purifiers gme

8/13/2019 Purifiers Gme

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CentrifugesThe centrifugal separator is used to separate liquids, for

example oil and water or liquids and solids, used onboardfor oil treatment.

Where a centrifuge is arranged to separate two liquids, it isknown as Purifier

Where a centrifuge is arranged to separate impurities andsmall amount of water from oil, it is known as Clarifier

The separation of impurities and water from fuel oil is essentialfor good combustion and to avoids damage (wear andbreakdown) to engine components.

The fuel oil required to be purified before used in engine forcombustion.

The lubricating oil in the engine system required to be purifiedcontinuously when engine is running since addition ofimpurities (wear particles, fuel oil,combustion productleakages,etc.,) and sludge formation.


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Principles of operation

When a volume of light oil is placedinto a tank contain a weir and a

quantity of water the fluids will tend toarrange themselves as shown above.The height of the water in the weirrises to a point governed by thevolume ( and thereby relative height)and specific gravity of the light oil.

As a oil/water mix is fed into the tankseparation begins with heavyparticulates falling to the base of thetank along with water which joins theother water excess overflowing the

heavy phase weir. Hopefully clear oilpasses over the light phase weir. Theproblem arises that to ensure their issufficient time to allow for full(separation of the oily mix the flow

would have to be very small relative tothe size of the tank.


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PRINCIPLE OF OPERATION

The principle of separation by which allcommercially available oil/ water separators

function is the gravity differential between oil andwater.

In oily water mixtures, the oil exists as a collectionof almost spherical globules of various sizes.

The force acting on such a globule causing it tomove in the water is proportional to the differencein weight between the oil particle and a particle of

water of equal volume.

This can be expressed as:

Fs = /6 D3

( w o)g


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FS = separating force.

w= density of water.

o = density of oil.D= dia. of oil globule.

g= acceleration due to gravity.


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Centrifuges

It was shown in the section on oil/waterseparators how liquids with a specific gravitydifference can be separated by gravity.

The process was expressed mathematically as

Fs = / 6 D3(pw- po) g


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Clearly in a standing vessel the acceleration cannotbe altered to enhance the separation force FS,

but by subjecting the operation to centrifugalforce the above expression can be replaced by

Fs = / 6 D3(pw- po) 2r

Where:= angular velocity

r = effective radius


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Both the rotational speed and the effective radiusare controllable within certain engineering

limitations.Thus if our standing vessel is replaced by a rotating

cylinder the separating force and hence the speedof separation can be increased. This, effectively, is

what happens in a centrifuge.

For many years marine centrifuges were designedfor batch operation,

that is the machines were run for a period duringwhich

solids accumulated in the bowl.


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The machine was stopped when the

accumulated solids began to impair itsperformance and the solidswere removed.

Batch centrifuging is still commonly used especiallyfor lubricating oil purification,

But many machines capable of continuous or semi-continuous sludge discharge are now at sea.


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The term separation is used to describe theseparation of two liquids.

In marine work these are usually oil and water.

A centrifuge arranged for the separation andcontinuous discharge of two liquids is called acentrifugal separator.

In a purifier arranged to operate as a centrifugalseparator any solids which are present will depositupon the side of the bowl, hence clarification takesplace at the same time as separation. In order fora purifier to operate as a centrifugal separator awater seal is necessary and this operates asfollows:


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Using the gravitation analogyFig. water is first fed into thetank at (a) until the level (b)

is reached, overflow of thewater will then take place at(b) and when this occurs thewater supply is stopped, asany additional water added

would not increase the levelof the water above (b).

Next oil is supplied into thetank at (a) and the oil willdisplace some of the water

in the tank, the amountdisplaced depending uponthe relative density of theoil.


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If overflow of oil takes place at(c) separation will occur, i.e.,water present in the oil will

settle out causing overflow ofan equal amount of water at(b) hence the quantity ofwater forming the seal

remains constant. Ifhowever, when oil is suppliedto the tank of sufficientlyhigh relative density to causethe oil-water interface level

(b) to reach (d), oil will bedischarged at (b) and no oilwill be discharged at (c). Thisis called loss of. seal.


