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Marine Auxiliary
MachineryChapter 1 Lesson 4
Positive Displacement Pumps
By Professor Zhao Zai Li
05.2006
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Learning objectives
After successfully completing this lesson, you will be
familiar with:
1: The operation of a reciprocating pumps
2: Single and double acting reciprocating pumps
3: The operation of several different rotary positive
displacement pumps
4: The relief valve In positive displacement pumps.
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RotaryReciprocating
Basic design
Positive displacement pumps have one of two basic designs either
Reciprocating or Rotary.
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Positive Displacement Pumpsversus Centrifugal Pumps
Positive displacement pumps differ from centrifugal pumps in
that they use mechanical energy to transfer fluid. If thedisplacement is on the forward stroke only, then the pump is
single acting.
This pump is double acting because it displaces liquid both on
the forward and return stroke.
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Positive Displacement Pumpsversus Centrifugal Pumps
Positive displacement pumps differ from
centrifugal pumps in that they use
mechanical energy to transfer fluid. If thedisplacement is on the forward stroke only,
then the pump is single acting.
This pump is double acting because it
displaces liquid both on the forward and
return stroke.
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Key features - reciprocating pumpdesign
High pressures Self-priming
External Priming
Cgnnectign
Pressure Fluctuatigns
Damper
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Key features - reciprocating pumpdesign
High pressures
Positive displacement pumps are capable of delivering very
high pressures.
These pressures can be much in excess of the design
pressure and so a relief valve must protect the pump.
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Key features - reciprocating pumpdesign
Self-priming
Positive displacement pumps are self-priming as the suction
and delivery are positively separated during pumping.
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Key features - reciprocating pumpdesign
Self-priming
Positive displacement pumps are self-priming as the suction
and delivery are positively separated during pumping.
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Key features - reciprocating pumpdesign
Damper Valve Relief valve
External PrimingCgnnectign
However, in order to reduce wear and ensure quick starting, an external
priming connection from above the pump should be available.
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Key features - reciprocating pumpdesign
Damper Valve Relief valve
Pressure Fluctuatigns
Reciprocating pumps can produce pressure fluctuations in the
discharge pipelines.
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Key features - reciprocating pumpdesign
A damper, either gas or air filled, can help to even out the
pressure fluctuations. The damper stores some of the energy
as the pressure rises, and releases this energy as the pressure
falls.
Damper
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Quick quiz
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Rotary pumps
Most positive displacement pumps in useare of the rotary type.
Rotary pumps operate in a circular motionand displace a constant amount of liquid
with each revolution of the pump shaft.This is accomplished by the pumping
elements moving in such a way as to expandvolumes to allow liquid to enter the pump.
These volumes are contained by the pump
until the geometry causes the elements tomove to reduce the volume and force theliquid out
The clearances of the elements within thepump separate the suction from the
discharge and are critical for efficientoperation.
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Operation of the radial pistonpump
You can see that the pistons rotate
eccentrically to the rotor axis. This causes
the cylinder volume to increase as the
pistons pass the suction port,and decrease
as the pistons pass the delivery port.
In some applications such as steering gear
applications,the pump can have a variable
discharge .This is achieved by increasing
and decreasing the eccentricity of thepistins thereby changing the amount of
cylinder volume increase.
The nature of the ports can also change by
moving the piston eccentricity past the
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Operation of the axial piston pump
2. and are forced back
in at outlet
1. Pistons withdraw
from bore at inlet.
PISTON
DRME
SWASH
OUTLET PORT
An axial piston pump is sometimes known as a swash plate pump.The principle is the same as for the Radial type.
An axial piston pump can also be used as a variable delivery pump.
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Interna! and external gear pumps
Gear pumps rely on the
meshing of gear wheels to
separate the suction anddelivery ports.
They come in two
configurations, either
Internal gear or Externalgear.
Internal gear pumps External gear oumos
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Internal gear pump
Internal gear pumps use the same principle as the external type.
