automatizaciontren molienda

8/13/2019 AutomatizacionTren Molienda

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1956 TWENTY-TI-II RD CONFERENCE 45

By W. R. CRAWFORDntroduction

Automatic control of continuous processes is not unknown inQueensland sugar mills. Where these controls have been installed theyhave been chiefly concerned with control of continuous chemical processessuch as the maintenance of a near-constant pH value in the limed mixedjuice and simple physical problems such as the maintenance of boilinglevels in evaporators and constant density of syrup.

The word automation has a different meaning. Althoughapparently of comparatively recent vintage, it is now used extensivelyin technical literature to describe the conversion to completely automaticoperation of any process, manufacturing or otherwise. As might beexpected, the greatest emphasis to-day is on automatic manufacturingin repetition work, since it is in this field that the greatest advantagesare to be gained. Nevertheless, automation will yield similar benefitswhen applied to any unit operation which is continually repeated andit is in this connection that it may be usefully applied in the sugar factory.quite well-known example of automation in sugar m lls is theuse of automatic juice scales. Although opinion seems to be divided as towhether this is even a desirable operation, it is certainly not an essentialone. There are, however, certain essential unit operations in a sugarmill which are continually repeated, and keeping to the theme of themilling train, outstanding examples are the movement of cane trucksto, and away from, the cane carrier, and the unloading of these trucks.With the high crushing rates in vogue to-day, and the type of labouravailable for the carrier operation, there can be little doubt that at leastpartial automation of truck handling operations is a most desirable andprofitable project.

In the past, little attention has been paid to the possibilities ofautomation and automatic control in the milling process and it is withthese matters that the writer deals in this paper. Perhaps the reasonfor the lack of control systems in this field is that modern automaticcontrol equipment tends to become more and more complicated andtechnical, and if used extensik~ely, equires the empIoyment of a highIyskilled technician for maintenance. Sugar mills are rugged places andmuch of the automatic control equipment available is not sufficientlyrobust for this application.

The mill engineer should remember, however, that in the past hehas probably used simple mechanical controls which are sufficientlyeffective for general purposes, and that those may be applied and adaptedt o o t h r needs For example automatic control of the exhaust mainpressure by a reducing valve is quite common, and the control valveis easily maintained. Let us consider, therefore, how simple mechanical


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146 TWENTY THI RD CONFERENCE- - 956A

and mechanical-electrical devices may be utilised to control the millingprocess, from the weighbridge and into the milling train and, by sodoing, lead to more economic and efficient milling.Mechanisation of Truck Handling

Under this heading three operations must be dealt with:-a) Feeding the cane trucks to the carrier,b) Unloading these trucks,c) Moving the empty trucks away from the carrier.

These operations must be integrated so that the trucks flow con-tinuously and feed the carrier at the required rate. Too often it isfound in Yueensland mills to-day that the crushing rate is determinedby the rate a t which the cane can be placed on the carrier. The efficientmill should have a feeding system which is capable of keeping pacewith the crushing plant under any circumstances.

The problems posed by a), b) and c) above, have been metsuccessfully in other industries, particularly the colliery industry, wherelarge numbers of comparatively small trucks are continually handled.Let us consider each of the above items separately and then see how theycan be integrated to give the desired final lesult.a) Feeding the Cane Trucks to the Carrier.-This operatibn normally

starts at the weighbridge where the rake of trucks is passed over theplatform by a rope and friction winch, the latter usually situated nearthe carrier and operated by the carrier hands. After weighing, therake is then pulled forward to the carrier by the same winch. Atintervals the rope must be detached and carried back to the weighbridgeto pull forward another rake. These operations are wasteful of man-power and the intermittent nature of the collection of rakes from theweighbridge militates against the continuity of cruslung. I t is only inrecent years that many mills have pensioned off the horse which draggedthe rope back to the weighbridge from the carrierA much more economic and effective system may be borrowed fromcolliery practice. Referring to Fig. 1 it can be seen that the winchand rope are replaced by two truck creepers, one on either side of theweighbridge. These creepers are endless chains, which run in a trenchbetween the rails, and t o which are attached, at suitable intervals, hornswhich engage the axles of the trucks. These horns are so constructedthat a following rake of trucks can overrun them. Each creeper isindependently motor driven, the unit feeding the weighbridge beingoperated by the weighbridge attendant. I t is desirable to fit a solenoidbrake on the motor driving this creeper, so that the attendant may