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Rotating the settling tank analogyin Fig. through 9Qgives thebelow figure in the diagramwhich shows separation taking

place in a centrifugalseparator, the principle beinganalogous to that ofgravitational separationdescribed above. Water is firstdelivered to the centrifuge andwhen discharge of water takesplace at (b) the water supply isshut off and then oil isdelivered at (a), some of thewater is displaced and when oil

and water are being separatelyand continuously discharged,the centrifugal separator isoperational.

The water dam ring or screws


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The water dam ring or screws(some purifiers use gravitydisc) is used to vary theposition of (b) and thechoice of dam ring should besuch that the oil-waterinterface is as near aspossible to (d) without oildischarge taking place at(b). This ensures as large a

quantity of oil in thecentrifugal separator aspossible, thus for a

given throughput rate the oilwill be in the separator for

as long a time aspracticable, enabling thecentrifugal force to givegood separation andclarification.


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The equilibrium equation for the centrifuge is:h1= h2x oil density .

where h1is the head of water,where h2 is the head of oil .When oil of high relative density is to be passed

through the centrifugal separator the dam ring

(or gravity disc) which would be fitted will bring(b) closer to the axis of rotation increasing h1without altering h2.

If oil of low relative density is to be passedthrough, the dam ring (or gravity disc)

would have to be such that (b) is moved awayfrom the axis of rotation reducing h1

without altering h2.


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control of the oil-water interface line, orequilibrium line, can be achieved in some

purifiers by variation of back pressure, henceno dam ring would required.


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BASIC CETRIFUGE

The basic centrifuge differs than that describedabove most obviously by sitting on its side. Inreality it takes the form of a round bowl a crosssection of which will show something like that

seen above. Gravity is replaced by centripetalforce as the bowl is spun at high revolutionsthereby creating very high g-forces.

A disc stack is incorporated to encourage alaminar flow increasing improving theseparation effect. Dirty oil is introduced via acenter line oil feed dip tube. The oil is led todistribution holes which are reflected in the discstack but not the dam.


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Basic Centrifuge


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The wide bowl type can retain more sludge beforeits performance is drastically impaired and canbe cleaned in situ. Thus it has no handlingproblems.

On the other hand settling characteristics in a widebowl machine are relatively poor towards thebowl centre and the distance the heavy phasehas to travel before reaching the bowl wall is

great.To overcome these problems a stack of conical

discs, spaced about 2-4 mm apart is arranged inthe bowl.

The liquid is fed into stack either from the outeredge of the discs or via distribution holestowards the periphery of the discs depending onthe make of machine, and flows along thespaces between adjacent plates.


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The plates then act as extended settling surface,the heavy phase particles passing through thelight phase and impinging on the under surfaces

of the discs

Once the particles have impinged on the discsurface they are unable to accumulate.

They slide out towards periphery.At the periphery the water globules and solid

particles leave the stack.

The solids pass to the bowl wall, water issandwiched between solids and oil, whichorientates towards bowl centre.

The oil/water interface is called e-line.


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To gain the fullest advantage from the disc stackthe e-line should be located outside it.

On the other hand, if the e-line is located outsidethe water outlet baffle (top disc) discharge of oilin water phase will take place.

Referring to gravity separation, height of the liquid

in the two legs will relate as under:l (e-l) ~ h(e-h), where l =density of oilh= density of water

A very similar arrangement is found in the

centrifuge.The equation now becomes

2l(e2l2) =2h(e

2h2)

Or h/ l = e2

l2

/ e

2

h2

Design considerations require e-line to be confined


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Design considerations require e-line to be confinedwithin limits.

Variations in oil density will be there.Therefore, means of varying h or l needed for

compensating this.Usually, h is varied by using gravity discs (dam

rings) of different diameters.Normally a table is provided for disc diameters

required.Alternatively, diameter Dh calculated fromDh = 2 [ l

2 l/ h+ e2 ( 1 - l/ h) ]

e can be taken as the mean radius of one thinconical plate and the heavy top conical plate(outlet baffle).

