Water Hydraulics - Wastewater Operators - page 64

(A) Piezometric surface (B) Piezometric surface

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HGL

HGL

Open valve

Closed valve

figure 2.10 Changes in head loss and piezometric surface when water
is flowing.

Water only rises to the water level of the main body of water when
it is at rest (static or standing water). The situation is quite different
when water is flowing. Consider, for example, an elevated storage tank
feeding a distribution system pipeline. When the system is at rest, with
all valves closed, all of the piezometric surfaces are the same height as
the free water surface in storage. On the other hand, when the valves are
opened and the water begins to flow, the piezometric surface changes.
This is an important point because, as water continues to flow down a
pipeline, less and less pressure is exerted. This happens because some
pressure is lost (used up) to keep the water moving over the interior sur-
face of the pipe (friction). The pressure that is lost is called head loss .

2.7.3.1 Head Loss

Head loss is best explained by example. Figure 2.10 shows an ele-
vated storage tank feeding a distribution system pipeline. When the valve
is closed (Figure 2.10A), all of the piezometric surfaces are the same
height as the free water surface in storage. When the valve opens and
water begins to flow (Figure 2.10B), the piezometric surfaces drop . The
farther along the pipeline, the lower the piezometric surface, because
some of the pressure is used up to keep the water moving over the rough
interior surface of the pipe. Thus, pressure is lost and is no longer avail-
able to push water up in a piezometer; this is the head loss.

2.7.3.2 Hydraulic Grade Line

When the valve shown in Figure 2.10B is opened, flow begins with
a corresponding energy loss due to friction. The pressures along the
pipeline can measure this loss. In Figure 2.10B, the difference in pres-
sure heads between sections 1, 2, and 3 can be seen in the piezometer
tubes attached to the pipe. A line connecting the
water surface in the tank with the water levels at sec-
tions 1, 2, and 3 shows the pattern of continuous
pressure loss along the pipeline. This is called the
hydraulic grade line (HGL) or hydraulic gradient of
the system.

Key Point: It is important to
point out that, in a static water
system, the HGL is always hori-
zontal. The HGL is a very useful
graphical aid when analyzing
pipe flow problems.