Head Losses in Pipe Flow

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This complex piping network on an oil tanker has many components that result in pressure loss.  The pressure at the exit of a pipe run will be less than that at the inlet due to these accumulated losses. 

By Hervé Cozanet (Tour on the ship) [GFDL, CC-BY-SA-3.0 or CC-BY-SA-2.0-fr], via Wikimedia Commons

Pressure or head losses in piping systems are due to the viscosity of the fluid, elevation changes, changes in diameter, valves, elbows, etc. Pressure losses can be substantial and engineers responsible for the design of a piping system takes these losses into account during the design process.

Pressure losses are frequently referred to as head loss.  This terminology comes from the units of the terms in Bernoulli’s equation.  One form of the equation has units of pressure while another equivalent form has units of distance or elevation.  Whether expressed in terms of pressure or head, the equations provide the same information.

Head losses can be classified into Major and Minor Losses.  Major losses are associated with the viscosity and associated shear stresses in the fluid.  These losses are often called frictional losses because of the shear stresses acting along the wall of the pipe.  Topics associated with major losses that are included in this chapter include frictional pressure losses, Darcey friction factor, Moody diagram, and the augmented Bernoulli’s equation.

Minor losses are associated with diameter changes, turning corners, etc.  Depending on the specific circumstances, the magnitude of minor losses can be of major importance.  Minor losses result from two or three-dimensional flow phenomena that exist within the pipe.  In this chapter, examples of these types of losses and the flow fields associated with them are discussed for sudden expansions and contractions.

 

Learning Objectives

Learn mechanisms that result in pressure loss.

Frictional Pressure Losses

Frictional Pressure Losses

Frictional losses in pipes are due to the viscosity of the fluid.

Darcy Friction Factor

The pressure drop is proportional to the length to diameter ratio of the pipe.

Moody Diagram

Moody Diagram

Augmented Bernoulli Equation

Augmented Bernoulli Equation

Expansion and Contraction Losses

Expansion and Contraction Losses