position the trucks on the weighbridge with ease.The carrier feed creeper is best controlled from a position near thecarrier.I t will be seen that with this arrangement, a rake of trucks deliveredto engage with the creeper A may be weighed and fed towards the canecarrier until the creeper is filled, without interfering with carrierfeeding.b) Udoadircg the Cane.-The truck creeper delivers trucks on to

a slope just in front of the tipping or raking position Fig. 2 . At this


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148 TWENTY THIRD CONFERENCE 956

point the trucks may be uncoupled and chains released, and beyondthis point, in normal mill practice, much manhandling occurs, requiringseveral carrier hands and slowing down the rate of feed.On the gravity feed slope Fig. 2) are three sets of retractablechocks and a pneumatic or hydraulic pusher P) which can engage withthe rear axle of the truck which is currently against the chocks A. The

chocks may also be operated either pneumatically or hydraulically.In the position shown, a truck has just been emptied, chocks Aand are in their holding position and chocks C are withdrawn. Thefollowing sequence of operations may then be brought about by a singlelever actuating suitable valves.

i) Move lever forward to withdraw chocks A and simultaneouslycause pusher P to push full truck on to tip, thereby pushingempty one on to gratrity removal slope when it engages withthe removal creeper.ii) Move lever to neutral position, withdrawing pusher and insertingchocks A.

iii) Move lever backwards to insert chocks C and remove chocks B,permitting a full truck to gravitate on to chocks A.iv) Move lever to neutral position re-inserting chocks B and remov-ing C allowing a truck to gravitate to B, or to be moved there

by the feed creeper.This completes a cycle of operations which may, literally, occupyonly a few seconds.These are the elements of one simple method of mechanising truckmoving and unloading. This will be most effective, as regards speed,where a tip is wed and one man could easily control all operations.If it is desired to make the feeding system completely automatic, thena fairly simple sequence controller may be installed with which a singlepush button would initiate and carry out the sequence of operationsoutlined above. Where a tip is used, the return of the tip to its normalposition could be used to energise the sequence controller.It may be considered, however, that these refinements are unneces-sary in the truck handling operation and, in any event, it would appeardesirable to retain manual control of the actual tipping operation.Pneumatic operation, because of smaller time delays, is preferableto hydraulic operation and standard pneumatic equipment is readilyavailable from a number of sources.c) emozhg Empty Tmcks.-As indicated in Fig. 2 this is doneby another truck creeper, also fitted with axle horns which may beoverrun by oncoming trucks. This creeper may be controlled by theattendant who couples and greases, or oils, the trucks, or it may becontrolled from the carrier station.The system which has been described above requires for its effectiveinstallation straight lengths of rail in front of the weighbridge andbetween weighbridge and carrier, and a reasonable length of straight

h e beyond the carrier. Even if these are not available, the methodof automatic handling on to and through the tip is still applicable.


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1956 TW EN TY TH RD CONFERENCE 149

Another labour saving device now largely used in coal mining isthe swivel coupling which enables trucks to be tipped without uncouplingthem from the train. I t would be a costly business to fit these couplingsto all the rolling stock of a mill, but, even so, the saving in labour costsmight well make it a sound proposition.f swivel couplings could be fitted, another avenue for automaticoperation is opened up for those mills installing feeder carriers at rightangles to the main carrier. This is an adaptation of the rolling tipperwhich is a German invention permitting tipping of coal trucks on to abelt while the train of trucks is in motion.

Fig. 3----Illustrating the principle of the rolling tipper for unloading cane.Referring to Fig. 3 a section of the rails between AA and CC istwisted so that the truck as it moves forward passes gradually fromits normal horizontal position at AA to the fully tipped position at RBand thence gradually back to its normal position at CC.Special means would have to be devised to retain the trucks onthe line, as, for example, a channel or angle section over the off-sidewheels as suggested in Fig. 3. This method of tipping would establishcompletely automatic feed, the rate of tipping being controlled by speedcontrol on a truck creeper moving the train towards the carrier. Arolling line of this type presents an intricate problem to the blacksmithin suitably shaping the rails; but, it is not insuperable, because therequired centreline of each rail may be drawn on the surface of a cylinderand the necessary operation is therefore a combined bending and twistingof each rail.


