Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
FIELD OF THB INVBNTION
2 The invention relates generally to a system for
3 controlling the loading of feed material onto an endless belt
4 conveyor train.
BACKGROUND OF THE INVENTION
6 The present invention has been developed in
7 connection with a conveyor train used to transport oil sand
8 from a mine site to a hydrocarbon extraction plant. It will
9 now be described in connection with that application, however
the system can easily be adapted for use in other
11 circumstances. It is to be understood, therefore that the
12 invention is not to be considered limited solely to the
13 application described herein.
14 Typically, a conveyor train system comprises a
number of endless belt conveyors, hereinafter termed
16 ~ conveyors~, serially arranged whereby the output load from
17 one conveyor is transferred as the input load to the next
18 conveyor in the train. Each conveyor, however, comprises a
19 discrete unit.
The individual conveyor comprises lengths of
21 flexible belts spliced together end to end to form the
22 endless belt. In high capacity systems, each such belt is
23 commonly formed of upper and lower rubber layers having
24 reinforcing steel cords sandwiched therebetween. The belt is
supported on a plurality of spaced, cushioned, anti-friction
26 idlers. One or more driven pulleys is provided at the
27 conveyor end for driving the belt. The driving pulleys are
28 normally powered by conventional electrical motors.
9~
1 Additional drive motors may be included, i required to cope
2 with variations in load. Tension means are lncorpoxated to
3 keep the belt taut.
4 For a system comprising belts of fixed length, it
S is common practice to run the conveyor train in a manner
6 such that each conveyor draws the same percentage of its
7 total rated power.
8 Where the load-carrying portion of any one belt
9 can vary however, a major disadvantage in such a system is
that overloading of a single conveyor can cause it to stall.
11 When this occurs, the whole train must be brought to a halt.
12 Restarting the train is difficult due to the great power
13 needed to get the loaded conveyors under way again.
14 Frequent stalling and restarting reduces productivity and
increases equipment wear.
16 In applicants' commercial open-pit oil sand mining
17 operation, carried out in the Fort McMurray region of
18 Alberta, about 300,000 tonnes per day of oil sand is mined
19 and conveyed to the extraction plant. Mining is carried out
using large draglines whlch excavate the oil sand to a depth
21 in the order of 40 metres. The draglines deposit the oil
22 sand in elongate windrows along the edge of the rectangular
23 pit. A bucketwheel reclaimer transfers the oil sand from
24 each windrow to a bridging conveyor, which feeds it onto the
first conveyor of a train. The train to the stacker may
26 consist of 2 to 4 conveyors extending around the perimeter
27 of the pit. In applicants' case, there are 4 draglines in
28 use, each supplying a separate train. The
29 - 3 -
7C~3
1 trains all terminate at a common zone adjacent the extraction
2 plant. Here the load from each train is transpoxted by an
3 inclined conveyor (or ~stacker~J and deposited on an arcuate
4 stack of oil sand. The extraction plant draws its feed from
these 4 stacks.
6 It will be noted:
7 - that the bucketwheel reclaimer works its way
8 along the length of the windrow and thus the
g point, at which its bridging conveyor deposits
lo the oil sand onto the first conveyor of the
11 train, varies;
12 - that the bucketwheel reclaimer deposits the oil
13 sand in discrete spaced bucket loads on the
14 transfer conveyor; and
- that the various conveyors of a train vary in
16 length. Exemplary lengths may range from 150
17 to 2500 metres.
18 Stated otherwise, the weight of the feed is not
19 distributed uniformly along the conreyors and the load-
bearing length of the conveyors in a train is also not
21 uniform.
22 It is a requirement of the train concept that the
23 velocity at which the various conveyors travel should be the
24 same. So any resistance to motion will automatically require
that the power drawn by the drive motors be increased, so as
26 to maintain a constant belt speed. There exists a maximum
27 safe level of electrical power that can be drawn from each
28 motor without causing it to stall. One seeks always to
29 optimize the loading of conveyors by running them at their
P3
1 maximum safe rated power draw. This is a power draw somewhat
2 below the maximum (stall) power level. ~aximum safe rated
3 power correlates with the upper limit of the load that can be
carried ~y the conveyor.
