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Mining Science, vol. 28, 2021, 189–200
Mining Science
(Previously Prace Naukowe
Instytutu Gornictwa Politechniki
Wroclawskiej, ISSN 0370-0798)
www.miningscience.pwr.edu.pl
ISSN 2300-9586 (print)
ISSN 2353-5423 (online)
Received September 29, 2020; Reviewed; Accepted September 28, 2021
COMPUTATIONAL MODEL
OF THE BASIC EFFICIENCY PARAMETERS
OF THE BUCKET WHEEL EXCAVATOR
WORKING IN A BLOCK WITH A VERTICAL OBSTACLE
Anna NOWAK-SZPAK*
Faculty of Geoengineering, Mining and Geology, Wroclaw University of Science and Technology,
Department of Mining, Na Grobli 15, 50-421 Wroclaw, Poland
Abstract: The article presents the results of the numerical experiments designed to determine the effect of
obstacles in the form of the drainage wells/piezometer on the basic efficiency parameters of the excava-
tor’s work. Simulation research on built model included defining the basic parameters of the excavator
working in the front block on stabile front (as a comparison) and on the non-stable front for two variants
of drainage infrastructure exposing.
Keywords: bucket wheel excavator, efficiency, computational model, open cast mining
1. INTRODUCTION
The main task performed by bucket wheel excavators is the process of generating a stream
of excavated material. During the excavators’ operation, productive states can be dis-
tinguished, when a stream of excavated material is generated, and unproductive states
related to manoeuvring movements necessary to maintain the continuity of work. The
condition for a productive state is the simultaneous occurrence of the main working
movement (rotation of the bucket wheel) and the lateral movement of the boom (rota-
tion of the excavator body). Nowadays, the speed of wheel rotation in excavators is
usually unregulated, i.e., it has a constant value. The rotation speed of the excavator’s
_________
* Corresponding author: anna.nowak-szpak@pwr.edu.pl (A. Nowak-Szpak)
doi: 10.37190/msc212814
A. NOWAK-SZPAK
190
body (swing mechanism) is controlled manually or automatically, according to a spe-
cific program and its value is limited by the maximum design value, given in the tech-
nical machine data. Unproductive states occur immediately after the completion of
full cutting with a given working range of the cutting wheel, at its specific location in
relation to the excavator’s body, and at a given excavator site, in the excavated
shortwall. The required manoeuvring activities to obtain the re-contact of the wheel
with working face: the infeed movement, performed after each slice has been cut, the
change of the terrace to a lower one, performed after excavating the slices in a given
terrace, and lifting the boom with the cutting wheel to the height of the first terrace
and thus, starting the mining of a new block advance.
In the BWE working process, stable and non-stable operations are distinguished.
The states of stable work are achieved in the working front with constant parameters
of a block in a quasi homogeneous soil medium. They are characterized by the fact
that the stream intensity of the bulk material over a longer period of time oscillates
close to its average value. The share of unproductive states during such work is the
smallest. Non-stable operation states occur in the excavator's working process at the
irregular front, forcing changes in the parameters of the excavator's working and
manoeuvring movements.
Examples of non-stable work states are:
work at the end of the working front (at the drive and return station) – the irregu-
larity of the front is mainly caused by cutting slices of variable width while
making an s-indentation into a new block,
starting block cutting below work level – the irregularity of the front is caused
by the cutting of slices of variable height,
selective operation – also involves the operation of variable heights as well as
forced breaks in the continuity of the stream, due to the time needed to change
conveyors on the switch,
work near obstacles, i.e., boulders, wells or piezometers – is caused by the in-
creased number of manoeuvring movements needed to exploit the block in which
the obstacle occurred.
1. COMPUTATIONAL MODELS
OF A BUCKET WHEEL EXCAVATOR WORK
Mining companies strive to reduce the energy used at each stage of the mine’s
production value chain, thanks to which it is possible to both, reduce costs and carbon
footprint (Kawalec et al. 2021; Kawalec, Król 2021, Suchorab, 2019). The computeri-
zation of mining works allows for the collection and analysis of more information
about the mining activity and thus, the identification of the key parameters of the
mine’s value chain (Pactwa, Woźniak, 2015). The literature mentions many simula-
Computational model of the basic efficiency parameters of the bucket wheel...