If oil is discharged in the water outlet the gravitydisc is too large.

Important variation in oil temperature will vary


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Important- variation in oil temperature will varythe S.G. and therefore, temperature must bekept constant.

To some extent the feed rate will effect the e-line:

Excessive feed should be avoided since quality of

separation deteriorates with increase ofthroughput.

To prevent oil passing out of the water side onstart up it is necessary to put water in the bowluntil water shows at the water discharge.

The bowl is then hydraulically sealed.

Principle of separation in centrifuge containing angled plate stack


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Principle of separation in centrifuge containing angled plate stack


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Principle of separation in centrifuge containingangled plate stack

Fluid moving between two plates has a velocitygreatest at mid point and minimum approaching theplates.

A particle entering into the plates will tend to bepushed upwards by the fluid flow. All the timecentrifugal force tends to retard the horizontalcomponent of the movement causing the particle to

approach the underside of the top disc. As itapproaches the fluid flow velocity reduces. Thecentrifugal force eventually overcomes the forceacting on the particle due to fluid movement and the

particle starts to move towards the outer rim.


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The centrifugal force acting on a particle isproportional to its mass therefore a smallparticle will tend to move further under the

influence of fluid flow. Indeed a particlesmall enough will be carried through theplates and out with the discharge. In thisway it can be seen that reducing the flowrate to a purifier will tend to increase thequality of the output.


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The important path taken by particles is that of thelimit size particle, which is the smallest to beremoved in the centrifuge.

Particles smaller than this pas out with the cleanoil.

Some of the factors affecting the limit size particlewould be:

1) Viscosity of the oil in the centrifuge, the higher itsvalue the greater will be the viscous drag on theparticles. Hence the oil should be pre-heated to as

high a temperature as practicable.

2) Disc spacing, diameter and inclination to thevertical.3) Speed of rotation of the purifier.


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4) Throughput.

If this is low the limit size particle will be small

and the oil discharged cleaner.If throughput is great the limit size particle will be

large.

However, if the oil contains appreciable quantitiesof water this would be effectively removed andthis form of contamination could be the reasonfor operating at a high throughput.


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Function of purifier

The following factors are of importance whenunderstanding the function of the purifier

Increasing the sg of the oil will tend to push

the interface outlet and cause overflow from theheavy phase outlet untill the equilibrium isrestored. Should the interface be moved so far asto breach the dam, oil will be issued from theheavy phase outlet and an alarm will sound.

The ideal position for the interface is to lie over thedistribution holes.


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Reducing the sg of the oil will tend to bring theinterface towards the axis, this reduces the forceof separation on the oil mix and reduces the

efficiency of the unit possibly leading tocontaminants and water carryover with the lightphase outlet

the "gravity" disc are changeable on virtually

all purifiers. Their centre bore is governed by thesg of the oil being centrifuged. The largest boreshould be used without risking overflow

The flow rate of a purifier should be set tooptimize removal of whole system impurities.

The lower the oil feed the greater the time forimpurity removal and the more efficient thepurification.


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The higher the rate the greater the amount ofsystem oil is treated per unit of time.

For a system such as main engine oil wherecontaminants are continuously being added tothe system.

As a rule of thumb the total volume of the systemshould pass through the purifier three timesevery 24 hours, this rate may be vary

depending on operational parameters. A

similarcalculation has to be made withfuel oil to ensure removal of water andsludges which may accumulate over time.


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De Laval Bowl Type CentrifugeThe stainless steel bow) of up to 06 m diameter is

mounted on a tapered spindle, the lower part of

which is fashioned into a sleeve which passesover a stationary spindle. The stationary spindlecarries two ball races which are provided for therotating spindle.

These bearings, the upper one serving as a thrustbearing, give a high degree of flexibility.

A constant speed electric motor supplies the motivepower for the oil suction and discharge pumps (iffitted) and the wheel and worm drive for the

centrifuge bowl.