However, it is not difficult to inadvertently
6 exceed this rating.
7 Conveyor power draw will fluctuate depending upon a
8 number of factors. For example~ the internal frictional
g losses associated with the conveyors are subject to numerous
10 variations. Additionally, external factors such as
11 fluc~uations in ambient temperature or differences in the
12 properties or grade of the oil sand per se will give rise to
13 variations in power draw requirements. Expanding further on
14 this latter point, the adhesive properties of the oil sand
vary with its grade. If the oil sand is sticky, it will
16 build up on the belt. This in turn affects the drag
17 characteristics of the belt and hence the power draw. Or
18 there may be density variations due to snow or rainfall pick-
19 up, which will alter the weight and adhesivity of the load.
20 It is to be emphasi~ed that the power draw fluctuations
21 caused by the above factors are not insignificant.
22 However, the single predominant factor leading to
23 rapid alterations in the power draw requirements resides in
24 the inherent inconsistency and intermittent nature of the
bucketwheel loading technique itself.
26 In using a bucketwheel reclaimer for loading the
27 bridging or first conveyor of the train, the operator has
28 only his experience and visual observation to guide him as to
29 the rate at which the feed material should be deposited on
i7~3
1 the conveyor.
2 When the system was irst put into use, the only
3 method of control was full on/full off. An observer in the
4 control tower at the systems delivery end simply radio-
signalled that the bucketwheel reclaimer should start or stop
6 adding feed to the firs~ belt of the system.
7 In order to assist the reclaimer operator to more
8 accurately gauge the optimum conveyor loading rate, a prior
9 art control system was utilized. This method involved
lo attaching wattage meters to the conveyor drive pulley motors
1l and monitoring the power draw requirements thereof. An
12 operator, located in a control tower positioned at the output
13 end of the conveyor train monitored the wattage draw signals
14 and instructed the reclaimer operator to 0ither reduce or
increase the feed rate depending upon whether an overload or
16 reduced power draw was observed.
17 However, this prior art method allows of only crude
18 control. The method fails to take into account the
19 following:
- the irregularity and intermittent nature of the
21 rate of loading the feed from the individual
22 buckets of the wheel;
23 - the time lag between the actual loading of the
2~ feed material onto the belt, its transportation
along the conveyor train, and the relaying of the
26 feedback message to adjust the loading rate
27 accordingly (this time lag may be of the order
28 of twelve minutesJ;
9t7 [?3
1 - the variation of effective length of the first
2 conveyor actually in use. This length will
3 change depending upon the position of the
4 reclaimer as it moves along the windrow; and
- the combined effects of irregular feed rate
6 and power draw fluctuations on the optimum
7 load each individual conveyor can carry.
