Friction loss

Friction loss refers to that portion of pressure lost by fluids while moving through a pipe, hose, or other limited space.

Causes

Friction loss has several causes, including:

*Frictional losses depend on the conditions of flow and the physical properties of the system.
* Movement of water molecules against each other
* Movement of water molecules against the inside surface of a pipe or the like, particularly if the inside surface is rough, textured, or otherwise not smooth
* Bends, kinks, and other sharp turns in hose or piping

In pipe flows the losses due to friction is of two kind first the skin-friction and the other is form-friction, the former one is due to the roughness in the inner part of the pipe where the fluid comes in the contact of the pipe material and the latter one is due to the obstructions present in the line of flow, it may be due to a bend or a control valve or anything which changes the course of motion of the flowing fluid.

Applications

While friction loss has multiple applications, one of the most common is in the realm of firefighting. With the advent of modern power-takeoff (PTO) fire pumps, pressures created can sometimes overwhelm the ability of water to flow through a hose of a given diameter. As the velocity of water inside a hose increases, so does the friction loss. This resulting increase occurs as an exponential rate, thus an increase in the flow by a factor of X will result in an increase in friction loss by a factor of X². For example, if you double the flow you will quadruple the friction loss. Ultimately, as the pressure created by a fire pump goes higher and higher the amount of water actually flowing through a hose to a given point lessens, threatening firefighting operations.

Formulas

The formula used most often in firefighting to express the amount of friction loss is:

FL = CQ²L

Where FL = friction loss (expressed in psi)C = coefficient of friction (based on the inside diameter of the hose and the inside jacket material)Q = flow rate in hundreds of gallons (gpm/100)L = Length of hose in hundreds of feet (L/100)