Friction loss

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

[edit] 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

[edit] 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.

[edit] 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)