Pumps

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PUMPS Prepared by

Md.Shamsuzzaman

Lesson Overview Lesson covers the type of pumps commonly installed on board ships, their constructions, working principles, characteristics, safety operation.

28/11/09

2

Learning Objectives        

Definition of Pump Classification of Pumps Working principles of positive displacement pumps Construction features of positive displacement pumps Working principles of rotordynamic pumps Construction features of rotordynamic pumps Characteristics of pumps Safe operation of pumps

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Tutorial 1. State the classification of pumps used for shipboard application.   2. With the aid of a labelled sketch, briefly explain the working principle of a typical  i)     reciprocating pump  ii)   gear pump  iii) vane pump 

Iv) centrifugal pump

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Tutorial 4.

5.

6.

Draw the pump performance (H-Q) curve for the centrifugal pump Explain how suction condition of the pump may be improved Explain the cause of efficiency drop in centrifugal pump.

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Tutorial 7.

8.

Briefly explain the differences between positive displacement and rotodynamic pumps.  With reference to the operation of a pump, explain the terms ‘self-priming’ and ‘non-self priming’.

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Definition Pump is a device that imparts energy to a fluid passing through it to enable the fluid to move from one point to another. In practice, pumps change both the velocity and the pressure passing through them. 28/11/09

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Energy Flow in Pumping System Pump losses Kinetic energy At Suction

+ Pressure energy At Suction

Kinetic energy At Discharge

+

Pump

Pressure energy At Discharge Energy flow from prime mover

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Types of Pumps Marine Pumps fall into two broad classes: 

Positive Displacement Pumps



Rotordynamic Pumps

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Positive Displacement Pumps Working Principles: 

Fluid is displaced from the suction to the discharge by the mechanical variation of the volume of chamber or chambers at every stroke or rotation of the pump



Volume of pump chamber alternately increases to draw the liquid in from suction pipe & then decreased to force the liquid out into the delivery pipe



This may be done by either a reciprocating motion of a piston or by a rotary motion of specially designed vanes, gears or screws

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Positive Displacement Pumps Characteristic 

Self-priming



All the valves at the discharge side of the pumps must be kept open prior starting



Failure to do so will cause rapid increase of fluid pressure, leading to failure at the weakest point in the system



Relief valve is always fitted in the system to avoid such failure

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Positive Displacement Pumps Subdivided into Two main categories: 

Reciprocating Pumps - where plunger or piston is reciprocated in a fluid cylinder - suitable for delivering small quantities at high pressure



Rotary Pumps ( Gear, Screw, Vane pumps ) - where the liquid is forced through the pump casing by means of screws, gears or vanes - used for delivering moderate quantity at moderate pressure

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Reciprocating Pump Main Components      

Cylinder Piston Piston rod Gland Suction valve Discharge valve

Pump may be of Single acting or Double acting type

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Single Acting Reciprocating Pump 

There is one suction & one discharge per cycle



Piston moves down during suction stroke



Causes low pressure to create & fluid to flow into cylinder by opening suction valve



Piston moves up during discharge stroke



Causes fluid to be compressed and pressurised



Discharge takes place by opening discharge v/v by high pressure fluid

SUCTION STROKE

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DISCH STROKE

14

Double Acting Reciprocating Pump 

There is two suction & two discharge per cycle



Piston moves from left to right (one stroke)



Piston compresses fluid above piston (discharge)



Piston draws fluid below piston (suction)

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Pressure Pulsation 



Pressure pulsations are inherent problem in reciprocating pump It causes pipes to surge & vibrate



Pipes may become loose and break under heavy strain



Accumulators are used to reduce the impact of pressure surges

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C

B

A

C

B

Pressure

A

Str

16

Accumulators 

Accumulators are air vessels or spring loaded device



Absorbs some energy during the pump discharge stroke



Releases this energy during low pressure period and raises the pressure in the pipe



Thus reduces the pressure pulsation by avoiding sharp drop in pressure

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Gear Pump 

It is a positive displacement pump



It consists of two meshing gears with one driving the other



Fluid flows between the casing and the gear teeth



Commonly employed for lube & fuel oil transfer



Must have relief valve installed in the system

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Screw Pump 

Screw pumps are positive displacement pumps



Screws are meshed together with one driving other



Fluid is displaced through the recesses between the screws and the casing



May have single, double or triple screws

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Vane Pump 

Vane pumps are positive displacement pumps



Rotors having slots for vanes



Centre of rotor is eccentric with casing



This causes vanes to move in and out as the rotor rotates



Causes change in volume in the respective chamber, similar to reciprocating pump

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Rotodynamic Pumps 

Often Known as dynamic pumps or centrifugal pumps



Centrifugal pumps are more suitable for delivery of large quantities at low discharge pressure



Are Non-self priming pumps



Looses suction and unable to pump once air gets into the pump system Must be primed before starting

 

Used on ship as G.S. pump, Ballast Pump, S.W. & F.W. cooling pumps

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Centrifugal Pump Main Components      

Impeller Impeller wear ring Volute Casing Shaft Ball bearing Gland

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Centrifugal Pump Working Principle 

Impeller rotates at high speed



Fluid enters through the ‘eye’ of the impeller



Fluid is thrown by centrifugal force from the centre (suction side) radially outwards to the periphery of impeller (discharge side)



High velocity fluid enters the stationary volute casing



Volute casing converts the kinetic energy of fluid into pressure energy at the discharge of the pump

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Centrifugal Pump Impeller Wear Ring 

Located on the casing at the impeller suction



Must have clearance to allow rotation of impeller



If clearance is too small, wear ring may come in contact with impeller due to misalignment, expansion



If clearance is too big, fluid flows back to suction side



Cause drop in discharge pressure & pump efficiency

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Centrifugal Pump 

Curve A shows the pump capacity, given in terms of flow rate (normally M3/hr), as a function of the head developed by the pump, given in terms of meters liquid column (mIc). This curve is called the pump characteristic.

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Centrifugal Pump 

Curve B shows the Net Positive Suction Head (NPSH) requirement for the pump as a function of flow‑rate.The NPSH requirement at any flow rate is the positive head of fluid required at the pump suction over and above the cargo's vapour pressure to prevent cavitation at the impeller. 28/11/09

26

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