8 It is to be noted that, in the present instance,
g the feed point of the system varies according to where, in
the mine, the oil sand is loaded onto the system. As well,
11 belt lengths are changed as the system is expanded to
12 accommodate an increase in the size of the pit. The conveyor
13 system is thus dynamic and variable. This differs from a
14 static system where the feed point and belt lengths are
fixed-
16 There exists, therefore, the need for a control
17 system functional to adopt itself to systematic fluctuations
18 and to optimize the load carried by the conveyor train whilst
19 not exceeding the permissible rated power draw for the drive
mOtors-
21 SUMMARY OF THE INVENTION
22 The present invention was based on the recognition
23 that:
24 - In every conveyor train, there exists a
limiting conveyor. That is to say, there is one
26 conveyor in the train which approaches
27 its maximum power draw requirements more
28 closely than do any of the remaining
~ ~5t~
1 conveyors. This conveyor is referred to
2 herein as the ~limitin~ conveyor'. As conditions
3 change, the conveyor in the tl-ain, which .is the
4 limiting conveyor, can chan~e (that is, at one
point in time the limiting conveyor may be the
6 first conveyor - at another point, it may be the
7 third conveyor);
8 - ~t is necessary to identify the limiting
g conveyor and to provide means for continuously
lo monitoring the power draw thereof to ensure
11 that overloading is avoided;
12 - Instantaneous weight readings of the feed
13 material being loaded onto the conveyor have to
14 be averaged out over pre-determined time periods,
in order to provide meaningful data. Due to the
16 irregularity of the deposition of feed material
7 onto the train from the b~lcketwheel reclaimer,
18 instantaneous weight readings as an indicia of
19 feed rate are valueless. Additionally, the
weight of feed has to be measured at the input
21 end of the train, to minimize errors arising from
22 time lags within the system; and
23 - A 'rolling average' of the total weight of feed
24 material on the limiting conveyor can be
computed in advance. By 'roll.ing' is meant that
26 several data points are averaged and, at fi~ed
27 intervals, the oldest point is withdrawn from
28 the computation and the most recent added. Thus
29 by computing a 'rolling average' it is possible
1 to predict the total load of feed material which
2 would be carried ~y an~ pre-determined section of
3 the train downstreamt given the current average
4 feed rate, and hence the current digging rate of
the reclaimer. A substantially direct cor-
6 relation exists ~etween the computed 'rolling
7 average' and the predicted power draw of the
8 limiting conveyor. As a result, a feedback
9 means can then be applied for controlling the
feed rate of material onto the train to there~y
11 optimize loading of the system and avoid
12 overloading of the limiting conveyor thereof.
13 ~roadly stated, the invention in an apparatus
aspect comprises: first means, associated with each endless
belt conveyor drive means, for substantially continuously
16 measuring the power draw by such drive means and producing
17 signals indicative thereof , whereby the conveyor in the
18 train which is the limiting conveyor may be identified;
19 second means, associated with the initial conveyor in the
20 train, for instantaneously measuring the weight of the feed
21 material deposited thereon and producing signals indicative
22 thereof; third means, associated with the weight measuring
23 and signalling means, for computing the average feed rate of
24 material deposited on the initial conveyor over a pre-set
period of time, utilizing the instantaneous weight
26 measurements sensed and producing signals indicative thereof;
27 fourth means, associated with the first and third means, for
28 projecting the computed current average feed rate to the
29 total load the limiting conveyor downstream will carry at
1 that feed rate, calculated as a rolling average o~ the load,
2 and producing signals indicative thereof; and fi~th means,
3 associated with the manually operated loading machine, for
4 receiving and displaying the signal produced from the fourth
means, whereby the feed rate addition to the first conveyor
6 may be altered in response to the fourth means signals, to
7 thereby control the rate at which feed material is being
8 deposited onto the conveyor train to avoid overloading the
g limiting conveyor.
In a method aspect, the invention comprises:
11 measuring the individual power draws of the drive means of
12 each conveyor of the train and utilizing said measurements to
13 establish which is the limiting conveyor of the train;
14 substantially continuously measuring the instantaneous weight
of feed material being added at the input end of the train
16 and producing signals indicative thereof; averaging, over a
17 pre-set period of time, the rate at which feed is added to
18 the initial conveyor; computing from said weight-added
19 signals a rolling average of the total load on the limiting
20 conveyor; and adjusting the feed rate in response to the
21 computed rolling average to ensure that the limiting conveyor
22 will not be overloaded.
23 DESCRIPTION OF THE DRAWINGS
24 Figure l is a schematic showing the mining
equipment used at applicants' open-pit oil sand mine site;
26 Figure 2 is a schematic of apparatus employed in
27 the practice of the present invention;
1Figure 3 is a flow chart showing the steps of the
2 method carried out by the apparatus of Figure 2;
3Figure 4a, 4b and 4c are a series of histograms
4 included to demonstrate the operability of the invention.
