When backflow prevention valves are installed on existing pipe scheduled systems, the friction losses of the device shall be accounted for when…

When a backflow prevention device is added to an existing pipe schedule system, it introduces a significant amount of friction loss. This loss occurs only when water is flowing. The pressure available from the water supply during flow is the residual pressure. Therefore, the friction loss of the new device must be subtracted from the available residual pressure of the water source to determine the effective pressure available to the sprinkler system. This ensures the system still has adequate pressure to operate as designed according to the pipe schedule tables.

B. system flow requirements.

The system's required flow (GPM) is based on the hazard classification and design area, not the components in the water supply line.

C. static pressure.

Static pressure is measured under no-flow conditions. The backflow preventer only causes pressure loss when water is flowing through it.

D. system GPM requirements.

This is functionally the same as option B. The device's friction loss impacts pressure delivery, not the fundamental water volume demand of the system.

1. NFPA 13, Standard for the Installation of Sprinkler Systems (2022 Edition). Annex A, Section A.29.3.1.2 states: "The adequacy of the water supply for a pipe schedule system should be verified prior to the design and installation of the system... The friction loss for all fittings and devices, such as backflow preventers and check valves, between the point of the flow test and the sprinkler riser should be calculated and accounted for in the water supply evaluation." This evaluation directly involves determining if the available residual pressure is sufficient after accounting for these losses.

2. Brock, Pat D. Fire Protection Hydraulics and Water Supply Analysis, 3rd Ed. (Fire Protection Publications, Oklahoma State University, 2015). Chapter 6, "Friction Loss Calculations," explains that the total pressure loss in a system is the sum of losses from pipe, fittings, and devices. It clarifies that the pressure loss from devices like backflow preventers must be subtracted from the available source pressure (residual pressure at a given flow) to determine the pressure available at the system riser (p. 135-137).