How do you estimate friction loss for an exterior attack line?

• A. Friction loss is negligible for exterior lines.
• B. Only use a friction loss table without considering length.
• C. Use hose size, length, and expected flow to estimate friction loss per 100 ft, then calculate required pump and nozzle pressures.
• D. By guessing based on experience.

Answer: (C)

When estimating friction loss on an exterior attack line, the amount of pressure drop you’ll see depends on the hose size, the length of hose laid, and the expected flow. The practical method is to determine the friction loss per 100 ft for the chosen hose size at the expected flow, multiply by the actual hose length, and then use that total friction loss to set the pump discharge and nozzle pressures. This approach links the hydraulics to the real setup: you know how much pressure will be lost in the hose and you can add the desired nozzle pressure (and any other small losses) to arrive at the pump discharge pressure needed to achieve the target nozzle pressure.

This matters because friction loss is not negligible and grows with both flow and length, and it varies with hose diameter. Relying on a friction loss table in isolation or guessing the pressures won’t account for the actual length or flow, which can lead to under- or over-pressurizing the line.