FRICTION LOSS FOR 1 3/4-INCH HOSE
For some years now the 1 3/4-inch handline has been working its way into the fire service as a replacement for the ever-faithful 1 1/2inch line. The 1 3/4-inch has some advantages over the 1 1/2-inch line, among them larger gpm flows, less friction loss, and less pump discharge pressure needed.
However, to the best of my knowledge there hasn’t been a fast and easy way to calculate friction loss in 1¾inch hose. Usually the pump operator either memorizes the friction loss for each gpm he’s flowing or carries a calculator with him. Here’s an easy-to remember method that I’ve developed and found to be accurate.
Turn the palm of your left hand up as you would for calculating friction loss in 2 1/2-inch hose. Assign numbers in order to your finger tips, with the thumb tip being number 1 and the pinky tip number 5. These numbers represent factors only, not gpm flows as they do in the rule of thumb for 2 1/2inch hose. Now number the base of your fingers, again starting with the thumb and working your way to the little finger: The base of the thumb gets the number 12 and the bases of all the other fingers get the number 9. Again, these numbers represent factors and not gpm flows. The thumb represents 100 gpm, the index 125 gpm, the middle 150 gpm, the ring 175 gpm, and the little finger 200 gpm.
To calculate the friction loss in a 100-foot section of 1 3/4-inch line, go to the finger that corresponds to the flow you are using and multiply the number at the tip with the number at the base of the same finger. For example, if you’re going to flow 150 gpm, multiply the number at the tip of your middle finger (3) with the number at the base (9). The friction loss in a 100-foot section of 1 3/4-inch line flowing 150 gpm is 27 psi.
Remember, though, that rules of thumb and formulas are not 100 percent accurate for any size hose. For greater accuracy in determining friction loss in the hose your department uses, run some tests on your own and then apply this five-finger method.