191
tion and identification programs related to bucket wheel excavators. The scope of
dependencies included in them is varied and depends on the objectives of the author’s
research. They concern the efficiency of the working process, the use of the excava-
tors’ potential (Nan et al. 2008; Rašić et al. 2016; Zhao-xue, Yan-long 2014; Gale-
takis, Roumpos 2015), maintaining the required safety as well as the reliability condi-
tions of the machine (Daničić et al. 2016; Ilić 2021). Most of the research related to
the technology of wheel excavators’ work focuses on determining the efficiency in a
single slice, with constant geometric parameters of the block, i.e., a constant height and
width of the haul itself and individual terrace (Kressner 2006; Kołkiewicz, Szatan 1993;
Brînaş 2021). Their common feature is the strictly deterministic character, as they are
based on functional dependencies between the parameters of the working front’s ge-
ometrical structures, the ranges and kinematics of the excavator’s working units, as
well as geotechnical and operational constraints.
As part of the implementation of the development project entitled: Mechatronic
system of control, diagnostics and security in opencast mining machines identification
and simulation software for bucket wheel excavators was prepared (Wygoda et al.
2013). It allows for the identification of a working front structure with a sublevel block
as well as calculating the basic efficiency parameters of excavators. Supplementary
data is available, containing information on the courses of individual phases calculat-
ed in the program, including the angular ranges of the cycle states and the time of
their implementation. They can be used to program the manoeuvring movements of
the machine for its autonomous operation (Poltegor-Instytut 2013). The created mod-
els do not
allow for the simulation and determination of the excavator’s work effects with an
unstablized front, forcing changes in the parameters of the excavator’s working and
manoeuvring movements, as is the case with unconventional shapes of the excavator,
e.g., in the case of an obstacle in the form of a boulder or a drainage well (Nowak-
-Szpak 2016).
Designing deeper and deeper open cast mines is associated with the need to protect
them against additional groundwater resources that may be activated from the in-
crease of effective infiltration in the area of the lowered water table (Szczepiński
2018). The forecasting size of the mine water inflow, and thus, the scale of the drainage
problem, the hydrostructural system and the conditions for groundwater supply are the
most important. This is especially visible in the case of the largest lignite open cast mine
in Poland, Bełchatów. The depth of the mining pits is 200 m in the Bełchatów Field
and 280 m in the Szczerców Field respectively (for which the planned maximum
depth is 330 m). The large-diameter pumping well system covers the area of several
thousand hectares. At the end of 2017, there were 819 drainage wells in operation
(253 Bełchatów Field; 566 Szczerców Field) and 969 piezometers (Bełchatów Field –
339; Szczerców Field – 630) (Stobiecki, Pierzchała 2018). Their number significant-
ly affects the work efficiency of bucket wheel excavators. Figure 1 shows the work
A. NOWAK-SZPAK
192
plan of the K45 excavator operating in the second overburden floor in the Szczerców
Field. There are over 30 wells/piezometers in the 3 km long and 100 m wide mining
front.
Fig. 1. Work plan of the bucket wheel excavator SchRs 4000.50 in the second overburden floor
of the Bełchatów lignite mine, Szczerców exploitation field
The article presents the results of simulation tests performed on an extensive computa-
tional model developed by the author. The introduced changes to the algorithms took into
account the increased number of unproductive states caused by the increased sequence of
movements in a single block advance forcing the exposure of the vertical obstacle.
2. COMPUTATIONAL MODEL OF THE BASIC PARAMETERS
OF AN EXCAVATOR’S WORK ON A NON-STABLE FRONT
WITH A VERTICAL OBSTACLE
The extensive computational model describes, in a mathematical manner, the tech-
nological operation consisting of selecting the masses in such a way as to reveal the
obstacle, enabling its elimination by appropriate mining services, while ensuring
the safety of people and machines. The excavator’s work in the vicini ty of a drain-
age well can be carried out in various ways and depends on the existing conditions,
but most of all – the subjectivism of operation control. Therefore, the mathematical
model includes only the technically justified components of the operation, neces-
sary to achieve the goal.
