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PUMPS

Prepared by

Md.Shamsuzzaman

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Lesson Overview

Lesson covers the type of pumps

commonly installed on board ships, theirconstructions, working principles,characteristics, safety operation.

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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 shipboardapplication.

2. With the aid of a labelled sketch, briefly explain theworking principle of a typical

i) reciprocating pump

ii) gear pump

iii) vane pump

Iv) centrifugal pump

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Tutorial4. Draw the pump performance (H-Q) curve

for the centrifugal pump

5. Explain how suction condition of the pumpmay be improved

6. Explain the cause of efficiency drop incentrifugal pump.

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Tutorial7. Briefly explain the differences between

positive displacement and rotodynamic

pumps.8. With reference to the operation of a pump,

explain the terms self-primingand non-self priming.

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Definition

Pump is a device that imparts energy to a

fluid passing through it to enable the fluidto move from one point to another.

In practice, pumps change both the

velocity and the pressure passing throughthem.

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Energy Flow in Pumping System

Pump

Kinetic energy

At Suction

+

Pressure energy

At Suction

Kinetic energy

At Discharge

+

Pressure energy

At Discharge

Energy flow from prime mover

Pump losses

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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 PumpsWorking Principles:

Fluid is displaced from the suction to the discharge

by the mechanical variation of the volume of chamberor chambers at every stroke or rotation of the pump

Volume of pump chamber alternately increases todraw 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 ofa piston or by a rotary motion of specially designedvanes, gears or screws

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Positive Displacement PumpsCharacteristic

Self-priming

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

Failure to do so will cause rapid increase of fluid

pressure, leading to failure at the weakest point in thesystem

Relief valve is always fitted in the system to avoidsuch failure

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Positive Displacement

PumpsSubdivided 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 ofscrews, gears or vanes

- used for delivering moderate quantity at moderate pressure

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Reciprocating PumpMain Components

Cylinder

Piston

Piston rod

Gland

Suction valve

Discharge valve

Pump may be ofSingle 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

SUCTIONSTROKE DISCHSTROKE

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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 belowpiston (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

Pressure

St

A

B

CB CA

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Accumulators Accumulators are air vessels

or spring loaded device

Absorbs some energy duringthe pump discharge stroke

Releases this energy duringlow pressure period andraises the pressure in thepipe

Thus reduces the pressurepulsation by avoiding sharpdrop in pressure

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Gear Pump It is a positive displacement

pump

It consists of two meshinggears with one driving theother

Fluid flows between thecasing and the gear teeth

Commonly employed forlube & fuel oil transfer

Must have relief valveinstalled in the system

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Screw Pump Screw pumps are positive

displacement pumps

Screws are meshed togetherwith one driving other

Fluid is displaced throughthe recesses between the

screws and the casing

May have single, double ortriple screws

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Vane Pump Vane pumps are positive

displacement pumps

Rotors having slots for vanes

Centre of rotor is eccentricwith casing

This causes vanes to movein and out as the rotor

rotates

Causes change in volume inthe 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 largequantities at low dischargepressure

Are Non-self priming pumps

Looses suction and unable topump 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 PumpMain 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 theimpeller

Fluid is thrown by centrifugal forcefrom the centre (suction side) radiallyoutwards to the periphery of impeller(discharge side)

High velocity fluid enters the

stationary volute casing

Volute casing converts the kineticenergy of fluid into pressure energyat the discharge of the pump

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Centrifugal Pump

Impeller Wear Ring

Located on the casing at theimpeller suction

Must have clearance to allowrotation of impeller

If clearance is too small, wearring may come in contact withimpeller due to misalignment,expansion

If clearance is too big, fluid flowsback to suction side

Cause drop in dischargepressure & pump efficiency

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Centrifugal Pump

Curve A shows thepump capacity, givenin terms of flow rate(normally M3/hr), as afunction of the headdeveloped by thepump, given in termsof meters liquid column(mIc). This curve iscalled the pumpcharacteristic.

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Centrifugal Pump

Curve B shows the Net

Positive Suction Head

(NPSH) requirement for

the pump as a function offlow-rate.The NPSH

requirement at any flow

rate is the positive head of

fluid required at the pump

suction over and above thecargo's vapour pressure to

prevent cavitation at the

impeller.