Page 148 - Fluid mechanics, heat transfer, and mass transfer

126 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS
TABLE 5.2 Additional Requirements for Different Pump Options

Option Details Additional Requirements
Impeller replacement with higher/lower diameter impeller of Possiblechange in drive; base frame; foundationrequiring additional
same or different design. space; change in speed; possible change in pump control system.
Addition of identical/lower/higher capacity pump parallel to Additional space; changes in drives, base frame, foundation; control
existing pump(s)/standby pumps of higher/lower capaci- system change; piping modifications; pump operation at different
ties for parallel operation. speed or with different impeller sizes; adding individual mini-
mum bypass system for each parallel pump.
Replacing existing pump(s) with pump(s) of higher capacity. Additional space; piping modifications; adding individual minimum
bypass system for each parallel pump; change in control system.
Replacing positive displacement pump with centrifugal Additional space; change in control system.
pump.

. What are the maintenance problems involved in pump & Operate at high speed so that they can be driven
performance and life? directly by electrical motors.
& Bent shafts and misalignment of the motor and pump & Give steady delivery, can handle slurries and take up
shafts. little floor space.
& If a bent shaft is installed into a pump and run, it will
Drawbacks
fail prematurely, leaving evidence and specific signs
on the circumference of close tolerance stationary & Single-stage pumps cannot develop high pressures
parts around the volute circle of the pump. The shaft except at very high speeds (10,000 rpm for instance).
will exhibit a wear spot on its surface where the close & Multistage pumps for high pressures are expensive,
tolerance parts were rubbing. A deflected shaft is particularly in corrosion-resistant materials.
absolutely straight when rotated in a lathe or dynamic & Efficiencies drop off rapidly at flow rates much
balancer. The deflection is the result of a problem different from those at peak efficiency.
induced either by operation or by system design. The & They are not self-priming and their performance
deflected shaft also will fail prematurely in the pump,
drops off rapidly with increasing viscosity.
leaving similar, but different evidence on the close
. “Thedischargelineofacentrifugalpumpcanbecomplete-
tolerance rubbing parts in the pump.
ly closed without damaging the pump.” True/False?
& Pump shaft and driver shaft alignments are very
& True. When discharge valve is closed, the fluid
important for long useful equipment life, and to
churns/recirculates inside the casing without ruptur-
extend the running time between repairs.
ing the discharge line although the pump overheats.
& Besides, good alignment reduces the progressive
. What are the difficult pumping applications that require
degradation of the pump.
highly demanding mechanical seals?
& One of the most important and least considered
& Crystallizing liquids.
points of correct alignment is the relationship with
& Solidifying liquids.
the power transmitted from the motor to the pump.
An almost perfect alignment with an adequate and & Vaporizing liquids.
new coupling transmits nearly 100% of the power of & Film forming liquids.
the motor, with some small losses. & High-temperature liquids.
. What are the symptoms and possible root causes for & Nonlubricating liquids.
centrifugal pump problems? & Dry-running the pump.
& Table 5.3 lists the symptoms and the possible causes
& Hazardous liquids.
for centrifugal pump problems.
& Gases and liquids that phase to gas.
. Give a summary of good and bad qualities of centrifugal
& Slurries.
pumps.
& Cryogenic liquids.
Good Qualities & High-pressure liquids.
& Simple inconstruction,inexpensive,availableina large & Vacuum conditions.
variety of materials and have low maintenance cost. & High-speed pumps.