Science of Water : Concepts and Applications

60 The Science of Water: Concepts and Applications
as the free water surface in the vessel, just as it would if a drinking straw (the piezometer) were left
standing in a glass of water.
When pressurized in a tank and pipeline system, as they often are, the pressure will cause the
piezometric surface to rise above the level of the water in the tank. The greater the pressure, the
higher the piezometric surface (see Figure 3.9). An increased pressure in a water pipeline system is
usually obtained by elevating the water tank.
√ Important Point: In practice, piezometers are not installed on pipelines or on water towers
because water towers are hundreds of feet high. Instead, pressure gauges are used that record
pressure of water in feet or in pounds per square inch.
Water only rises to the level of the main body of water when it is at rest (static or standing water).
The situation is quite different when water is fl owing. Consider, for example, an elevated storage
tank feeding a distribution system pipeline. When the system is at rest, all valves closed, all the
piezometric surfaces are the same height as the free water surface in storage. In contrast, when the
valves are opened and the water begins to fl ow, the piezometric surface changes. This is an important
point because as water continues to fl ow down a pipeline, less and less pressure is exerted. This
happens because some pressure is lost (used up) keeping the water moving over the interior surface
of the pipe (friction). The pressure that is lost is called head loss.
HEAD LOSS
Head loss is best explained by an example. Figure 3.10 shows an elevated storage tank feeding a
distribution system pipeline. When the valve is closed (Figure 3.10a), all the piezometric surfaces
are the same height as the free water surface in storage. When the valve opens and water begins
to fl ow (Figure 3.10b), the piezometric surfaces drop. The farther along the pipeline, the lower the
piezometric surface, because some of the pressure is used up keeping the water moving over the
rough interior surface of the pipe. Thus, pressure is lost and is no longer available to push water up
in a piezometer; this is the head loss.
HYDRAULIC GRADE LINE
Pressure applied
Piezometric surface
FIGURE 3.9 A container under pressure where the piezometric surface is above the level of the water in the
tank.
When the valve is opened in Figure 3.10, fl ow begins with a corresponding energy loss due to
friction. The pressures along the pipeline can measure this loss. In Figure 3.10b, the difference in
pressure 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 sections 1–3 shows the
pattern of continuous pressure loss along the pipeline. This is called the hydraulic grade line (HGL)