BY LAWRENCE P. HAWKINS

Pump operators are responsible for delivering the required water flow [gallons per minute (gpm)] to the nozzle team with a nozzle reaction (NR) that can be safely handled by available firefighters. This can be done if operators can control pressure at the nozzle, since nozzle pressure (NP) largely determines volume flow and NR.

Friction Loss

The fire engine pump creates a mechanical energy force (pressure) measured in pounds per square inch (psi) to pump water. As water courses through the hose, it experiences friction created by rough hose linings, couplings, gravitational forces, and turbulent flow. Friction converts mechanical energy to heat energy, so by the time water reaches the nozzle it has lost a good part of its pressure. It is the operator’s responsibility to know the friction loss (FL) so he may control NP by adjusting pump discharge pressure (PDP). So, you can see the importance of friction loss measurement.

Another reason measurements are so important is that standard industry estimates for FL calculations are outdated and seriously overstated. Consider the equation: FL = c × (gpm/100)2 × L/100.

The “c” term is the so-called friction loss factor and measures the pressure loss per 100 feet. This is multiplied by the square of the volume flow (gpm) and hose length (L) to get total friction loss. The “c” factor is a handy way of comparing results from different tests and flow rates. By measuring friction loss and volume flow, we can determine the “c” factor. For example, 200 feet of 1¾-inch hose with a smooth bore 7⁄8-inch tip and 50-psi NP pumping at 161 gpm has a 49-psi friction loss. This translates to a “c” factor of 9.5 psi per 100 feet.

I have been gathering “c” factors from known fire department tests and research studies.1 Though the analysis is somewhat anecdotal, the results are instructive; “c” factors varied from c=8 to c=13, with a modal point of c= 9.5. The National Fire Protection Association (NFPA) uses an estimate of c=15.5 for 1¾-inch hose, as do many fire departments as well as hose manufacturers in their published FL tables. These “book” operations are not representative of line operations.

What happens if you use c=15.5 and the actual is c=9.5? At c=15.5, the FL for 200 feet of 1¾-inch hose is calculated to be 80 psi, or 40 psi per 100 feet. To get an NP of 50 psi, discharge pressure would be set at 130 psi. But if the actual “c” was 9.5 or 49 psi, the NP would be 81 psi. You would be pumping 205 gpm with a NR of 97 pounds of force. Clearly, the pump operator needs accurate FL measures to properly do the job.

So, how do you directly measure FL? There are protocols established by the NFPA, but basically they appear as in Figure 1. The test is performed on level ground. Two pitot gauges (calibrated for accuracy) are installed in the hose—one near the discharge gate and one near the nozzle. During testing, the difference between the pitot gauges measures FL in psi.

Static Test

Before conducting the FL test, run the pump to fill the hose and close the nozzle. The readings on the two pitot gauges should be the same; otherwise, there is an elevation d