Lesson 23HEAD LOSS

• DEFINE the terms head loss, frictional loss, and minor losses.

• DETERMINE friction factors for various flow situations using the Moody chart.

• CALCULATE the head loss in a fluid system due to frictional losses using Darcy’s equation.

• CALCULATE the equivalent length of pipe that would cause the same head loss as the minor losses that occur in individual components.

Head Loss

• A measure of the reduction in the total head (sum of elevation head, velocity head and pressure head) of the fluid as it moves through a fluid system.

• Unavoidable in real fluids.

• Caused by • Friction between the fluid and the walls of the pipe• Friction between adjacent fluid particles as they move relative to one another;• Turbulence caused whenever the flow is redirected

• Frictional loss - That part of the total head loss that occurs as the fluid flows through straight pipes.

• Head loss is directly proportional to – the length of pipe, – the square of the fluid velocity,– the friction factor.

• The head loss is inversely proportional to the diameter of the pipe

D

Lv f Loss Head

2

Friction Factor• Depends on – Reynolds number– Relative roughness of the pipe’s inner surface

• Obtained from the Moody Chart

D Roughness lativeRe

Darcy’s Equation

• Used to calculate frictional head loss

• Two forms– Losses in the system associated with the length of

the pipe.– “Minor” losses

Frictional Head Loss

where:

f = friction factor (unitless)L = length of pipe (ft)D = diameter of pipe (ft)v = fluid velocity (ft/sec)g = gravitational acceleration (ft/sec2)

g2D

LvfH

2

f

Minor Losses

• Losses in pipelines due to – Bends,– Elbows– Joints– Valves

• Often more important than the losses due to pipe friction• For turbulent flow, the head loss varies as the square of the velocity. • Expressed by means of a loss coefficient (k)

g2

vkH

2

f

Equivalent Pipe Length• Found by setting the two forms of Darcy’s

Equation equal to each other and solving for L to get

• Can also solve for k to get

f

DkLeq

D

Lfk eq