Page 137 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 115

plodes the gas bubbles, changing the state of under an earlier question, making the metal sur-
the liquid from gas into liquid. When cavitation face permanently activated for the chemical at-
is less severe, the damage can occur further tack. Thus, in this way even in case of slight
down toward the eye of the impeller. A careful cavitation it may lead to considerable damage to
investigation and diagnosis of point of the im- the materials. The rate of erosion may be accen-
peller erosion on impeller, volute, diffuser, and tuated if the liquid itself has corrosive tendencies
so on can help predict the type and cause of such as water with large amounts of dissolved
cavitation. oxygen to acids.
➢ The extent of cavitation erosion or pitting de- . Summarize the most severe damaging effects of pump
pends on a number of factors such as presence of cavitation.
foreign materials in the liquid, liquid tempera- & Pitting marks on the impeller blades and on the
ture, age of equipment, and velocity of the internal volute casing wall of the pump.
collapsing bubble. & Premature bearing failure.
➢ Figure 5.10 is a photograph of blade damage due to
& Premature mechanical seal failure.
cavitation of a mixed flow pump impeller.
& Shaft breakage and other fatigue failures in the
& Mechanical deformations: Apart from erosion of
pump.
pump parts, in larger pumps, longer duration of
. Suggest a remedy to suppress cavitation in a centrifugal
cavitation condition can result in unbalancing (due
pump.
to unequal distribution in bubble formation and
& Add a bleed system: An effective method to suppress
collapse) of radial and axial thrusts on the impeller.
cavitation in centrifugal pumps is to bleed some of
This unbalancing often leads to following mechan-
the high-pressure fluid from the pump discharge back
ical problems:
to the suction side, through essentially tangential-
➢ Bending and deflection of shafts.
entry nozzles that give the fluid a rotating motion
➢ Bearing damage and rubs from radial vibration.
before entry into the impeller eye.
➢ Thrust bearing damage from axial movement.
. “Cavitation problems are more severe for water com-
➢ Breaking of impeller check-nuts. pared to hydrocarbon liquids.” True/False? Comment.
➢ Seal faces damage, and so on. & True. Specific volume of hydrocarbon vapors is very
& These mechanical deformations can completely small compared to that of water vapor. Water occu-
wreck the pump and require replacement of parts. pies 1600 times its volume on vaporization whereas
The cost of such replacements can be huge. hydrocarbon liquids on vaporization occupy much
less volumes. Volume reduction on implosion of
& Cavitation corrosion: Frequently, cavitation is
vapor bubbles is very large for water creating much
combined with corrosion. The implosion of bub-
higher destructive forces.
bles destroys existing protective layers, as stated
. “Flow restrictions can cause cavitation damage.” True/
False? Illustrate with an example.
& True. In heat exchanger tubes, internal tube cavita-
tion erosion can occur in the area of sudden contrac-
tion at the tube sheet as shown in Figure 5.11.
. What is pump vapor locking? How is it detected?
& When there is no flow from a running pump, but flow
resumes after it is shut down and restarted, there is
more likelihood of vapor lock in the pump. This
occurs when a quantity of vapor or gas, as little as
10% by volume, enters the pump casing and becomes
trapped in the eye of the impeller, thereby blocking
the inflow of the fluid.
. The reason for this accumulation of the gas in the eye is
because the centrifugal forces generated by the impeller
vanes cause the fluid with higher density, that is, liquid
to be thrown outward from the eye while the gas
FIGURE 5.10 Typical blade damage due to cavitation of a mixed
flow pump. remaining concentrated in the center of the eye. When