for every 28 inches of elevation can amount to almost 7 psi loss in just a 2- story building. A high-rise building with an elaborate fire sprinkler system cannot tolerate losses in supply pressure and is therefore highly concerned with the effects of elevation in the supply system.