Water Hydraulics - Wastewater Operators - page 74

hydraulic grade line (hgl) —Recall that the hydraulic grade line
(shown in Figure 2.15) is a line connecting two points to which the
liquid would rise at various places along any pipe or open channel if
piezometers were inserted in the liquid. It is a measure of the pres-
sure head available at these various points.
Note: When water flows in an open channel, the HGL coincides with the
profile of the water surface.
laminar and turbulent flow —Laminar flow is ideal flow, or water par-
ticles moving along straight, parallel paths in layers or streamlines.
Moreover, in laminar flow the water has no turbulence and there is
no friction loss. This is not typical of normal pipe flow because
the water velocity is too great, but it is typical of groundwater flow.
Turbulent flow (characterized as normal for a typical water system)
occurs when water particles move in a haphazard fashion and con-
tinually cross each other in all directions, resulting in pressure
losses along a length of pipe.
Slope (gradient) —Slope is the head loss per foot of channel.
Specific energy (e) —Sometimes called specific head , specific energy
is the sum of the pressure head ( y ) and the velocity head ( v 2 /2 g ).
The specific energy concept is especially useful in analyzing flow
in open channels.
Steady flow —Steady flow occurs when the discharge or rate of flow at
any cross-section is constant.
uniform and nonuniform flow —Uniform flow occurs when the depth,
cross-sectional area, and other elements of flow are substantially
constant from section to section. Nonuniform flow occurs when
the slope, cross-sectional area, and velocity change from section
to section. The flow through a Venturi section used for measuring
flow is a good example.
varied flow —Flow in a channel is considered varied if the depth of flow
changes along the length of the channel. The flow may be gradually
varied or rapidly varied (when the depth of flow changes abruptly)
as shown in Figure 2.16.
2.10.3 Major head loss
Major head loss consists of pressure decreases along the length of
pipe caused by friction created as water encounters the surfaces of the
pipe. It typically accounts for most of the pressure drop in a pressurized
or dynamic water system.
2.10.3.1 Components of Major Head Loss
The components that contribute to major head loss are roughness,
length, diameter, and velocit y.