Environmental Engineering Reference
In-Depth Information
Figure 9.4
Cavitation damage near the outlet side of a globe valve
3 MINOR LOSSES
Flow passing through valves, orifices, elbows, transitions, and other pipe fittings
generates an increase in turbulent eddies. Head loss associated with the dissi-
pation of the turbulent eddies is referred to as minor loss. The term
minor loss
can be somewhat of a misnomer in some cases such as short piping systems
where the friction loss is relatively small and the minor losses are responsible
for the majority of the system head loss. Minor losses are represented as a loss
coefficient (
K
) multiplied by the appropriate pipe velocity head:
K
V
2
2
g
Q
2
2
gA
2
m
H
m
=
=
K
(11)
In equation (11),
A
m
is the pipe flow area at the inlet of the minor loss element.
Minor loss coefficients are generally available in most hydraulic engineering
reference topics (Miller 1990). Some minor loss coefficient data are presented
in Table 9.2. The loss coefficients for fittings vary with material, manufacturer's
method of fabrication, and installation. Minor loss coefficients for tees also vary
with the flow direction and percent flow distribution between the branches.
The total system head loss (
H
l
) can be calculated using the following equation
provided the appropriate friction and minor loss coefficients are used.
fL
D
2
gA
p
+
Q
2
K
2
gA
2
m
=
CQ
2
H
L
=
(12)
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