They show plots of the frequency (% of operating time) versus
6 the conveyor load (kt/h). The rate capacity varies depending
7 on ambient conditions. For the examples of Figures 4a and 4c
8it is 6400 kt/h, and for 4b it is 7300 kt/h. Figures 4a and
9 4b exemplify cases where the conveyor is operated using
conventional methods. In 4a, output is fairly high ~79%J but
11 the rated capacity is often exceeded. In 4h, to keep maximum
12 load at or below the rated capacity, production is reduced
13 (64%). Figure 4c shows an example of the conveyor being
14 operated using the method of the present invention. In this
15 case, the histogram shows that most events occur at or just
16 below the rated capacity. Production is high (86%) but the
17 system is never overloaded; and
18Figure 5 shows plots of weightometer response
19 versus time as (a) raw load cell output (b) filtered load
20 cell output (c~ filtered load cell output averaged over 4
21 second intervals to eliminate the effect of intermittent feed
22 as it is deposited by individual buckets of the BWR, and (d)
23 is a rolling average produced from the average readings to
24 predict the load that will be carried by the limiting
25 conveyor.
7~;~
DE:SCRIP ION OF l'HE PREFERRED EMBODIMENl'
2 ~aving reference to Figure 1, it shows a
3 conventional oil sand mine in which the present invention may
4 be utili~ed. It should be understood that although the mine
system set forth is operated in four essentially equal
6 quadrants, it is described herein particularly with respect
7 to a single quadrant.
8 Nore particularly, the mine system comprises a
9 dragline 10 which excavates the oil sand at the pit face to
generate a windrow 12. A bucketwheel reclaimer 14 transfers
1l the oil sand from windrow 12 onto the endless belt conveyor
12 train 16. Conveyor train 16 carries the oil sand to a
13 stacker 17 which transfers the material onto a stack 18. A
14 control tower 13 is centered within the stacking area. The
mined sand is conveyed from the stacking area to the
16 hydrocarbon extraction plant (not shown)~
17 The conveyor train 16 shown comprises four
18 conveyors 16a, 16b, 16c, 16d, serially arranged as shown in
19 Figure 1. Each conveyor was driven ~y a driven pulley
powered by one or more electric motors 19. Typically, the
21 train layout comprises the following:
22 Length Travel l'ime Number of
23 (Meters) (Minutes) Motor Drives
24 Conveyor 16a2,500 9 4
Conveyor 16b 300
26 Conveyor 16c2, 500 8 4
27 Conveyor 16d1,000 4 3
12
g~
l In accordance with the invention, and as shown in
2 Figure 2, watta~e meters 22 were electrically connected to
3 each drive motor l9 (only one of which is illustrated~ to
4 monitor the power drawn by each drive system. I'he wattage
meters 22 were conveniently located in the control tower 13.
6 From the tower, a radio transmitter 24 sent a signal
7 indicative of power draw to a radio receiver 26 located in
8 the buc~etwheel reclaimer 14. The operator in the bucketwheel
g reclaimer 14 would instruct a computer as to which conveyor
10 in the pertinent train was the limiting conveyor.
ll A continuous-sensing weightometer 23 was associated
12 with the bridge conveyor lS of the bucketwheel reclaimer 14.
13 More particularly, roller 30 was arranged to continuously
14 contact the underside of the belt 32 of the bridge conveyor
15. A pivoting arm 3~ supported the roller 30 and
l6 communicated with a load cell 36. As roller 30 moved
l7 upwardly and downwardly in response to the amount of material
18 loaded on belt 32, a signal indicative of the weight was
19 transmitted from the load cell 36. The weightometer 23 was
constructed by the inventors. Meters serving the same
21 purpose are widely available in commercial form. The load
22 cell was manufactured by Strainsert as model FL7.5U (CJ-
23 2SKWT.
24 A signal conditioner 38 received the output signal
transmitted from the load cell 36. Signal conditioner 38
26 electronically filtered out noise and variables which would
27 influence the true weight readings. It further functioned to
28 amplify the output signal. The signal conditioner 38 used
29 was model TSC 17-1121 option l manufactured by Acrotech.
