Civil Engineering Reference
In-Depth Information
V
2
H
K
(26-8)
L
L
2
g
where:
H
L
headloss, ft (m)
K
L
headloss coefficient, dimensionless
V
velocity, ft / sec (m / s)
g
Acceleration of gravity
32.174 ft / sec
2
(9.82 m / s
2
)
Table 26-3 lists typical values of losses for a variety of hydraulic components in terms
of velocity head. These losses are in addition to pipe friction losses.
A simple example can be used to illustrate the concepts based on a typical piping
system to a clarifier, as shown in Figure 26-4.
Entrance
The entrance configuration is commonly a pipe in a tank wall, such as a reservoir
outlet box, chemical mixing box, flow split box, junction box, and so forth. The intake
may be a sluice gate, a simple sharp-edged entrance, or a streamlined bell-mouthed
entrance. The designer must decide which type to use, depending upon upstream head,
the importance and cost of headloss, velocities, and downstream condition. If headloss
is critical and must be kept to a minimum, the radius of the bell-mouthed entrance
should be
D
/ 7, with
D
diameter of the pipe downstream.
The entrance may include a sluice gate or slide gate that is used for isolation of
downstream units. For example, if the gate is large enough that it does not constrict
the opening, the headloss coefficient
(K
L
) can be estimated to be approximately 2.0
times the velocity head. If the sluice gate is partially submerged, the
K
L
value will be
approximately 2.5.
The values of
K
L
vary within the published literature, and median values are shown
in Table 26-3. The designer should adopt a set of
K
L
values for each condition and
use them in a consistent manner.
Increasing and Reducing Components
Headloss in pipes that are enlarged or expanded can be approximated by:
V
2
V
2
H
K
1
2
(26-9)
L
L
2
g
where:
V
1
velocity at the entrance to the enlargement, ft / sec (m / s)
V
2
velocity at the exit from the enlargement, ft / sec (m / s).
In this case, values for
K
L
depend on the expansion rate, as shown in Figure 26-5.
(Fig. 26-5 also includes coefficients for sudden contractions.)
