Friction loss

Friction loss refers to that portion of pressure lost by fluids while moving through a pipe, hose, or other limited space. In mechanical systems such as internal combustion engines, it refers to the power lost overcoming the friction between two moving surfaces.

Causes

Friction loss has several causes, including:

• Frictional losses depend on the conditions of flow and the physical properties of the system.
• Movement of fluid molecules against each other
• Movement of fluid 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 are of two kinds: skin-friction and form-friction. The former is due to the roughness of the inner part of the pipe where the fluid comes in contact with the pipe material, while the latter is due to obstructions present in the line of flow--perhaps a bend, control valve, or anything that changes the course of motion of the flowing fluid.

Firefighting 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 at a quadratic 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, doubling the flow through a hose 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. Conversely, friction loss can restrict the distance which water can be lifted during fire department drafting operations.

External links

• Pipe pressure drop calculator for single phase flows.
• Pipe pressure drop calculator for two phase flows.
• Open source pipe pressure drop calculator.

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