assignment 3- pd pump
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Assignment 3 BIO 3073 Bioprocess Plant & Equipment
INTRODUCTION
Pump is a device used to move liquids orslurries. A pump moves liquids from
lower pressure to higher pressure, and overcomes this difference in pressure by addingenergy to the system (such as a water system). A gas pump is generally called a
compressor, except in very low pressure-rise applications, such as in heating, ventilating,
and air-conditioning, the equipment is known asfans orblowers.
Diagram of pump
Positive displacement pumps can be further classified as either rotary-type or
reciprocating-type.A positive displacement pump causes a liquid to move by trapping a
fixed amount of fluid and then forcing (displacing) that trapped volume into the discharge
pipe.
Example of Positive-displacement Pump
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http://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Slurryhttp://en.wikipedia.org/wiki/Slurryhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/HVAChttp://en.wikipedia.org/wiki/HVAChttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Slurryhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/HVAChttp://en.wikipedia.org/wiki/HVAC -
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BASIC OPERATING PRINCIPLE
Positive-displacement Pump has an expanding cavity on the suction side of the
pump and a decreasing cavity on the discharge side. Liquid is allowed to flow into the pump as the cavity on the suction side expands and the liquid is forced out of the
discharge as the cavity collapses.
By definition, PD pumps displace a known quantity of liquid with each revolution
of the pumping elements (i.e., gears, rotors, screws, vanes). PD pumps displace liquid by
creating a space between the pumping elements and trapping liquid in the space. The
rotation of the pumping elements then reduces the size of the space and moves the liquid
out of the pump.
ADVAN
TAGES & DISADVANTAGES
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SELECTION OF POSITIVE DISPLACEMENT PUMP
Selection of a positive displacement (PD) rotary pump is not always an easy
choice. There are four common types of PD pumps available: internal gear, externalgear, timed lobe, and vane. Most PD pumps can be adapted to handle a wide range of
applications, but some types are better suited than others for a given set of circumstances.
The first consideration in any application is pumping conditions. Usually the need
for a PD pump is already determined, such as a requirement for a given amount of flow
regardless of differential pressure, viscosity too high for a centrifugal pump, need for
high differential pressure, or other factors.
Inlet conditions, required flow rate, differential pressure, temperature, particle
size in the liquid, abrasive characteristics, and corrosiveness of the liquid must be
determined before a pump selection is made.
A pump needs proper suction conditions to work well. PD pumps are self-
priming, and it is often assumed that suction conditions are not important. But they are.
Each PD pump has a minimum inlet pressure requirement to fill individual pump cavities.
If these cavities are not completely filled, total pump flow is diminished. Pumpmanufacturers supply information on minimum inlet conditions required. If high lift or
high vacuum inlet conditions exist, special attention must be paid to the suction side of
the pump.
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NTERNAL GEAR PUMPS
The crescent internal gear pump has an outer or
rotor gear that is generally used to drive the inner or
idler gear (Figure 1). The idler gear, which is smaller
than the rotor gear, rotates on a stationary pin and
operates inside the rotor gear. The gears create voids as
they come out of mesh and liquid flows into the pump.
As the gears come back into mesh, volumes are
reduced and liquid is forced out the discharge port.
Liquid can enter the expanding cavities through the rotor teeth or recessed areas on the
head, alongside the teeth. The crescent is integral with the pump head and prevents
liquids from flowing to the suction port from the discharge port.
The rotor gear is driven by a shaft supported by journal or antifriction bearings.
The idler gear contains a journal bearing rotating on a stationary pin in the pumped
liquid. Depending on shaft sealing arrangements, the rotor shaft support bearings may run
in pumped liquid. This is an important consideration when handling an abrasive liquid
and can wear out a support bearing.
The speed of internal gear pumps is considered relatively slow compared to
centrifugal types. Speeds up to 1,150 rpm are considered common, although some small
designs operate up to 3,450 rpm. Because of their ability to operate at low speeds,
internal gear pumps are well suited for high-viscosity applications and where suction
conditions call for a pump with minimal inlet pressure requirements.