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The bowl rotates under operating conditions at50008000 rev/min depending upon size.

This gives a lower centrifugal settling force than isto be found in the tubular bowl type ofcentrifuge.

To compensate for the lower centrifugal settlingforce, stainless steel conical discs carried by

splines on the distributor are fitted to reducethe settling distance.

To operate the centrifuge as a purifier, it is firstbrought up to operating speed, supplied with

fresh water then the oil to be purified isdelivered to the distributor by the inlet pump.As the oil passes down the distributor it israpidly brought up to the rotational speed of thepurifier by the radial vanes provided for this

purpose.


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The oil passes from the distributor through the spacebetween the bottom plate and bowl to supply holes.

From the supply holes the oil is fed to the spaces

between discs through the distribution holes in thediscs.Separation and clarification takes place between the

discs.Water and sludge moving radially outwards pass along

the under surface of the discs and the purified oilmoving radially inwards passes over the upppersurface of the discs.

Water and sludge are eventually discharged at (a) andthe purified oil at (b)

. (See Fig. ).


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If the centrifuge is to be operated as a clarifier,no water seal is porovided and the bottom

plate and discs have no supply and distributionholes.

Discharge of the oil takes place at (b) sludgeand solids collect upon the bowl wall.

Since there is no water seal in a clarifier morebowl space is available for the oil, hence

there will be available a greater centrifugalsettling force due to the increased radii.

shows a centrifuge arranged for clarification.


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Self cleaning centrifuge


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Self cleaning centrifuge


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Self-Cleaning PurifierFig. shows diagrammatically the method of

sealing and sludge ejection for a self-cleaning

purifier.Bowl sections A, B and C, are all keyed to the

central drive spindle, B and C, are secured sothat they cannot move vertically whereas A is

free.The purifier is first brought up to operating speedand water is then supplied to space D throughsupply port G.

Due to centrifugal force the water pressure inspace D moves A vertically to form a seal at thebowl periphery.

Water and then oil would next be supplied to thepurifier in the usual way.


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When the purifier requires to be cleared ofsludge the oil supply is shut off and watersupply is changed over from G to F supplyport.

The hydraulic pressure created in space E is

sufficient to open the spring-icaded valvesand the water from space D will - togetherwith water from space F - be dischargedand A will fall, the bowl seal will now be

broken and the sludge ejection will takeplace.


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Alfa-Laval intermittent discharge centrifugeFigure shows a centrifuge bowl capable of being

programmed for periodic and regular dumpingof the bowl contents to remove the sludge build-up. The sludge discharge takes place through anumber of slots in the bowl wall.

Between discharges these slots are closed by the

sliding bowl bottom, which constitutes an inner,sliding bottom in the separating space.The sliding bowl bottom is forced upwards against

a seal ring by the pressure of the operatingliquid contained in the space below it.


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This exceeds the counteracting downwardpressure from the process liquid, because theunderside of the sliding bowl bottom has a

larger pressure surface (radius R1), than itsupper side (radius R2.Operating liquid is supplied on the underside of

the bowl via a device known as the paring disc.This maintains a constant operating liquidannulus (radius R3) under the bowl, as itspumping effect neutralizes the static pressurefrom the supply.

When the sludge is to be discharged, operating

liquid is supplied through the outer, wider supplytube so that if flows over the lower edge of theparing chamber (radius R4) and continuesthrough a channel out to the upper side of asliding ring, the operating slide.

Between discharges the operating slide is pressed


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Between discharges, the operating slide is pressedupwards by coil springs.

It is now forced downwards by the liquid

pressure, thereby opening discharge valves fromthe space below the sliding bowl bottom so thatthe operating liquid in this space flows out (b).

When the pressure exerted by the operating liquidagainst the underside of the sliding bowl bottom

diminishes, the latter is forced downwards andopens, so that the sludge is ejected from thebowl through the slots in the bowl wall. Anyremaining liquid on the upper side of theoperating slide drains through a nozzle g (c).This nozzle is always open but is so small thatthe outflow is negligible during the bowl openingsequence.

