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Darcy’s Equation | Up
Thermodynamics Heat Transfer and Fluid Flow Volume 3 of 3 | Next
TABLE 1 Typical Values |
HEAD LOSS
Fluid Flow
Minor Losses
The losses that occur in pipelines due to bends, elbows, joints, valves, etc. are sometimes called
minor losses. This is a misnomer because in many cases these losses are more important than
the losses due to pipe friction, considered in the preceding section. For all minor losses in
turbulent flow, the head loss varies as the square of the velocity. Thus a convenient method of
expressing the minor losses in flow is by means of a loss coefficient (k). Values of the loss
coefficient (k) for typical situations and fittings is found in standard handbooks. The form of
Darcy’s equation used to calculate minor losses of individual fluid system components is
expressed by Equation 3-15.
(3-15)
Hf
kv
2
2g
Equivalent Piping Length
Minor losses may be expressed in terms of the equivalent length (Leq) of pipe that would have
the same head loss for the same discharge flow rate. This relationship can be found by setting
the two forms of Darcy’s equation equal to each other.
f
L v2
D 2 g
k
v2
2 g
This yields two relationships that are useful.
(3-16)
Leq
kD
f
(3-17)
k
f
Leq
D
Typical values of Leq/D for common piping system components are listed in Table 1. The
equivalent length of piping that will cause the same head loss as a particular component can be
determined by multiplying the value of Leq/D for that component by the diameter of the pipe.
The higher the value of Leq/D, the longer the equivalent length of pipe.
HT-03
Page 34
Rev. 0
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