In order to correctly define possible technological operations in a single block
advance with an obstacle, it is necessary to define the limits of the manoeuvring
movements’ path performed by the excavator. Since the computational model is to
determine the difference in the achieved efficiency parameters during stable and
non-stable work, it was assumed that the external boundaries of the single block, in
which the obstacle occurred, will be the same as the single block in which there is
no obstacle. Thus, the operation begins at the end of the work cycle on a block
where the excavator’s work is stable and ends at the beginning of the stable work
Computational model of the basic efficiency parameters of the bucket wheel...
193
cycle on a new block advance.
The boundaries of the manoeuvring path inside the block are determined by the lo-
cation of the obstacle. The following parameters were adopted to identify it in a three-
-dimensional space:
the boom deflection angle at which the obstacle is located at the moment the
excavator starts cutting (
prz0),
the distance of the excavator’s axis from the obstacle’s axis at the moment the
excavator starts cutting (Lprz_0),
The parameter supplementing the information about the obstacle is its diame-
ter (dp).
Taking the aforementioned information into account, algorithms that allow for the
precise determination of the obstacle’s location were built and thus, the internal bounda-
ries of the operation of exposing the wells in each of the strips are able to be calculated.
Based on the observations as well as consultations with excavator operators, two vari-
ants of the excavator’s operation were developed in a carrier with an obstacle.
Variant I call for the vertical obstacle in the form of a well to be removed (cut off) sepa-
rately in each terrace. Thus, in the horizontal structure of the terrace, the phases of
the technological process implementation have been identified (Fig. 2). In the first phase,
the excavator works as in the case of a stable operation, i.e., it selects successive slices
with the full width of the block. The boundary of phase I is the unveiling of the well to the
extent sufficient to be mined. As the obstacle divides the terrace into two parts, the slices
on the outer (phase 2) and then the inner (phase 3) side of the block are selected in the next
stages of the process. The number of slices processed in these phases is determined by the
necessity to fully expose it in the analysed stage and to keep the advance between the
phases. Such preparation of the obstacle allows it to be safely accessed from the side of the
front block and cut by a section corresponding to the height of the terrace, by authorized
services. After removing the obstacle, the excavator continues to select the
A. NOWAK-SZPAK
194
Fig. 2. Technological operation phases of the excavator’s work on the single block advance
with an obstacle in variant 1
masses in the terrace (phase 4). The sample diagram of the technological process devel-
oped according to the assumptions of variant I is shown in the figure (Fig. 2).
In Variant II, four phases were also separated in the horizontal structure of a sin-
gle block, but a different sequence of their selection is assumed, so that they are
characterized by a geometry different than in variant I. The work in the first
phase proceeds as in the case of a stable operation, i.e., the width of the block.
The sequence calls for that the first phase slices to be excavated in all terraces
(Fig. 3). After the obstacle is exposed (in the section corresponding to the height
of the upper step), strips are selected in all steps between the outer border of the
block and the obstacle (phase 2) next, between the inside of the block and the obsta-
cle (phase 3), and in the last stage, the remaining part of the block with obstacle is
selected (phase 4).
Computational model of the basic efficiency parameters of the bucket wheel...
195
Fig. 3. Technological operation phases of the excavator’s work on the single block advance
with an obstacle in variant 2
3. SIMULATION TESTS OF EXCAVATOR WORK
IN A SINGLE BLOCK ADVANCE WITH AN OBSTACLE
In this chapter, simulation studies were carried out as an example of the created model’s
practical application. The conducted numerical experiments were aimed at determining
the impact of the occurrence of an vertical obstacle in the form of a drainage well/
piezometer of the excavator’s work achieved basic parameters.