13
1 Slip rings 40 were mounted in the bucketwheel
2 reclaimer 14. The amplified signal from the conditioner 3~
3 was passed via slip rings 40 to a computer 42. Attached to
4 computer 42 was a monitor screen 44.
It is to ~e noted that the computer 42 waB
6 "associated" with or operatively interconnected with the
7 wattage meters 22 (through the medium of the reclaimer
8 operator) and with the load cell 36.
9 ~ithin the internal memory of the computer were two
programs which enabled the computer to process the incoming
11 data to provide a feedback arrangement for controlling the
12 rate at which feed was loaded onto the conveyor train and
13 hence for controlling the digging rate.
14 More particularly, the first program caused the
computer to average the instantaneously sensed weight of oil
16 sand loaded onto the bridge conveyor 15 over a pre-determined
17 time interval. This produced a measure indicative of the
18 average current feed rate .
19 The second program was adapted to utili~e the
average current feed rate data and the transit time of the
21 limiting conveyor as the input data, to compute a 'rolling
22 average' of the load on the limiting conveyor. The transit
23 time was determined by dividing the length of the conveyor by
24 its velocity.
Both first and second programs were written
26 utilizing conventional programming steps. Programs of the
27 second type should take account of any variations in belt
28 slope.
1 The steps executed are illustrated in Figure 3.
2 ~ith load added to the system held constant for a length of
3 time required to fill the entire system, the wattages drawn
4 by the drive motors of the individual conveyors of the
conveyor train were monitored or measured, using meters 22.
6 The conveyor which drew the highest percentage of the maximum
7 power available from its drive motor(sJ 19, was established
8 as the limiting conveyor of the train. rhe bucketwheel
9 reclaimer operator was advised of the identity of the
limiting conveyor. The length of the loaded portion of the
11 limiting conveyor was utilized as input to the second
12 program.
13 'The instantaneous weight of feed material deposited
14 on the bridging conveyor 15 was determined as described
hereabove. The current average feed rate was computed
16 utilizing program 1. The computed rate was utilized as input
17 data for program 2.
18 As stated earlier, the second computer program then
19 computed the projected rolling average of the weight o~ feed
material onto the limiting conveyor from the input data.
21 The results from the first and second computer
22 programs were displayed to the bucketwheel reclaimer
23 operator. The power draw, monitored in the control tower,
24 indicated whether the limiting conveyor was loaded below
capacity. Based on this, the operator was instructed to
26 adjust the digging rate if required. The correctness o~ the
27 adjustment was determined by the new wattage draw. rrhe
28 adjustment was determined by the effect of a discrete change
29 in digging rate on the load carried by the limiting conveyor
1 as given by the rolling average program.
2 Example of Operation
3 l. Determine critical conveyor:
4 B~R operator digs at constant average feed rate until
entire conveyor train is loaded at that feed rate.
6 Control tower power meters are examined to identify
7 conveyor which is draw.ing the highest percent of its
8 allowed power. This is the critical conveyor.
9 2. Set rolling average period~
Identity of critical conveyor is radioed to B~R
11 operator, who inputs corresponding loaded length to
12 program Z. Computer now displays rolling average of
13 feed on loaded length of critical conveyor.
14 3. Set desired digging rate:
If power draw is low compared with available power,
16 control tower radios BWR operator to increase digging
17 rate~
18 or
19 If power draw is too high or if less feed is required,
control tower radios BWR operator to crease digging
21 rate.
16
,3
1 BWR operator increases or decreases digging rate las
2 required) and maintains new average rate until critical
3 conveyor has been loaded at that feed rate.
4 Control tower radios BWR operator with further
corrections as a few iterations may be required.
6 4. Naintain desired digging rate and conveyor power draw:
7 The desired digging rate, as determined by step 3 is
8 maintained by the B~R operator keeping a constant
g rolling average feed rate.
