Environmental Engineering Reference
In-Depth Information
2.1 WhaT is WaTer hydrauliCs?
The word “hydraulic” is derived from the Greek words hydro
(“water”) and aulis (“pipe”). Originally, hydraulics referred only to the
study of water at rest and in motion (the flow of water in pipes or chan-
nels). Today, it is taken to mean the flow of any liquid in a system. What
is a liquid? In terms of hydraulics, a liquid can be either oil or water.
In the fluid power systems used in modern industrial equipment, the
hydraulic liquid of choice is oil.
Some common examples of hydraulic fluid power systems include
automobile braking and power steering systems, hydraulic elevators,
and hydraulic jacks or lifts. Probably the most familiar hydraulic fluid
power systems in water/wastewater operations are those used on dump
trucks, front-end loaders, graders, and earth-moving and excavation
equipment. In this text, though, we are concerned with liquid water.
Many find the study of water hydraulics difficult and puzzling (espe-
cially the licensure examination questions), but it is not all that mys-
terious in that it involves practical applications of the basic principles
of water physics. Because water/wastewater treatment is based on the
principles of water hydraulics, concise, real-world training is necessary
for operators who must operate the plant and for those sitting for state
licensure/certification examinations.
2.2 basiC ConCePTs
Air Pressure (at sea level) = 14.7 pounds per square inch (psi)
The relationship shown above is important because our study of
hydraulics begins with air. A blanket of air, many miles thick, surrounds
the Earth. The weight of this blanket on a given square inch of the Earth's
surface will vary according to the thickness of the atmospheric blanket
above that point. At sea level, the pressure exerted is 14.7 pounds per
square inch (psi). On a mountaintop, air pressure decreases because the
blanket is not as thick.
The relationship shown below is also important:
1 ft 3 of water = 62.4 lb
Note that both cubic feet and pounds are used to describe a volume of
water. A defined relationship exists between the two units of measure-
ment. The specific weight of water is defined relative to a cubic foot.
One cubic foot of water weighs 62.4 lb; however, this relationship is true
only at a temperature of 4°C and at a pressure of one atmosphere, con-
ditions referred to as standard temperature and pressure (STP). One
atmosphere = 14.7 psi at sea level and 1 ft 3 of water contains 7.48 gal-
lons. The weight varies so little that, for practical purposes, this weight
is used for temperatures ranging from 0 to 100°C.
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