Internal gear pumps are made to close tolerances and are damaged when pumping
large solids. These pumps can handle small suspended particulate in abrasive
applications, but gradually wear and lose performance. Some performance loss is restored
by adjusting the pump end clearance. End clearance is the closeness of the rotor gear to
the head of the pump.
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Figure 1.Internal gear pumps are
ideal for high-viscosity liquids, but
they are damaged when pumping large
solids.
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EXTERNAL GEAR PUMPS
External gear pumps are similar inpumping action to internal gear pumps in that
two gears come into and out of mesh to produce
flow (Figure 2). However, the external gear
pump uses two identical gears rotating against
each other. Each gear is supported by a shaft
with bearings on both sides of each gear.
Typically, all four bearings operate in the
pumped liquid.
Because the gears are supported on both sides, external gear pumps are used for
high pressure applications such as hydraulics. Usually, small external gear pumps operate
at 1,750 or 3,450 rpm and larger versions operate at speeds up to 640 rpm.
The design of external gear pumps allows them to be made to closer tolerances
than internal gear pumps. The pump is not very forgiving of particulate in the pumped
liquid. Since there are clearances at both ends of the gears, there is no end clearance
adjustment for wear. When an external gear pump wears, it must be rebuilt or replaced.
External gear pumps handle viscous and watery-type liquids, but speed must be
properly set for thick liquids. Gear teeth come out of mesh a short time, and viscous
liquids need time to fill the spaces between gear teeth. As a result, pump speed must be
slowed down considerably when pumping viscous liquids.
The pump does not perform well under critical suction conditions. Volatile liquids
tend to vaporize locally as gear teeth spaces expand rapidly. When the viscosity of
pumped liquids rises, torque requirements also rise, and pump shaft strength may not be
adequate. Pump manufacturers supply torque limit information when it is a factor.
LOBE PUMPS
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Figure 2.External gear pumps (shown
is a double pump) are typically used for high-
pressure applications such as hydraulics.
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Lobe pumps (Figure 3) are similar to
external gear pumps in operation, except the
pumping elements or lobes do not make
contact. Lobe contact is prevented by external
timing gears. Pump shaft support bearings are
located in the timing gear case. Since the
bearings are out of the pumped liquid, pressure
is limited by bearing location and shaft
deflection. There is not metal-to-metal contact
and wear in abrasive applications is minimal.
Use of multiple mechanical seals makes seal construction important.
Lobe pumps are frequently used in food applications, because they handle solids
without damaging the pump. Particle size pumped can be much larger in lobe pumps than
in other PD types. Since the lobes do not make contact, and clearances are not as close as
in other PD pumps, this design handles low viscosity liquids with diminished
performance. Loading characteristics are not as good as other designs, and suction ability
is low. High-viscosity liquids require considerably reduced speeds to achieve satisfactory
performance. Reductions of 25% of rated speed and lower are common with high-
viscosity liquids.
Lobe pumps are cleaned by circulating a fluid through them. Cleaning is
important when the product cannot remain in the pumps for sanitary reasons or when
products of different colors or properties are batched.
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Figure 3.Lobes in lobe pumps do not
make contact, because they are driven by
external timing gears. This design handles low-
viscosity liquids.
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Flexible Member
This principle is similar to the Vane principle except the vanes flex rather than
slide. The fluid pumping and sealing action depends on the elasticity of the flexible
members. The flexible members may be a tube, a vane, or a liner. Figure 5 shows a
flexible vane pump.
Diagram of Flexible Member type
Circumferential Piston
Fluid is carried from inlet to outlet in spaces between piston surfaces. Rotors
must be timed by separate means, and each rotor may have one or more piston elements.
Diagram of Circumferential Piston type
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Single screw pumps
Single screw pumps are commonly called progressive cavity pumps. They have a
rotor with external threads and a stator with internal threads. The rotor threads are
eccentric to the axis of rotation.
