Page 130 - Fluid mechanics, heat transfer, and mass transfer

108 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

& For low temperature applications, especially for . If NPSH A is low (<3 m), what modifications may be
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liquids involving high boiling points, P is not critical recommended for the pump system to prevent cavita-
and assumed to be negligible. tion problems?
. On what factors NPSH of a centrifugal pump depend? & Oversizing the pump suction line and the impeller
& Speed of rotation. eye.
& Inlet area of the impeller. . “NPSH A should always be >NPSH R .” True/False?
& Number and types of vanes. & True.
. What are the ways by which NPSH A can be increased? . What is NPSH R3 ?
& Raise liquid level on the suction side by either & NPSH R3 is defined as the cold water pump head drop
raising level in the tank or raising suction side tank of 3%, if NPSH A ¼ NPSH R , that is, pump head will
itself. be reduced by 3% in case of pumping cold water over
that obtainable theoretically, which is the accepted
& Lower the pump (install it in a sump or trench).
industry standard.
& Pressurize the suction side tank.
. What is the effect of excessive increase in NPSH A over
& Reduce friction losses in suction piping by increasing
NPSH R on impeller life?
pipe diameter, reducing valves (e.g., replace globe
& Maximum pump damage rate due to erosion to the
valves by gate or butterfly valves by ball valves) and
impeller occurs if NPSH A is in the range of two to
fittings (e.g., eliminate some elbows or change stan-
three times the NPSH R3 .
dard elbow by long radius elbow) or changing pipe
material with that with smooth surface (e.g., plastic . What are NPIP R and NPIP A ?
or stainless steel pipe). & NPIP R is the required NPIP and is the difference
& Use a booster pump. between the inlet pressure and the vapor pressure
(corrected to the center line of the pump inlet port)
& Subcool the liquid.
necessary for the pump to operatewithout a reduction
. What is the effect of altitude of a pump installation
of flow.
above sea level on NPSH A ?
& The Hydraulic Institute defines the minimum re-
& NPSH A decreases with altitude as barometric pres-
quired pressure (or equivalent NPIP R ) as the pressure
sure decreases.
where 5% of the flow reduction occurs due to
. What are the factors on which NPSH R for a given
cavitation.
application depends?
& NPIP A is the available NPIP.
& NPSH R isdependentonpumpdesign,forexample,on
. What are the effects of low NPIP A ?
suction area of the impeller including double suction,
& NPIP A should always be more than NPIP R .
shape and number of vanes, area between vanes, eye
diameter, specific speed, shaft diameter, and so on. & For positive displacement pumps, an increase in
NPIP A has no effect on volumetric efficiency as long
& Normally NPSH R is provided by the pump
as NPIP A is greater than NPIP R .
manufacturers.
& Low values of NPIP A may result not only in
. How is NPSH R reduced?
flow reduction but also in significant pressure
& Use lower speeds. This needs installing a larger size
spikes, noise, vibrations, and possible damage to the
impeller to handle a given pumping task.
pump.
& Use double-suction impeller. A double-suction im-
. “NPSH for hot/boiling liquids is high.” True/False?
peller design needs only two-thirds as much NPSH as
Comment.
a similarly rated single-suction design.
& False. Hot/boiling liquids have high vapor pressures
& Use large impeller eyearea or increase suction nozzle
that promote vaporization of the liquid in the suction
size.
line, especially in the eye of the impeller where
& Use oversize pump.
pressure is very low.
& Use several smaller pumps in parallel.
. What is the common recommended range for NPSH A ?
& Use a booster pump.
& 1.2–6.1 m of liquid.
& Use inducers ahead of conventional impellers. In-
. What is priming in a pump operation?
ducer is a low head axial-type impeller with few
& The process of introducing fluid into the pump casing
blades located in front of conventional impeller.
to improve sealing of pump parts on starting and
& Polish suction throat and pathway to the impeller.
expelling air from it is called priming.