Environmental Engineering Reference
In-Depth Information
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operation
—”Simple and quiet” best describes the operation of a
centrifugal pump. An operator-in-training with a minimum amount
of experience may be capable of operating facilities that use cen-
trifugal-type pumps. Even when improperly operated, the rugged
construction of the centrifugal pump allows it to operate (in most
cases) without major damage.
•
maintenance
—The amount of wear on the moving parts of a cen-
trifugal pump is reduced and its operating life is extended because
its moving parts are not required to be constructed to very close
tolerances.
•
Self-limited pressure
—Because of the nature of its pumping action,
the centrifugal pump will not exceed a predetermined maximum
pressure. Thus, if the discharge valve is suddenly closed, the pump
cannot generate additional pressure that might result in damage
to the system or could potentially result in a hazardous working
condition. The power supplied to the impeller will only generate a
specified amount of head (pressure). If a major portion of this head
or pressure is consumed in overcoming friction or is lost as heat
energy, the pump will have a decreased capacity.
•
adaptable to high-speed drive systems
—Centrifugal pumps can
make use of high-speed, high-efficiency motors. In situations where
the pump is selected to match a specific operating condition that
remains relatively constant, the pump drive unit can be used with-
out the need for expensive speed reducers.
•
Small space requirements
—For most pumping capacities, the
amount of space required for installation of the centrifugal-type
pump is much less than that of any other type of pump.
•
fewer moving parts
—The rotary rather than reciprocating motion
employed in centrifugal pumps reduces space and maintenance
requirements due to the fewer number of moving parts required.
Although the centrifugal pump is one of the most widely used pumps, it
does have a few disadvantages:
•
additional equipment is needed for
priming
—The centrifugal pump
can be installed in a manner that will make it self-priming, but it is
not capable of drawing water to the pump impeller unless the pump
casing and impeller are filled with water. This can cause problems,
because if the water in the casing drains out the pump ceases
pumping until it is refilled; therefore, it is normally necessary to
start a centrifugal pump with the discharge valve closed. The valve
is then gradually opened to its proper operating level. Starting the
pump against a closed discharge valve is not hazardous provided
the valve is not left closed for extended periods.
•
air leaks affect pump performance
—Air leaks on the suction side
of the pump can cause reduced pumping capacity in several ways. If
the leak is not serious enough to result in a total loss of prime, the
pump may operate at a reduced head or capacity due to air mixing
with the water. This causes the water to be lighter than normal and
reduces the efficiency of the energy transfer process.
