Page 136 - Fluid mechanics, heat transfer, and mass transfer
P. 136
114 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS
. What are the causes for pump suction cavitation? and to the local shock wave that occurs as each
& Clogged suction pipe. bubble collapses. Formation and collapsing of bub-
bles will alternate periodically with the frequency
& Suction line is too long.
resulting out of the product of speed and number of
& Suction line diameter is too small.
blades. Pump cavitation can produce various vibra-
& Suction lift is too high.
tion frequencies depending on the cavitation type,
& Valve on suction line is only partially open.
pump design, installation and use. The excessive
. What are the remedies to prevent suction cavitation? vibration caused by cavitation often subsequently
& Remove debris from suction line. causes a failure of the seal of the pump and/or
bearings. This is the most likely failure mode of a
& Move pump closer to source tank/sump.
cavitating pump.
& Increase suction line diameter.
. What types of pump damage occurs due to cavitation?
& Decrease suction lift requirement.
& Cavitation erosion or pitting:
& Install larger pump running slower that will decrease
➢ During cavitation, the collapse of the bubbles
the NPSH R by the pump.
occurs at sonic speed ejecting destructive mi-
& Increase discharge pressure.
crojets of extremely high velocity (up to
& Fully open suction line valve.
1000 m/s) liquid strong enough to cause extreme
. Under what circumstances cavitation occurs on the erosion of the pump parts, particularly impel-
discharge side of a pump? lers. The bubble is trying to collapse from all
& Discharge cavitation occurs when the pump dis- sides, but if the bubble is lying against a piece of
charge head is too high where the pump runs at or metal such as the impeller or volute it cannot
near shutoff. collapse from that side. So the fluid comes in
from the opposite side at this high velocity and
. What are the symptoms for pump discharge cavitation?
bangs against the metal, creating the impression
& High noise levels.
that the metal was hit with a ball pin hammer.
& High discharge gauge reading.
The resulting long-term material damage begins
& Low flow. to become visible by so-called pits that are
. What are the causes for pump discharge cavitation? plastic deformations of very small dimensions
& Clogged discharge pipe. (order of magnitude of micrometers). The dam-
age caused due to action of bubble collapse is
& Discharge line is too long.
commonly referred to as cavitation erosion or
& Discharge line diameter is too small.
pitting.
& Discharge static head is too high.
➢ Cavitation erosion from bubble collapse occurs
& Discharge line valve is only partially open.
primarily by fatigue fracture due to repeated
. What are the remedies to prevent pump discharge bubble implosions on the cavitating surface, if
cavitation? the implosions have sufficient impact force. The
& Remove debris from discharge line. erosion or pitting effect is quite similar to sand
& Decrease discharge line length. blasting. High head pumps are more likely to
suffer from cavitation erosion, making cavitation
& Increase discharge line diameter.
a high-energy pump phenomenon.
& Decrease discharge static head requirement.
➢ The most sensitive areas where cavitation ero-
& Install larger pump that will maintain the required
sion has been observed are the low pressure sides
flow without discharge cavitating.
of the impeller vanes near the inlet edge. The
& Fully open discharge line valve.
cavitation erosion damages at the impeller are
. How can one determinewhether the noise produced by a more or less spread out. The pitting has also been
pump is due to cavitation or bearing problem? observed on impeller vanes, diffuser vanes, im-
& Todistinguishbetweenthenoiseduetoabadbearingor peller tips, and so on. In some instances, cavi-
cavitation, operate the pump with no flow. The disap- tation has been severe enough to wear holes in
pearance of noise will be an indication of cavitation. the impeller and damage the vanes to such a
. What condition causes vibration during pump opera- degree that the impeller becomes completely
tion? Explain. ineffective.
& Vibration is due to the uneven loading of the impeller ➢ Near the outside edge of the impeller, pressure
as the mixture of vapor and liquid passes through it, builds to its highest point. This pressure im-
