In NFPA 13 hydraulic calculations, why is the residual pressure at the most remote area critical, and how is it determined?

• A. It is only used for initial pump sizing and has no relation to remote areas.
• B. It must meet the system design pressure to ensure adequate flow; determined by adding static pressure, pump head, friction losses along the most remote path, and velocity head, then subtracting system demand.
• C. It is equal to the static pressure.
• D. It is determined by the color of pipes.

Answer: (B)

The essential idea is that the farthest, hydraulically remote area must still have enough pressure to deliver the required fire sprinkler flow. That residual pressure is the pressure available at the most remote point when the system is flowing, and it must meet the system design pressure so that the sprinklers can operate properly.

To determine it, start with the pressure available at the source (static pressure and any pump head). As water travels to the most remote area, subtract the energy losses along that path due to friction in the piping and the velocity head of the moving water. Then subtract the system demand—the pressure required by the sprinklers at that remote area. The result is the residual pressure at the most remote point. If this residual meets or exceeds the design pressure, the remote area is adequately served; if not, adjustments to pump capacity, pipe sizing, or reducing demand are needed.