Science of Water : Concepts and Applications

66 The Science of Water: Concepts and Applications
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must have the energy to provide this lift. A pump is said to be in a suction lift condition any
time the center (eye) of the impeller is above the water being pumped (see Figure 3.13).
Suction head—A pump is said to be in a suction head condition any time the center (eye)
of the impeller is below the water level being pumped. Specifi cally, suction head is the distance
between the suction water level and the center of the pump impeller when the pump
is in a suction head condition (see Figure 3.13).
Velocity head—The amount of energy required to bring water or wastewater from standstill
to its velocity. For a given quantity of fl ow, the velocity head will vary indirectly with the
pipe diameter. Velocity head is often shown mathematically as V 2 /2g (see Figure 3.13).
Total dynamic head—The total energy needed to move water from the centerline of a
pump (eye of the fi rst impeller of a lineshaft turbine) to some given elevation or to develop
some given pressure. This includes the static head, velocity head, and the head loss due to
friction (see Figure 3.13).
WELL AND WET WELL HYDRAULICS
When the source of water for a water distribution system is from a groundwater supply, knowledge
of well hydraulics is important to the operator. Basic well hydraulics terms are presented and
defi ned, and they are related pictorially (see Figure 3.14). Also discussed are wet wells, which are
important in both water and wastewater operations.
WELL HYDRAULICS
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Static water level—The water level in a well when no water is being taken from the groundwater
source (i.e., the water level when the pump is off; see Figure 3.14). Static water level
is normally measured as the distance from the ground surface to the water surface. This is
an important parameter because it is used to measure changes in the water table.
Pumping water level—The water level when the pump is off. When water is pumped out of
a well, the water level usually drops below the level in the surrounding aquifer and eventually
stabilizes at a lower level; this is the pumping level (see Figure 3.14).
Drawdown—The difference, or the drop, between the static water level and the pumping
water level, measured in feet. Simply stated, it is the distance the water level drops once
pumping begins (see Figure 3.14).
Cone of depression
FIGURE 3.14 Hydraulic characteristics of a well.
Discharge
Ground surface Pump
Static water level
Zone of influence
Drawdown
Pump water level