In this case the driven gear has internal teeth and meshes with
the idler, which has external teeth.
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External gear pump
The External gear pump has two
gear wheels, one that is driven bythe motor and the second, or idler,which is driven by the meshing ofthe gears.
The gears carry the liquid around
the outside and the meshing of theteeth cause the liquid to bedischarged.
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Screw type pumps
Twin-screw pumps Three-screw pumps
Screw type pumps usually have either two or three screws. Theycan operate at high speeds and give a smooth discharge withalmost no pulsations.
They come in two configurations, either Twin-screw pump or
Three-screw pump.
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Twin-screw pumps
Due to the imbalance of forces, this type of pump often hasthe suction arranged at the centre of the Shdft screws andthe delivery at either end, and is often driven through timinggears, which keep the screws from contacting each other.
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Three-screw pumps
The Three-screw pump is usually
arranged so that the central screwdrives the two outer screws.
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Progressive cavity pumps
A special type of screw pump is the single screw or progressive cavitypump. This pump has a single screw rotating in stator. The stator is madean elastomer material.
The stator has an internal helix of different pitch to the rotor. The geommeans that the rotor moves vertically whilst turning, so the meshing of throtor and stator causes the cavity to progress to thedischarge.
The screw is an interference fit in the stator, so this pump must alwayshave liquid present to prevent overheating and failure.
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Quick quiz
Which part of a progressive cavity pump is called a stator?Click on the stator.
If you are not sure, go to the previous screen to refresh your memory.
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Vane pumps
Flexible vane pumps
Vane pumps come in two configurations,either Flexible Vane Pump or Sliding VanePump.
Sliding vane pumps
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Flexible vane pumps
The rotor is made of an
elastomer, and as the vanes
pass the cam they deflect,
decreasing the cavity and so
discharging the liquid.
CasingRotor
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Sliding vane pumps
The vanes are a sliding fitin slots cut in the rotor. Therotor turns eccentric to thecasing, causing the cavities todecrease in volume.
Vanes can be forced out bycentrifugal force, use springs
or utilise the pressure of thedischarge liquid.
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Quick quiz
Where are the vanes located?Click on the vanes.
If you are not sure, go to the previous screen to refresh your memory.
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Lesson test
In this test you must answer some questions from the current
lesson. You must take the test to complete the lesson.
Please read the "General rules for tests" if you're not familiarwith taking tests on the computer.
Click the Next-button if you are ready to take the test.
Click the Previous-button if you want to repeat any part of
the lesson before taking the test. Click the Menu-button if you want to repeat the lesson from
Its beginning.
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Question 1
In order to pump liquids Positive displacement pumps rely
on?
A) Fast speed of operation
B) Small clearances within the pump
C) Change of volume of the pump chambers
D) Non-return valves
E) I do not know
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Question 2
Positive displacement pumps can only vary their delivery
rate by changing the speed of the drive motor.
A) True
B) False
C) I do not know
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Question 3
Positive displacement screw or vane pumps are used for oily
water separator supply because?
A) They are self priming
B) They have a minimum of moving parts
C) They deliver a constant flow
D) They do not cause oil droplet size to change
E) I do not know
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Question 4
Positive displacement pumps require a relief valve because?
A) It can regulate the delivery pressure.
B) The pump gives a constant flow rate.
C) It allows priming liquid to return to the suction.
D) A high pressure could overload the motor.
E) I do not know.
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Question 5
Positive displacement pumps require a relief valve because?
A) It can regulate the delivery pressure.
B) The pump gives a constant flow rate.
C) It allows priming liquid to return to the suction.
D) A high pressure could overload the motor.
E) I do not know.
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Question 6
Positive displacement pumps are ideal for fuel transfer
duties because?
A) They can deliver high pressures
B) They have very small clearances
C) They can handle large volumes of vapours and gases
D) They require the pumped liquid to be used for
lubrication
E) I d t k