The simulation tests were carried out for the following geological and mining con-
ditions of the SchRs 4000.50 bucket wheel excavator:
Parameters of the excavated material: overburden,
workability class II/III,
group 4;
Block parameters:
– number of terraces (bands) s = 3 szt.,
– block height HZ = 15 m,
– block width BZ = 100 m,
– height of individual terraces (bands) hs1 = 4 m,
hs2 = 6 m,
hs3 = 5 m,
– the length of a single excavator ride wdp = 1.0 m,
– number of slices np = 15,
– turning angle of rotor boom in a sublevel towards the inside lateral slope
A. NOWAK-SZPAK
196
kw = 80,
– front slope angle αc = 60,
– side slope angle αb = 45,
– inside wedge band angle α0z = 30,
– outside wedge band angle α0w = 90;
the time needed to cut and remove the obstacle Tup = 45 min.
Assumed parameters for the location of the obstacle, at the moment of starting the
mining of a single block:
obstacle diameter dp = 0.711 m.
the boom swing angle at which the obstacle is located
p_0 = 0,
distance of the excavator’s axis from the axis of the obstacle Lprz_0 = 86 m.
The method adopted to determine the impact of a vertical obstacle is to compare
the obtained effects of such work with the effects of work on the stable front. There-
fore, in the first stage, a simulation was carried out on the work in which there was no
obstacle. The computational model specifies the geometrical parameters according to
which the uninterrupted operation of the excavator will take place. Since the work on
the stable front is characterized by constant geometrical parameters, each of the 15 slices
will have the same parameters of the technological model, i.e., widths (BZi, Bwi, Bzi),
turning angle of rotor boom in a sublevel towards the inside and outside lateral slope
(
wi,
zi) and cutting radius (PUi).
The total time needed for such work was estimated at 4 hours 18 minutes and
31 seconds (Table 1). The productive time during which the output stream is generat-
ed is 88% (03:47:42). The total unproductive time, which includes the time needed to
perform manoeuvring and steering activities, is 12% (00:30:49). The effective capaci-
ty was determined at the level of 7,427 m3/h.
Calculations for a single block advance according to the assumed variants of work
in a carrier with a vertical obstacle showed that the number of slices to be selected in
single terrace will increase from 15 to 25 in variant I and 41 in variant II (Fig. 4).
Table 1. Summary of the basic parameters of the excavator’s work
in the single block advance on the stable front
Basic parameters
Symbol
Unit
Value
Volume of the excavating block
Vz
[m3]
32 000.95
Total operating time
Tz
[s]
[hh:mm:ss]
15 511
04:18:31
– productive time
Tp
[s]
[hh:mm:ss]
13 662
03:47:42
– unproductive time
Tu
[s]
[hh:mm:ss]
1 849
00:30:49
Efficiency values
– effective
Qe
[m3/h]
7 427
Computational model of the basic efficiency parameters of the bucket wheel...
197
– technical
Qt
[m3/h]
8 301
– theoretical
Q0
[m3/h]
11 040
Performance indicators:
– technical utilization rate
0
[–]
0.89
– theoretical utilization rate
e
[–]
0.67
Fig. 4. The number of slices in successive phases of operations in individual terraces
according to variant I (left) and variant II (right)
For such defined conditions for the implementation of technological operations,
the basic parameters of the mining process were simulated (Table 2). The values show
that working in a single block advance with an obstacle will reduce the effective effi-
ciency to 80% in variant I and 76% in variant II. The total operating time will be ex-
tended by 3813 seconds (1 hour 3 minutes and 33 seconds) and 4982 seconds (1 hour
23 minutes and 2 seconds), respectively. The largest share in the extension of work-
ing time are periods of unproductive states, which in both variants increased almost
threefold (variant I – 296%, variant II – 351%).
Based on the conducted estimates, the precise time after which subsequent sec-
tions of the drainage well are uncovered (in each terrace) was also determined. In
the case of variant I, it is 01:14:07 (episode in 1st terrace), 03:24:44 (episode in 2d
terrace) and 05:03:44 (episode in 3rd terrace). In variant II, this time is more con-
centrated because the expected moments of unveiling the next episodes occur after
4:32:58 (episode in 1st terrace), 4:56:38 (episode in 2nd terrace) and 5:20:39 (epi-
sode in 3rd terrace).
