Page 160 - Fluid mechanics, heat transfer, and mass transfer

138 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

& With magnetic drive pumps, however, the bearings . What are the advantages of vertical pumps?
on the impeller shaft are lubricated by the fluid that & Liquid level is above the impeller and thus pump is
may not be an appropriate lubricant. In addition, self-priming.
when the pump runs dry, or operates at very low
& Shaft seal is above liquid level and so not wetted by
flows, the lubricant tends to disappear and the bear-
pumped liquid with sealing task simplified.
ings will overheat. Any solids in the fluid will be
& Used for large capacity applications.
detrimental to the bearing reliability.
& Vertical submersible turbine pumps are used as pipe-
& Magnets are also temperature sensitive and will de-
line boosters as well as for deep well service.
magnetize if exposed to temperatures exceeding their
. What are the disadvantages of vertical pumps?
upper limit.Anyupsetconditions,such asrunning the
pump dry or against a closed discharge valve. & Require intermediate or line bearings when shaft
length is more than three meters, to avoid shaft
& To provide some degree of protection against this
resonance problems.
problem, the material of the magnets should be

selected to be able to handle 12–25 C above the & These bearings must be lubricated.
expected maximum operating temperature. & Since all wetted parts must be corrosion-resistant,
& One application for such pumps is use in liquid metal low-cost materials may not be suitable for shaft,
pumping in nuclear power plants. column, and so on.
. What is the disadvantage of magnetic drive pumps? & Costly maintenance since pump is larger and more
difficult to handle.
& Low efficiencies due to internal heat generation.
. What is a turbine pump?
& Load for refrigeration applications will be high be-
cause of heat generation characteristics. & This uses mixed flow-type impellers, giving partly
axial and partly centrifugal flows.
. Give a comparison of magnetic drive pumps and canned
motor pumps. & These pumps are usually vertical pumps.
& Table 5.6 presents a comparison of magnetic drive & Turbine pumps are characterized as having bearings
and canned motor pumps. lubricated with the pumped liquid.
. What is a vertical pump? & These pumps are popular in multistage construction.
& Pump with a vertical shaft, having a minimum length & The impellers discharge into a vertical support col-
of 20 m, from drive end to impeller. umn housing the rotating shaft.

TABLE 5.6 Comparison of Magnetic Drive and Canned Motor Pumps
Selection Parameter Magnetic Drive Pump Canned Motor Pump
Safety on failure Has only one sealed liner and if it ruptures, fluid Two boundaries exist, the can and the motor
escapes into the atmosphere housing. If the can ruptures, the motor
housing takes over as a barrier
Viscosity Has limitation for use with high fluid viscosities Fluid is warmed inside the motor section al-
lowing the pumping of higher viscosity liquid

Temperature Limited to applications between 35 and 400 C Can be used at temperatures between 130 and
540 C

Maximum operating pressures Can thickness limits the maximum pressure Pressures are independent of the can thickness
because support can be furnished outside of
the gap
NPSH R Better NPSH characteristics Poorer NPSH characteristics due to warm up of
liquid inside pump
Sensitivity to solids More sensitive to solids, especially with ferrous Available with slurry designs, which isolate the
solids in the fluid motor section from the pumped fluids
Noise levels Higher due to fan on the motor and additional Lower. Especially quiet operation
bearings in the couplings
Starting problems Extreme care must be taken in applying the torque Easy starting
requirements to these units
Cost Higher Lower
Cost of repair Less More
Foundation Required Not required