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utomatic Carrier ControlThere can be little doubt that in the interests of good cane millingit is desirable to have automatic control of the feed to the first unitof a milling train. Benefits which may be derived from such anapplication include :

a) a more uniform feed,6) a reduction in labour costs,c) the elimination of personal judgment,d) the control of incipient knife chokes.

Most automatic carrier control systems installed up to the presenttime have been chiefly concerned with prevention of knife chokes. Oneor two mill engineers in Queensland have had the initiative to couplethis carrier control system with automatic feed control and have metwith considerable success. Carrier control systems, so far installed, aremostly electrically operated and the writer knows of only one instancewhere a mill engineer has a form of carrier speed control on a steam-driven carrier, based on overloading of the knives.

The writer believes that in devising existing carrier control s3rstemsemphasis has been placed on the wrong factor and that the prime functionof automatic carrier control should be the supply of a constant anduniform feed to the milling train. Prevention of knife chokes should besupplementary. Completely new electrical control installations arecostly and can hardly be justified for the single purpose of preventingchokes, when this can be done by placing the cane on the carrier uniformlyand in the proper state for kniving.Feed control by carrier speed coxtrol will be only partially effectivewith existing mill layouts unless a feed chute of very large capacity isavailable to provide storage between carrier and mill, because mostexisting antichoking controls involve intermittent delivery of preparedcane from carrier to hopper.Those who have attempted automatic feed control have used feedblanket thickness, as measured by a Killer type finger plate, as thecontrol criterion. This is not always entirely satisfactory because ofvariations in the degree of preparation, the type of cane and the facttha t the control point is usually so close to the feed opening of the mill.The writer believes that a system based on constant weight of canedelivered to the mill would lead to greater uniformity in milling, withconsequent improved results in extraction. At the same time it ispointed out that, if both sets of preparatory knives were well downthe carrier, a wide Killer type plate resting on the feed above the kniveswould be of assistance in initiating carrier control to give a more uniform

feed to the mills. I t is believed that in one instance this has been done.Without wishing to detract in any way from the excellent pioneerwork on electrical methods of carrier speed and antichoking control onwhich existing installations have been based, the writer wishes to discussmethods which will enable mills to retain their existing steam drives.The steam engine is a most flexible machine and, with some justification,many engineers are reluctant to dispense with it in favour of an electricdrive. Let us consider the matter in sections and assume steam-drivenknives and carrier.


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1956 TWENT\I THIRD CONFERENCE 151a) latichoking Control.-Carrier speed control to prevent knifechokes is based on overloadmg of the knives. In electrical installationsthe measure of overloading is motor current and from this criterionstem the relays resistances and switch gear which slow down the carrierin steps as the knife load progressively increases. I t may be thoughtthat it is difficult to find a similar criterion for steam drives but thisis not the case in practice.

Consider the average throttle-governed high speed reciprocatingengine used for driving cane knives. The speed is usually controlled bya modified Hartnell type centrifugal governor; but if the load buildsONT40L IR SUPP Y

SRVO VALVEW U A L VALVE SERVO OPERATEDCARRIER WINCHT W R O n = uWL

Fig 4 An automatic control for cane carriers driven by steam enginesup steadily or suddenly a point will be reached where despite fullthrottle the engine speed starts to fall and if the load is not relieved theengine will stop. This is the condition arising from knife chokes andit is clear that if the knife load were controlled by maintaining a lowerlimit to the knife speed then choking would never occur.

