BY KARL STAGGS
When it comes to supply-ing hoselines on the fireground, we all can handle the standard configurations. Most of us have pumped preconnects often enough or have standard operating guidelines that address our pump discharge pressures. When it comes to supplying nonstandard hoseline configurations, most of us rely on rules of thumb or have “cheat sheets” to determine our initial pressures until we have time to figure out actual friction loss (FL) for the setup.
One night when preparing to teach a hydraulics class at the local fire academy, I was reviewing a pump operator’s handbook and came across the hand method for 21/2-inch lines and later found a hand method for 13/4-inch lines in a different book. I could not find a similar method for three-inch lines. So I started to play with numbers and count fingers. I think my 10-year-old daughter thought that I had lost my mind, sitting at the kitchen table muttering about coefficients, Q-squared, and doing something similar to Itsy Bitsy Spider.
After some trial and error, here is the result. As with the hand method for the 21/2-inch line, starting with the thumb, the number at the base of each finger represents flow in hundreds of gallons per minute (gpm). The thumb is 1 (100 gpm), the index finger is 2 (200 gpm), the middle finger is 3 (300 gpm), and so forth. The half hundred gpm figures (i.e., 150, 250) are assigned to the spaces between the fingers.
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Starting again with the thumb, the tip of each finger is assigned a number. The thumb is 1, the index finger is 1.5, the middle finger is 2, the ring finger is 3, and the pinkie is 4.
To determine the friction loss per 100 feet of hose at the desired flow, select the finger with the desired flow, and multiply that number by the number at the tip of the same finger. For example, to determine the FL for a line flowing 400 gpm, you would use the numbers assigned to the ring finger (4) and (3): 4 2 3 = 12. Thus, for a line flowing 400 gpm, the FL is 12 psi per 100 feet.
Likewise, to determine the FL for a 200-gpm flow, use the numbers assigned to the index finger: 2 2 1.5 = 3. Thus, the resulting FL for a 200-gpm flow is 3 psi per 100 feet.
However, to determine the FL for half-hundred gallon flow, take the preceding lower whole-hundred-gallon flow number and multiply it by the tip number of the next higher finger. For example, for a flow of 250 gpm, multiply the preceding lower whole-hundred gpm (200 gpm, or 2) by the tip number of the next finger (2): 2 2 2 = 4. Thus, for a 250-gpm flow, the FL is 4 psi per 100 feet.
KARL STAGGS, a 10-year fire service veteran, is a firefighter with the Midland (TX) Fire Department and serves at Station 5, which specializes in rescue. He is an instructor in hydraulics and rope rescue at the Midland College Regional Fire Academy.
