Friction loss refers to the amount of energy that water loses as it travels through a hoseline, the couplings, and the nozzle. This loss of energy must be compensated for by the pump operator to ensure that the nozzles are being supplied with the correct operating pressure. In order to determine the amount of friction loss in a given hoseline, a mathematical formula is used.
Things You'll Need
 Calculator
 Water flow meter
 Friction loss coefficient table

The formula to determine friction loss is Q squared x c x L=FL. Friction loss is measured in pounds per square inch (psi). The following is an example of some of the values needed to determine friction loss: A fourinch hoseline at 150 feet long, flowing 200 gallons per minute.

Determine the amount of water being flowed per minute: The flow is measured in hundreds of gallons per minute (gpm). "Q" squared is represented in the example as 200 gpm. This value is always squared because friction loss varies with the square of the water flow. This is a constant that must be accounted for in this manner when determining this value.

Using a frictiton loss coefficient table, locate the appropriate coefficient based upon the diameter of the hose being used. The value representing "c" is .2. This is the standard coefficient for a fourinch hoseline.

Add up the total length of the hose being used in hundreds of feet. Inour example, 150 feet represents the value of "L" in the friction loss formula.

Replace the variables with the appropiate values and then multiply those values accordingly to determine the amount of friction loss in the hoseline.
=(400/100)2x.2x(150/100)
=(4)2 x .2 x 1.5
=(16) x .2 x 1.5
=4.8 psi
Based upon our example, the amount of friction loss would be 4.8 pounds of pressure per square inch.
Tips & Warnings
 Today, various equipment comes complete with flow meters that automatically compensate for the friction loss occurring in the line and only require the pump operator to set the pressure.
 The presence of valves or appliances in the hoseline will also affect the friction loss formula. If you are not using an automated flow meter, you should refer to the appliance manufacturer's specifications to determine the loss of water pressure of each of these appliances and factor them into your calculations.
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References
 Essentials of Firefighting and Emergency Response; T.R. Koonce, III; 2004