On completion of sludge discharge, the coil springsi f th ti lid d (d) th


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p g g , p gagain force the operating slide upwards (d), thusshutting off the discharge valves from the spacebelow the sliding bowl bottom.

Operating liquid is supplied through the outer,wider tube, but only enough to flow to the spacebelow the sliding bowl bottom and force thelatter upwards so that the bowl is closed. (If toomuchliquid is supplied, it will flow into the

channel to the operating slide and the bowl willopen again.)

The outer, wider inlet is now closed while theinner, narrower one is open (e).

The paring disc counter-balances the staticpressure from the operating liquid supply, andthe bowl is ready to receive a further charge ofoil.


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The situation is identical with that shown inthe first illustration of the series but withthe difference that the sludge dischargecycle is now accomplished.

Periodically the purifier bowl should be

stripped and thoroughly cleaned. It isimportant to remember that this is aprecision built piece of equipment, whichhas been carefully balanced and all partsshould be treated with the utmost care.


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purifierSELF CLEANING PURIFIER


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Fuel oil purifiers are fitted with a slidingbowl bottom which is raised or lowered bya water operating system for self cleaning.Lubricating oil purifiers are sometimes selfcleaning. Manual cleaning may be

preferred so that the solids can beexamined and also because use may beintermittent and the extra expense not

justified. The section (Fig. 35) shows one

operating system for a self cleaningpurifier.


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While oil is passing through the purifier, the slidingbowl bottom is held up in the position shown bythe operating water beneath it.

The sliding bottom seals the bowl by beingpressed against the sealing ring in the rim of thecover.

Solids from the oil are thrown outwards by

centrifugal force and collect against the bowlperiphery.At intervals dictated by either a timer or choice,

the oil feed is turned off and the bowl opened to

discharge the solids.There are a number of discharge ports around thebowl. At the end of the discharge, the bowl isclosed and after the liquid seal has been re-established, the oil feed is continued.

D i l i th t d b th t


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During normal running, the pressure exerted by the waterunder the sliding bottom, is sufficient to keep it closedagainst the pressure from the liquid in the bowl.

The operating water tank maintains a constant head ofwater to the paring disc which acts like a pumpopposing this head, provided that the radius of the

liquid annulus remains constant.If evaporation or leakage causes a slight water loss, the

reverse pumping effect of the paring disc is reduced andwater from the operating tank brings the quantity ofwater in the paring chamber back to the correct radius.The operating slide prevents loss of water from beneaththe sliding bowl, by closing the drain holes.

T di h th lid fi t th il f d i l d


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To discharge the solids, first the oil feed is closedand then the solenoid valve is opened.

This allows water from the high pressure line to

flow into the paring chamber.The water enters from a point nearer the centre

than the normal radius at which the operatingwater is maintained.

This extra water (indicated by the arrow) fills theparing chamber until it runs over the lip and viathe drilling in the bowl body, into the openingchamber immediately above the operating slide.

Water in the operating chamber, builds up apressure due to centrifugal force

(despite small loss through the drain nozzle) whichpushes the operating slide down against thesprings beneath it.


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As soon as the operating slide begins to

move downwards, the drain holes open andthe high pressure operating water

under the sliding bowl escapes rapidly.Pressure exerted by liquid in the bowl

forces the bottom down and solids aredischarged through the ports.

When all of the operating water has drained fromthe underside of the sliding bottom and


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the underside of the sliding bottom anddischarge of solids is complete, then with thesolenoid valve closed the operating slide is

moved back up by the springs to close the drainholes.To raise the sliding bottom, the chamber under it

must be filled with operating water.The filling is completed quickly by a short opening

of the solenoid valve.When the chamber is filled and pressurized the

paring chamber will start to fill.At this point the solenoid is closed to prevent

overflow and a second opening.The radius of the liquid annulus is then maintained

by the operating tank and paring discarrangement.


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