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Marine Auxiliary Machinery

Chapter 1 Lesson 4

Positive Displacement Pumps

By Professor Zhao Zai Li05.2006

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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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Rotary Reciprocating

Basic design

Positive displacement pumps have one of two basic designs either Reciprocating or Rotary.

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Positive Displacement Pumps versus Centrifugal Pumps

Positive displacement pumps differ from centrifugal pumps in that they use mechanical energy to transfer fluid. If the displacement 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 Pumps versus Centrifugal Pumps

Positive displacement pumps differ from centrifugal pumps in that they use mechanical energy to transfer fluid. If the displacement 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 pump design

High pressures Self-priming

External Priming

Cgnnectign

Pressure Fluctuatigns

Damper

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Key features - reciprocating pump design

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 pump design

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 pump design

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 pump design

Damper Valve Relief valve

External Priming Cgnnectign

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 pump design

Damper Valve Relief valve

Pressure Fluctuatigns

Reciprocating pumps can produce pressure fluctuations in the discharge pipelines.

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Key features - reciprocating pump design

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 use are of the rotary type. Rotary pumps operate in a circular motion and 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 expand volumes to allow liquid to enter the pump. These volumes are contained by the pump until the geometry causes the elements to move to reduce the volume and force the liquid out The clearances of the elements within the pump separate the suction from the discharge and are critical for efficient operation.

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Radial and Axial Piston

Rotary piston type positive displacement pumpsare found in hydraulic systems. There are twotypes: radial piston and axial piston

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Operation of the radial piston pump

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 the pistins thereby changing the amount of cylinder volume increase.

The nature of the ports can also change by moving the piston eccentricity past the pump centre line.

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Operation of the axial piston pump2. 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 and delivery ports. They come in two configurations, either Internal gear or External gear.

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 by the motor and the second, or idler, which is driven by the meshing of the gears. The gears carry the liquid around the outside and the meshing of the teeth cause the liquid to be discharged.

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Screw type pumps

Twin-screw pumps Three-screw pumps

Screw type pumps usually have either two or three screws. They can operate at high speeds and give a smooth discharge with almost 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 has the suction arranged at the centre of the Shdft screws and the delivery at either end, and is often driven through timing gears, which keep the screws from contacting each other.

Due to the imbalance of forces ,this type of pump often has the suction arranged at the centre of the shaft screws and the delivery at either end ,and is often driven through timing gears,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 screw drives the two outer screws.

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Progressive cavity pumps

A special type of screw pump is the single screw or progressive cavity pump. This pump has a single screw rotating in stator. The stator is made of an elastomer material. The stator has an internal helix of different pitch to the rotor. The geometry means that the rotor moves vertically whilst turning, so the meshing of the rotor and stator causes the cavity to progress to thedischarge. The screw is an interference fit in the stator, so this pump must always have 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 Vane Pump.

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.

Casing Rotor

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Sliding vane pumps

The vanes are a sliding fit in slots cut in the rotor. The rotor turns eccentric to the casing, causing the cavities to decrease in volume. Vanes can be forced out by centrifugal force, use springs or utilise the pressure of the discharge 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 familiar with 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 3Positive 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 don’t know