4_Positive Displacement Pumps

8/11/2019 4_Positive Displacement Pumps

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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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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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Self-priming

Positive displacement pumps are self-priming as the suction

and delivery are positively separated during pumping.


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Self-priming

Positive displacement pumps are self-priming as the suction

and delivery are positively separated during pumping.


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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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Damper Valve Relief valve

Pressure Fluctuatigns

Reciprocating pumps can produce pressure fluctuations in the

discharge pipelines.


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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

4_Positive Displacement Pumps

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