The Importance of Friction Loss and Flow Testing

Issue 9 and Volume 23.

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Another reason measurements are so important is that standard industry estimates for FL calculations are outdated and seriously overstated.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 difference. Use this difference to adjust the FL measurements.

FL and Flow Test

Now open the nozzle and increase the PDP until the pitot gauge at the nozzle reaches a predetermined pressure—for example, 40 psi. Read the pressure of the pitot gauge at the discharge gate and record the numbers. The difference is the FL at an NP of 40 psi. Repeat the test for NPs of 50, 60, and 70 psi and record the results. Also use a flowmeter to measure volume flow at each NP.

Next calculate the theoretical volume flow (gpm) based on the equation: gpm = 29.72 × Dt2 × NP1/2 where Dt is the diameter of the nozzle tip in inches. If the measuring instruments are accurate, a comparison of measured flow to theoretical flow should be very close. We now have all the data needed for a simplified pump chart. By selecting a combination of gpm and NR, a pump chart gives you the required nozzle and discharge pressure. Don’t assume that the same type of hose on another discharge port has the same flow rate and friction loss. Test the other hose, and if the results are the same, you will have more confidence in the manufacturer’s quality control. You can easily interpolate between various pressures. Also, for 100 feet, the FL would be one half of that in the table. You can also calculate the table for a 15⁄16-inch nozzle tip. You will get more volume at a higher NR. For fog/combination nozzles, the gpm and nozzle reaction can be accurately calculated using an algorithm I have developed.2

Resources

1. Fire Protection Research Foundation, “The Determination of Fire Hose Friction Loss Characteristics”; October, 2013.

2. Hawkins, L.P., “A Review of Research into Firematic Operations”; October, 2017.

LAWRENCE P. HAWKINS is a retired businessman and volunteer firefighter. He served as second assistant chief with the Continental Village (NY) Fire Department and is currently an exterior firefighter with the Chelsea (NY) Fire Company.