Page 134 - Fluid mechanics, heat transfer, and mass transfer

112 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

➢ Voided areas increase the amount of energy re- 5.1.2 Cavitation
quired to force the fluid through the passage.
& Mechanical losses in a pump are caused by viscous . What is cavitation? What are the causes and the effects
disk friction, bearing losses, seal or packing losses, of cavitation?
and recirculation devices. & Cavitation occurs in liquid when bubbles form and
➢ Bearings, lip seals, mechanical seals, and pack- implode in pump systems or around impellers.
ings, all consume energy and reduce the efficiency Pumps put liquid under pressure, but if the pressure
of the pump. Small pumps are particularly of the liquid drops or its temperature increases, it
susceptible. begins to vaporize, just as boiling water. The bubbles
that form inside the liquid are vapor bubbles, gas
& Close tolerances on thewear rings have a tremendous
bubbles, or a mixture of both.
effect on the efficiency of a pump, particularly for
pumps with a low specific speed (N s < 1500). & Vapor bubbles are formed due to the vaporization of
the liquid being pumped, at a point inside the pump
➢ If the clearance between the impeller and the
where the local static pressure is less than the vapor
casing sidewall is too large, disk friction can
pressure of the liquid. A cavitation condition induced
increase, reducing the efficiency.
by formation and collapse of vapor bubbles is com-
➢ Bearings, thrust balancing devices, seals and
monly referred to as vapor cavitation.
packing all contribute to frictional losses. Most
& Gas bubbles, by contrast, are formed due to the
modern bearing and seal designs generate full
presence of dissolved gases in the liquid that is being
fluid film lubrication to minimize frictional losses
pumped. In many situations, the gas dissolved is air.
and wear.
& Vapor cavitation bubbles get carried in the liquid as it
➢ Frequently, recirculation devices such as auxil-
flows from the impeller eye to the impeller tip, along
iary impellers or pumping rings are used to
the trailing edge of the blade. Due to the rotation of
provide cooling and lubrication to bearings and
the impeller, the bubbles first attain very high veloc-
seals. Similar to the main impeller, these de-
ity, then reach the regions of higher pressure. The
vices pump fluid and can have significant power
pressure around the bubbles begins to increase until
requirements.
they collapse. This process is an implosion (inward
& Internal leakage occurs as the result of flow between
bursting). Hundreds of bubbles implode at approx-
the rotating and the stationary parts of the pump, from
imately the same point on each impeller blade.
the discharge of the impeller back to the suction.
& The bubbles collapse in such a way that the surround-
➢ The rate of leakage is a function of the clearances
ing liquid rushes to fill the void, forming a liquid
in the pump. Reducing the clearances will de-
microjet. The microjet subsequently ruptures the
crease the leakage but can result in reliability
bubbles with such a force that a hammering action
problems if mating materials are not properly
occurs.
selected.
& Figure 3.17 illustrates bubble collapse.
➢ Some designs bleed offflows from the discharge to
& After bubbles collapse, a choke wave emanates
balance thrust, provide bearing lubrication, or to
outward from the point of collapse. This choke wave
cool the seal.
is what is actually heard and called cavitation.
& Impeller diameter influences efficiency. There
& The collapse of the bubbles also ejects destructive
will be an efficiency reduction with a reduction
microjets of extremely high velocity, up to 100 m/s,
in the impeller diameter. For this reason, it is not
causing abnormal sounds, vibrations, and extreme
recommended to reduce the impeller size by more
erosion of the pump parts, pitting, and denting in the
than 20%.
metal of the casing and the impeller blades, as
& Viscous liquids generally affect efficiency. As the
illustrated in Figure 5.8.
viscosity of the fluid goes up, generally the efficiency
& Figure 5.9 shows the potential cavitation damage
of most pumps goes down.
areas of a pump impeller.
& Slurries with low solids concentrations (less than
& It has been estimated that during collapse of bubbles
10% average) classified by size and material gener- 4
pressures of the order of 10 bar develops.
ally exhibit no adverse effect to pump efficiency.
& Apart from erosion of pump parts, cavitation can also
Sanitary and wastewater pumps that handle high
result in imbalance of radial and axial thrusts on the
solids, have two or three blades on a specially de-
impeller, due to lack of symmetry in the bubble
signed impeller that gives lower efficiency.
formation and collapse.