Page 163 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 141

& Such use is justified if large pump sizes are involved & Seals: Rotating shafts must have proper sealing
for providing steam lines and condensate handling methods to prevent leakage without affecting process
facilities that are expensive than power supply. efficiency negatively.
& Steam turbines are used on larger vital sparepumpsin ➢ Seals can be grouped into noncontacting seals
some plants during power failure and consume ex- and mechanical face seals. Noncontacting
cess steam generated in the process or to provide low- seals are often used for gas service in high-
pressure exhaust steam, from the turbines, required speed rotating equipment. Mechanical face
for heating in the process. seals provide excellent sealing for high leakage
protection.
. What are the different types of variable speed drives
used for pumps? & Bearings: Factors to take into consideration while
choosing a bearing type include shaft speed range,
& Two-speed type: For simple pumping not requiring
maximum tolerable shaft misalignment, critical
high accuracy.
speed analysis, and loading of impellers.
& Direct current type: These are among the first vari-
➢ Bearing styles include cylindrical bore, cylindrical
able speed drives using wound motors.
bore with dammed groove, lemon bore, three lobe,
& Variable voltage type.
offset halves, tilting pad, plain washer, and taper
& Variable frequency type: Most reliable and efficient.
land.
& Wound rotor regenerative type: Gives one of the best
& Materials: Pump material is often stainless steel.
efficiencies.
Material should be chosen to reduce costs and main-
. Give a summary checklist for troubleshooting a pump. tain personnel safety while avoiding materials that
& Has a temporary suction strainer been provided for will react with the process liquid to create corrosion,
pump start-up? erosion, or liquid contamination.
& Has adequate valving been provided for pump main-
tenance or removal? Is there a check valve to prevent 5.1.6 Vacuum Pumps
excess reverse flow?
& Does the pump have a valved drain connection for . “Subatmospheric pressures can be divided into four
maintenance work? regions.” Name them, giving pressure ranges involved.
& Is there provision for venting noncondensables? & Rough vacuum 760–1 Torr.
& Has all auxiliary instrumentation been provided as & Medium vacuum 1–10 3 Torr.
specified? & High vacuum from 10 3 to 10 7 Torr.
& Is there any discharge pressure gauge for monitoring & Ultrahigh vacuum 10 7 Torr and below (Note: 1 Torr
pump performance? ¼ 1 mmHg; 1 mm ¼ 0.001 Torr.)
& Is there a relief valve to prevent overpressurization of . Give examples of applications of rough, medium, high,
a positive displacement pump? and ultrahigh vacuum in chemical process industry.
& Have proper measures provided to tackle weather & Rough vacuum: Polymer reactors, vacuum distilla-
problems? tion columns, vacuum dryers, and so on.
. What are the essential variables involved in pump & Medium vacuum: Degassing of molten metals, mo-
specifications? lecular distillation, freeze drying, and so on.
& Based on the application in which a pump will be & High and ultrahigh vacuum: Production of thin films,
used, the pump type, service, and operating condi- mass spectrometry, low-temperature research, sur-
tions, the specifications of a pump can be determined. face physics research, nuclear research, and space
& Casing connection: Volute casing efficiently converts simulation.
velocity energy impacted to the liquid from the & Rough to ultrahigh vacuum: Semiconductor
impeller into pressure energy. A casing with guide applications.
vanes reduce loses and improve efficiency over a . What are the ranges of vacuum obtainablewith different
wide range of capacities and are best for multistage types of vacuum producing equipment?
high head pumps.
& Table 5.7 lists ranges of pressures obtainable with
& Impeller details: Closed-type impellers are most different vacuum producing equipment.
efficient. Open-type impellers are best for viscous . Which of the following piston vacuum pumps are
liquids, liquids containing solid matter, and general
capable of producing higher vacuum than the other?
purposes.
Give the ranges of vacuum obtainable for each type?