An automatic alternator
High-water alarm
Motor switches
Automatic starters
The centrifugal type of sewage pump has
nonclogging impellers. It can be installed in either a
horizontal configuration or in a vertical configuration
with a suction lift. The centrifugal pump can be placed
in either a wet or dry pit and is equipped with a float or
diaphragm for automatic operation.
ADVANTAGES AND DISADVANTAGES OF
CENTRIFUGAL PUMPS.The advantages of
centrifugal pumps include simplicity, compactness,
weight saving, and adaptability to high-speed prime
movers.
One disadvantage of centrifugal pumps is
their relatively poor suction power. When the pump
end is dry, the rotation of the impeller, even at high
speeds, is simply not sufficient to lift liquid into the
pump; therefore, the pump must be primed before
pumping can begin. For this reason, the suction lines
and inlets of most centrifugal pumps are placed below
the source level of the liquid pumped. The pump can
then be primed by merely opening the suction stop
valve and allowing the force of gravity to fill the pump
with liquid. The static pressure of the liquid above the
pump also adds to the suction pressure developed by
the pump while it is in operation. Another dis-
advantage of centrifugal pumps is that they develop
CAVITATION. Cavitation occurs when the velocity
of a liquid increases to the point where the consequent
pressure drop reaches the pressure of vaporization of
the liquid.
When this happens, vapor pockets, or
bubbles, form in the liquid and then later collapse when
subjected to higher pressure at some other point in the
flow. The collapse of the vapor bubbles can take place
with considerable force. This effect, coupled with the
rather corrosive action of the vapor bubbles moving at
high speed, can severely pit and corrode impeller
surfaces and sometimes even the pump casing. In
extreme instances, cavitation has caused structural
failure of the impeller blades. Whenever cavitation
occurs, it is frequently signaled by a clearly audible
noise and vibration (caused by the violent collapse of
vapor bubbles in the pump).
Several conditions can cause cavitation, not the
least of which is improper design of the pump or
pumping system. For example, if the suction pressure
is abnormally low (caused perhaps by high suction lift
or friction losses in the suction piping), the subsequent
pressure drop across the impellers may be sufficient to
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reach the pressure of vaporization. A remedy might be
to alter the pump design by installing larger piping to
reduce friction loss or by installing a foot valve to
reduce suction lift.
Cavitation can also be caused by improper
operation of the pump. For instance, cavitation can
occur when sudden and large demands for liquid are
made upon the pump. As the liquid discharged from
the pump is rapidly distributed and used downstream, a
suction effect is created on the discharge side of the
pump. Think of it as a pulling action on the discharge
side that serves to increase the velocity of the liquid
flowing through the pump. Thus, as the pressure head
on the discharge decreases, the velocity of the liquid
flowing across the impellers increases to the point
where cavitation takes place. Perhaps the easiest way
to avoid this condition is to regulate the liquid demand.
If this is not possible, then increase the suction
pressure by some means to maintain pressure in the
pump under these conditions.
OPERATION AND MAINTENANCE OF
CENTRIFUGAL PUMPS.The
operating
procedures and maintenance schedules for
centrifugal pumps are generally similar to those of the
other pumps we have discussed previously.
Centrifugal pumps are also fitted with stuffing boxes
and various types of bearings that, of course, require
periodic maintenance and inspection. Always refer to
the manufacturers instructions and locally prepared
maintenance schedules for operating and
maintenance procedures.
One operating practice is common to nearly all
types of centrifugal pumps. Unlike positive displace-
ment pumps, the discharge stop valve on centrifugal
pumps must. be CLOSED before starting the pump.
This action allows the pump to work against the sealed
discharge and builds up an effective pressure head
before attempting to move and distribute the liquid
downstream. After the pump is up to speed and the
discharge valve is opened, the pump continues to
maintain that pressure head unless the operating
conditions are altered. Note that there is no danger of
hydraulicking while the pump is run with the discharge
closed. If the centrifugal pump were to continue
operation with the discharge sealed, it would simply
build up toward its maximum discharge pressure. It
would then begin to churn the liquid; that is, the
discharge pressure would overcome the suction
pressure and the liquid would continually slip back to
the suction side of the pump. Nothing more would
happen, except the pump would build up heat, since the
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