Originally Posted by

**KuhShise**
Basic nozzle flow equation, where Q = flow(GPM), d = diameter of smooth bore nozzle(inches), Np = nozzle pressure(PSI)

Q = 29.87 x d x d x Sq. Rt. of Np

For a 1” tip operating at 50 psi nozzle pressure the flow will be:

Q = 29.87 x 1 x 1 x Sq. Rt. of 50 = 29.87 x 7.07 = 211 gpm.

Cross check your knowledge with a calculator using ¾”, 7/8” and 15/16” smooth bores and you should find the following: 130, 162, and 186 gpm.

Applying these flows to the 250 ft. layout of 1 ¾” hose you described…..

The “K” formula looks like this: Fl = K x Q x Q x L

Where: Fl is the pressure drop through the hose, K is a factor for the diameter of the hose(12 for 1 ¾”), Q is the flow in 100’s of GPM’s or flow / 100, and L is the number of 100 ft. joints of hose.

For the 1” smooth bore with a flow of 211 gpm, the loss in the hose will be:

Fl = 12 x 2.11 x 2.11 x 2.5 = 133 psi

Then the Engine pressure for a 1” smooth bore nozzle on a 250 ft. length of 1 ¾” hose should be:

Ep = Fl + Np or 133 + 50 = 183psi

Do the calculations for each of the other sizes and you should find engine pressures of: 100, 128 and 154 psi for the above sizes of smooth bore nozzles. There are some other things going on inside most pump panels. I suggest you open the inspection door to your pump compartment, get a hand light and begin to trace some piping on your preconnect lines. Each time you see a 90 degree elbow, imagine that you see an additional 15 feet of piping. I have seen some plumber’s nightmares over the years, and it is not unusual to need engine pressures that are from 15 to 40 psi higher than the calculations for hose alone. As others have suggested, try to borrow a flow meter and measure the actual flow at the calculated pressure. If it is not adequate, raise the engine pressure until you flow the proper amount from each line. Then prepare a sheet with the correct discharge pressures, and tape it to the pump panel as a reminder to the pump operator.

Combination nozzles (unless low pressure) require 100 psi at the base of the nozzle. If they are selectable gallonage, then use the set gallonage to figure the friction loss in the hose line. Add this to the standard nozzle operating pressure to get the correct engine pressure needed.

Automatic nozzles present a unique set of problems for firefighters and pump operators. Because of the automatic adjustment of the nozzle baffle, the stream will always have a good looking arc and reach, but might not open enough to allow the proper flow to reach the fire. Automatic nozzles need to be periodically tested for performance, cleaned and lubricated. In the last annual test conducted on 39 nozzles there were eight (8) that were not properly functioning. If using TFT nozzles, I would urge you to go the their web site and read the information on how to maintain and lubricate their automatic nozzles.