It follows that any device which by regulating carrier speed willprevent the knife speed falling below a predetermined value providesan antichoking control.To provide an automatic control of this type for steam-drivencarriers is comparatively simple and one suggested method is illustratedin Fig. 4 I t makes use of a servo operated throttIe valve on the carrierwinch. The servo valve is supplied with compressed air and the positionof i ts piston is controlled by a small sensitive centrifugal governor driven

by the knife shaft. A sleeve around the servo piston is attached by aflexible steel cable through a suitable linkage to the spindle of the carrierthrottle valve.If the knife speed drops below a predetermined value the governorsleeve falls causing the piston of the servo valve to move down. Thisaction feeds compressed air to the top of the piston of the throttle valvecausing it to commence to shut thereby reducing carrier speed. At the


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same time the flexible cable operating through the linkage pulls downthe sleeve over the servo piston and, as i t overruns the piston, the supplyof air to the throttle valve is cut off, thereby bringing the valve to restat a new opening. As long as the knife speed continues to fall, thethrottle valve will continue to close. When the knives free themselvesfrom the cane and the speed begins to rise, the reverse action takesplace, the air escaping from the throttle valve piston to permit it tobe opened by the compression spring. There is a neutral position forthe servo valve which is determined by the governor setting. I t willbe noted that in the suggested arrangement carrier speed is controlledonly in the downward direction.

This air-operated servo system has many advantages. I t is easyto maintain and will give positive continuous reduction in carrier speeddown to a stop. The only disadvantage of the system is in the flexiblecable link between the servo valve and the throttle valve; but, withreasonable precautions, this should not prove troublesome.

In a system of this type, failure of the air supply causes the throttlevalve to take up the fully open position and the control of the winchmay then be taken over by a manually operated throttle valve in serieswith the servo-operated valve as suggested in Fig. 4Although this servo system has been described, it will be clear thatany governing device which will reduce the carrier speed in an attemptto keep knife speed constant will do the same or a similar job. Manyother simple devices can be suggested, such as a bank of paralleledsolenoid valves in the steam supply line to the winch, which are energisedby a tacho-generator driven by the knife shaft and which cut out insequence as the knife speed falls.At the moment it has been assumed that, apart from this anti-choking control, the speed of the winch is manually controlled byvalve in front of the antichoking control valve, and a system for linkingthis antichoking control with automatic feed control will be consideredlater. b) utomatic eed Colztro1.-As already pointed out, the systems

already in use are only partially effective when combined with anti-choking control unless a very large feed hopper is available. In manymills it is virtually impossible to fit longer chutes and the method tobe described has been devised to overcome this difficulty.The general scheme is shown diagrammatically in Fig. 5 Themain cane carrier is shortened without altering its height this is possiblein most mills) and between the feed chute and the nose of the carriera short horizontal apron feeder is inserted. This short carrier is com-pletely supported on a framework pivoted on the supporting structureat it s end nearest to the mill. The other end is supported on adjustable

springs and dashpots are fitted as indicated. This carrier may be drivenfrom the mill or by a motor, whichever is expedient; but, it must notbe driven from the main carrier. The end supported by the springs isconnected by a simple linkage to an air servo valve of the type alreadydescribed and this actuates the throttle valve on the winch.The throttle valve should have a spindle of adjustable length sothat , with its operating piston in mid-position, the speed of the canecarrier may be adjusted to give the required crushing rate. When thisspeed has been established, the springs and servo valve connections are


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1956 TWENTY TH RD CONFERENCE 153

adjusted to place the servo valve piston and sleeve in the neutral position.The servo valve will now respond to varying loads on the feeder carrierin an endeavour to maintain the load on that carrier at a constant value.The stiffness of the spring chosen must ensure that the full travelof the servo valve is obtained within the permitted limits of load onthe feeder carrier. Knowing the crushing rate and permitted variationsand the feeder carrier speed and dimensions an approximation to thespring stiffness required can be calculated and the spring arrangementmay then be designed to give variable stiffness within a selected rangeof this. This arrangement allows easy adjustment to suit changes nthe crushing rate which may be desirable from time to time.