Table 2. Summary of the basic parameters of the excavator’s work in the single block advance
on the unstabile front according to variant I and variant II
Basic parameters
Symbol
Unit
Variant I
Variant II
Volume of the excavating block
Vz
[m3]
31 995
31 995
Total operating time
Tz
[s]
[hh:mm:ss]
19 324
05:22:04
20 493
05:41:33
– productive time
Tp
[s]
[hh:mm:ss]
13 846
03:50:45
14 003
03:53:22
– unproductive time
Tu
[s]
[hh:mm:ss]
5478*)
01:31:18*)
6490*)
01:48:10*)
A. NOWAK-SZPAK
198
Efficiency values
– effective
Qe
[m3/h]
5961
5620
– technical
Qt
[m3/h]
8301
8301
– theoretical
Q0
[m3/h]
11 040
11 040
Performance indicators:
– technical utilization rate
0
[–]
0.72
0.68
– theoretical utilization rate
e
[–]
0.54
0.49
*) The unproductive time includes 45 minutes (2700 seconds) needed for the liquidation (cut-off) of
a fragment of the drainage well.
Fig. 5. Instantaneous efficiency of the bucket wheel excavator (time series) in single block advance:
a) stabile front, b) unstabile front according to variant I, c) unstabiled front according to variant II
Anticipating such information allows for the prior planning of the necessary auxil-
iary works and the organization of relevant mine services for the removal of drainage
infrastructure. The parameterized models of the exploitation technology thus make it
possible to consider many possible variants, compare them and decide which of the
working scenario is the most appropriate in terms of the availability of these services.
In order to perform a more detailed analysis, a simulation was carried out, in which
it was assumed that the time needed to cut and remove the obstacle (Tup) was 0 sec-
onds. Such an assumption made it possible to estimate the actual differences resulting
from the increased number of manoeuvring movements performed by the excavator.
Computational model of the basic efficiency parameters of the bucket wheel...
199
In both variants, there are significant differences in the obtained operating parameters.
The estimated time of unproductive states significantly reduced the total cutting time
to 16 624 seconds (4 hours 37 minutes and 4 seconds) in variant I and 17 793 seconds
(4 hours 56 minutes and 33 seconds) in variant II.
The change in the time of excavation of the single block advance significantly in-
fluenced the obtained effective work efficiency, which was determined at the level of
6872 m3/h (variant I) and 6473 m3/h (variant II). This means that the efficiency of the
excavator’s work caused by additional manoeuvring movements resulted in a decrease
in efficiency to 93% and 87%, respectively.
The energy consumption to excavate single block advance while working on the
stable front (estimated on the basis of active power) is 15.08 MWh. Extending the
selection time of all slices while working on non-stable front with an obstacle results
in increased energy consumption, which according to variants I and II is 16.16 MWh
and 17.30 MWh, respectively.
4. CONCLUSIONS
The existing bucket wheel excavator simulation software allows only the programming
of stabilized work – i.e., the working front with constant parameters of a block. The
article proposes an innovative approach to the preparation of short-term work plans
for a bucket wheel excavator operating on an unstable front, forcing changes in the
parameters of the excavator’s working and manoeuvring movements.
Thanks to the developed algorithms, it is possible to analyse variant manoeuvring
movements to exploit the block in which the obstacle occurred. Based on the results
obtained, the most appropriate scenario of carrying out this process may be selected.
This decision is now left to the machine operator, and is solely based on his subjective
decisions.
The use of the developed models allows a rational evaluation of the variant, based
on the obtained performance parameters and the energy consumption needed to select
a given block. It also provides the operator with a choice, due to the availability of the
necessary auxiliary works. The simulations of the manoeuvring movements performed
allow for the determination of the exact times in which their participation is neces-
sary. Anticipating such information makes prior planning and the organization of rel-
evant mine services for the removal of drainage infrastructure possible.
Further development of the presented method will consist of testing the developed
model in real mine conditions. This requires an earlier compilation of the developed
software with a GPS system installed on a bucket-wheel excavator as well as its con-
trol systems.
A. NOWAK-SZPAK
200
ACKNOWLEDGEMENTS
The author would like to express sincere gratitude to Professor Lech Gładysiewicz from the Faculty of Ge-
oengineering Mining and Geology, Wrocław University of Science and Technology, for his useful critiques
of this research work and the employees of Poltegor Institut for their substantive support during research.
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