Diagram of Single Screw type
Multiple screw pumps
Multiple screw pumps have multiple external screw threads. These pumps may be timed
or untimed
Diagram of Multiple Screw type
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VANE PUMPS
Sliding vane pumps (Figure 4) operate quite
differently from gear and lobe types. A rotor with
radial slots, is positioned off-center in a housing bore.
Vanes that fit closely in rotor slots slide in and out as
the rotor turns. Vane action is aided by centrifugal
force, hydraulic pressure, or pushrods. Pumping action
is caused by the expanding and contracting volumes
contained by the rotor, vanes, and housing.
Vanes are the main sealing element between the suction and discharge ports and
are usually made of a nonmetallic composite material. Rotor bushings run in the pumped
liquid or are isolated by seals.
Vane pumps usually operate at 1,000 rpm, but also run at 1,750 rpm. The pumps
work well with low-viscosity liquids that easily fill the cavities and provide good suction
characteristics. Speeds must be reduced dramatically for high-viscosity applications to
load the area underneath the vanes. These applications require stronger-than-normal vane
material.
Because there is no metal-to-metal contact, these pumps are frequently used with
low-viscosity nonlubricating liquids such as propane or solvent. This type of pump has
better dry priming capability than other PD pumps. Vane pumps can run dry, but are
subject to vane wear.
Abrasive applications require the proper selection of vane material and seals.
Vane pumps have fixed end clearances on both sides of the rotor and vanes similar to
external gear pumps. Once wear occurs, this clearance cannot be adjusted, but some
manufacturers supply replaceable or reversible end plates. Casing liners are a low-cost
way of restoring pump performance as wear occurs. Unlike lobe pumps, vane pumps
cannot handle solids.
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Figure 4.Vane pumps have better dry
priming capability than other positive
displacement pumps.
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TROUBLESHOOTING
Pump Problems can be either caused by:
1. Mechanical Problem with the Pump
To determine if this noise is a mechanical problem with the pump drains it, close
both suction and discharge valves and run the pump briefly. If the noise continues
you have a mechanical problem. The mechanical noise can be caused by:
i. Debris in the Impeller
ii. Impeller Rubbing
iii. Impeller out of Balance
iv. Bent or Twisted shaft
v. Bad bearings
vi. Coupling Misalignment
vii. V-Belt sheave Misalignment
viii. Pipe Stress
In any case the problem can be corrected by taking it apart and simply fixing it by
either replacing the damaged parts or correcting the installation. Items Required:
a. Pump Installation Manual
b. Pump Operation Manual
c. Pump Maintenance Manual
d. Parts List
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2. Pump System Problem
However if the Noise goes away after draining the pump etc. then the noise is
caused by the Pumping System.Pump System Noise is usually caused by:
a. Cavitation
b. Vortexing
In order to troubleshoot a pump system problem the following tools are required:
i. Combination Vacuum/Pressure Gauge - To Check Pump Suction
Reading
ii. Pressure Gauge - To Check Pump Discharge Pressure Reading
iii. Amp Meter - To Check Horsepower Load
iv. Tachometer - To Check Pump Speed
v. Pump Performance Curve - To Check all Readings Against the
Expected Pump Performance
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MAINTENANCE
The maintenance for positive-displacement pump similar in many ways to that of
centrifugal pumps. Alignment is one common concern, since PD pumps are oftenpedestal-mounted and have close-tolerance components that wear or bind if misaligned.
Some key considerations are:
If the pump is belt driven, the belt tension is important. The rule of thumb
is that the belt should be able to be depressed in midlength. The faces of
both pulleys also need close alignment; a long straightedge can be used as
described for checking a coupling.
The length of the service life of PD pumps often depends on how clean the
pumped liquids are; therefore, filters and strainers should be scrupulously
maintained, and the air line feeding double diaphragm pumps monitored for
contamination.
Motors rarely need maintenance today. The ball bearings are often double
sealed for the life of the bearing; if they are not, care should be taken to avoid
the dangers of overgreasing.
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