Page 151 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 129

. What are the ranges of operating pressures and capac- . How are flow pulsations eliminated/reduced?
ities of reciprocating pumps? & Liquid, being incompressible, cannot dampen pulsa-
4
& Pressures: 7 10 kPa or higher. tions that are characteristic with positive displace-
3
& Capacities: up to 250 m /h. ment pumps. Therefore, air or gas, being compress-
ible, is filled in a surge chamber on the discharge side
. “The discharge line of a positive displacement pump
can be closed without damaging the pump.” True/ of a pump/compressor to cushion out/dampen
pulsations.
False?
& For gas flow, a combination of a surge chamber
& False. A positive displacement pump delivers a fixed
located close to pulsation source and a constriction
quantity of the fluid with each stroke. If discharge
in the line between surge chamber and metering
line is closed, the delivered fluid increases pressure
element.
with each stroke and finally damages the discharge
line so that the fluid escapes through the ruptured & A liquid level gauge is desirable to permit checking
line. the amount of air/gas in the chamber.
& As a precaution, a relief valve is installed on the & Use of multiple cylinders, for example, duplex and
discharge line so that when pressure increases be- double-acting pumps reduces amplitude and fre-
yond the set pressure of thevalve, thevalve opens and quency of pulsations in discharge lines.
relieves the pressure, thus preventing damage to the . What are the differences in amplitude and frequency of
pipe. pulsations between positive displacement pumps/com-
. What is overall or mechanical efficiency of a positive pressors and centrifugal pumps/compressors?
displacement pump? & In general, pulsations generated by positive displace-
& It is the ratio of the useful hydraulic power transmit- ment pumps/compressors are of low frequency and
ted to the fluid leaving the pump to the total power high amplitude whereas those generated by centrif-
absorbed by the pump: ugal pumps/compressors are of high frequency and
low amplitude.
h ¼ FHP=BHP: ð5:20Þ . What is slip of a pump?
& Fraction/% loss of capacity relative to the theoretical
. Define (i) volumetric efficiency and (ii) mechanical
capacity that is based on the displacement volume of
efficiency for a reciprocating pump.
the cylinder due to the stroke of piston/plunger.
& Volumetric efficiency: Ratio of actual discharge to
that based on piston displacement. Slip ¼ð1 h Þ ð5:21Þ
vol
& Mechanical efficiency: Ratio of energy supplied to
the fluid to the energy supplied to the pump. where
. Which of the two pumps, centrifugal or positive dis-
Actual amount of liquid pumped
placement, has higher efficiency? Why? h vol ¼
& There will be considerable internal recirculation in a Theoretical capacity based on
piston movement:
centrifugal pump leading to unproductive work,
whereas in a positive displacement pump, there will
& Slip varies 2–10% of displacement, average being
not be any recirculation, and once fluid enters pump
3%.
casing it will be discharged, barring small amount of
. Why is it necessary to use a relief valve set to open at a
slip through clearances that are very small compared
safe pressure on the discharge side of a positive dis-
to those in a centrifugal pump.
placement pump?
. What are the overall efficiencies obtainable in a positive
& Each stroke of the pump discharges a finite quantity
displacement pump?
of fluid to the discharge line, unlike in centrifugal
& 85–94%.
pumps in which considerable fluid recirculation oc-
. “Plunger pumps are always single acting.” True/False?
curs in the pump casing.
& True.
& If a valve on the discharge side is closed, pressure
. What are the sources of pulsations in flow? goes on increasing with each stroke in a positive
& In discharge lines in reciprocating pumps/compres- displacement pump.
sors because of flow (during discharge) and no flow & This pressure increase leads to discharge pipe rupture
(during suction) strokes, pulsations in flow occur. if pressure increases beyond design pressure of the
& In lines supplying steam to reciprocating machinery. pipe.