3LA-D CdWUfR PIWTU) AT ONE ENAND SUPPOFtTED ON S P R ~T ~ E R .D ~ L A C E M U I T UE TO VARY Q

E SPRINt3.Y

Fig. 5 A system of automatic feed control to No mill.The length of balanced feeder carrier is a matter on which no definiterecommendation can be given; but it may be said that the objects areto provide a reasonable storage of cane to allow for slowing up of the

main carrier to avoid knife chokes while a t the same time attemptingto provide a constant feed to the mill. I t is likely that a length of tento twelve feet would give very satisfactory results in the majority of cases.This arrangement will work most satisfactorily when the secondset of preparatory knives is situated some distance down the cane carrierthereby avoiding the projection of prepared cane which occurs when theknives are situated right on the nose of the carrier.For those mills which have feed chutes of reasonable length orwhere it is thought that the installation of the weighing feeder is notjustified the same servo operated system may be used actuated from aKiller plate in the feed hopper. The dashpot is an essential feature inall applications of this type otherwise the winch engine will hunt violently.c ) Combined Feed nd Antichoking Control.-The two systemsdescribed above may be combined readily to provide an automatic feedcontrol system with an overriding antichoking control. n its most

efficient and inexpensive form the dual arrangement uses only one throttlevalve on the carrier winch and an interlocking device is used to ensurethat when the knives start to slow down control is taken away from


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the feed control system. Otherwise peculiar results might be producedbecause as the carrier slowed down to overcome knife overloading thefeeder control would try to speed it up again.

Probably one of the simplest methods of interlocking is thatillustrated by Fig. 6.As indicated both servo valves are connected to the throttle valveand in this case two tiny solenoid valves are inserted one each in therespective air lines from the servo valves to the throttle valve. Thevalve on the feed control system is normally open while that on the

SECnONSHOWING YJITCHfS

ONE N O I I W L L Y OPENSERVO V LVE ONL NORM LLY CU SWFT

SOLENOlO V LM S L E N O I O V WENORM LLY OPEN NORM LLY CLOSE

IR SUPPLY IR SUPPLY

LMLRGUJCVM NU L V LVE

Y K C W N I C L LINKFig. 6 A system for combined feed and anti choking control.

antichoking control is normally closed. I t follows that when the knivesare running at normal speed or higher the throttle valve is isolated fromthe antichoking control and the carrier speed is completely controlledby the feed control system.

Above the governor lever on the antichoking control are two micro-switches and B Fig. 6. The switch A has normally open contactsand is connected to the circuit of solenoid SA. I has normally closedcontacts and is in the circuit of SB. The switches are adjusted forposition so that with the governor lever in the position of normal knifespeed the contacts of are closed and those of B open i a . valveSA is open and valve SB is closed. When the knife shaft commencesto slow down the governor lever drops and the valve SA closes while SB


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1956 TWENTY TH R CONFERENCE 55

opens, thereby transferring carrier control to the antichoking system.The flexible strip connections between the switches and the governorlever permit adjustment in the allowable drop in knife speed beforechange of control, and also allow for some increase in knife speed withoutdamage to the switches.?Vhen the incipient choke has been overcome and knife speed returnst o normal, control is returned to the feed system by the closure ofSBand the opening of SA.Although this arrangement with the solenoid isolating valves hasbeen suggested, other methods may be used to interlock the two controls;but, the above method is so simple and positive that improvementappears to he difficult.

onclusionThe subject of mechanisation of truck handling and cane unloadingis a most important one at the high crushing rates of the present day.Mechanisation, complete or partial, will enable a mill to feed the carriera t any required rate with a much smaller labour force at the carrier.One system, widely used in the colliery industry, has been describedand other suggestions have been made.Cane carrier controls to mitigate knife chokes and to maintain moreuniform mill feeding have been discussed and a mechanical system hasbeen advanced for discussion. I t would be much less costly than someexisting electrical systems and would require no special maintenance.The writer believes that knife choke control should be supplementaryto mill feed control and, if the two are combined, effective feed controlcan only be obtained by dividing the carrier into two parts or by havinga very large feed chute. Despite the discussion of antichoking controlsthe writer is still of the opinion that these controls are merely expedientsnecessitated by incorrect placing of the cane on the carrier and that,

i the cane is placed on the carrier in the proper tangled condition asfrom rakes or a feeder carrier, the necessity for antichoking controlswill not arise.

Sugar Research ImtituteMackay.

automatizaciontren molienda

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