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IntroductionUnfortunately. a fairly high incidence ofstructural
damage or failure of bulk materialshandling systems is experienced
in the miningindustry (Krige, 2012), notwithstandingdesign
compliance with appropriate standards.Improved structural safety is
in the interest ofall employees and also facilitates steadycompany
earnings. Catastrophic failures maycause injuries or fatalities and
inevitably causesignificant business interruptions since
bulkmaterials mines are usually operated on acontinuous basis with
scheduled maintenanceintervals.
This paper specifically addresses rail-mounted mobile bulk
materials handling(BMH) equipment such as stackers, reclaimers,and
ship loaders, and focuses on designshortcomings pertaining to
controls, protectionsystems, and integration across
engineeringdisciplines. ISO 5049-1 (InternationalOrganization for
Standardization, 1994) isinternationally recognized and
utilizedthroughout the industry (Krige, 2012) for thedesign of
mobile BMH equipment. Compliancewith this standard means that the
designer hasmet the design obligation, notwithstandingthat the
limitations of the standard are widely
recognized (Krige, 2012; Morgan, 2012).Where equipment damage or
failure occurs,potential disputes between the owner andsupplier are
not easily resolved when the lattercan prove that the equipment
design met therequirements stipulated in the standard orclient
specification.
Although highly skilled and experienceddesign engineers are
usually involved in thedelivery of mobile BMH equipment,
recentfailures of machines designed in first-worldcountries by
reputable original equipmentmanufacturers (OEMs) support claims in
theliterature that the skills shortage crisis in theengineering
industry is yet to be resolved(Hays, 2012; Kaspura, 2011; Gardner,
2011).Failures cannot always be attributed to design-related issues
only. A wide range of factorsmay contribute to failures, including
materialquality, manufacturing, commissioning, abuse,etc. The
fast-track nature of most miningprojects nevertheless puts pressure
onequipment suppliers to provide new designswith a minimum of
engineering effort, and thismay be exacerbated by the scarcity of
designengineering resources. The drive towards morecost-effective
designs may result in lessconservative designs which leave
littletolerance for unexpected loading conditions orpossible future
upgrades. Furthermore, thelack of a proper systems design
approachrestricts the extent of integration betweenprotection
systems limits and structural ormechanical strength. The risk of
failure isoften not understood when controls arewilfully
over-ridden or have not yet beencommissioned.
Avoiding structural failures on mobile bulkmaterials handling
equipmentby M.J. Schmidt* and B.W.J. van Rensburg†
SynopsisBulk materials handling systems are extensively used in
the mining andminerals industry, where a fairly high incidence of
structural failure isexperienced, notwithstanding design compliance
with appropriatestandards. A number of case studies are explored to
demonstrate howinsufficient controls or protection systems have
contributed to structuralfailures on mobile bulk handling
equipment. The importance of designintegration across engineering
disciplines is highlighted. The revision ofISO 5049-1 (1994) is
proposed to provide specific rules and guidelinespertaining to
machine protection systems. It is further recommended thatthe
structural design engineer of the original equipment
manufacturer(OEM) fulfils a more prominent role during the final
acceptance andhandover of mobile bulk handling equipment, with
specific reference toprotection systems.
Keywordscontinuous bulk handling equipment, machine protection
system,structural failure, ISO 5049-1 (1994).
* Anglo American Coal.† Department of Civil Engineering,
University of
Pretoria.© The Southern African Institute of Mining and
Metallurgy, 2015. ISSN 2225-6253. Paper receivedDec. 2013;
revised paper received Sep. 2014.
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Avoiding structural failures on mobile bulk materials handling
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The aim of this paper is to recommend actions to improvethe
overall safety of mobile BMH equipment by focusing onaspects
specifically related to the design integration andcommissioning of
protection systems and controls. Threetypical case studies have
been selected from an assortment ofmobile BMH machine failures in
order to illustrate thesignificant impact that inadequate
protection systems andlack of design integration across engineering
disciplines hadon these failures.
Design standardsStandards related to the design of mobile MBH
include:
1. ISO 5049-1 (1994) Mobile equipment for continuoushandling of
bulk materials – Part 1 Rules for thedesign of steel structures
(International Organizationfor Standardization, 1994)
2. FEM SECTION II (1992) 2 Rules for the design ofmobile
equipment for continuous handling of bulkmaterials, Document 2.131
/ 2.132 (De La FederationEuropeenne de la Manutention, 1992)
3. AS 4324.1 (1995) Mobile equipment for continuoushandling of
bulk materials - General requirements forthe design of steel
structures (Standards Associationof Australia, 1995)
4. DIN 22261 (2006) Excavators, spreaders and auxiliaryequipment
in opencast lignite mines (German Institutefor Standardization,
2006).
ISO 5049-1 (1994), FEM SECTION II (1992), andAS4324-1 (1995)
focus on the design of the steel structuresand some mechanical
aspects associated with mobile BMHequipment. Although additional
parts were initially plannedfor all of these standards, which would
address mechanical,electrical, and other aspects, these were never
published.With the exception of DIN 22261, which is not
commonlyutilized (Schmidt, 2014), the standards available to
themobile BMH equipment industry are therefore silent on rulesand
requirements for machine protection systems. Byimplication, it is
therefore left to the equipment supplier toprovide protection
systems that are deemed adequate toensure the safety of any
equipment supplied.
AS 4324-1 (1995) is currently under revision and it isenvisaged
that the revised standard will be published in May2015 (George,
2014). Additional parts, which will addresselectrical and controls
aspects, are planned for publicationwithin the next two years.
Case studiesCase study 1 – collapse of a portal
reclaimerBackgroundPrior to failure, the machine had been in
production use forseveral months, although commissioning of the
collisionprotection system had not been completed. The
generalarrangement of a typical portal reclaimer is shown in Figure
1.
At the time of the collapse, the designed reclamation ratewas
exceeded by approximately 30%. The stockpile proximityprobes
appeared to not be working, resulting in unexpectedlyhigh digging
forces which led to the failure of majorstructural connections as
shown in Figure 2.
Key findings from the investigationThe lateral resistance of the
machine was insufficient towithstand the forces generated within
the structure whenexcessive digging was experienced. Proximity
probes,detecting the stockpile height, are fitted to ensure that
thedigging depth of rake buckets is maintained within theprescribed
limits. These devices did not function properly orhad not yet been
commissioned, so were switched off,resulting in excessive digging
forces (Anon., 2007; Krige,2012).
Electric drive motors are equipped with protection relaysto
limit the electrical current that can be drawn duringoperation,
i.e. the applied system torque can be limited.Industry practice
suggests that the overload protection is setto a value of 5–10%
above the peak system design load(Bateman, 2013). The protection
study report, compiledsubsequent to the failure, indicated that the
motor protectionrelay setting on the reclamation drives was at a
default valueof 2 instead of 1.05 (Anon., 2007). Furthermore,
themechanical design for the scraper drive system dictated
aninstalled motor power requirement of 154 kW, which impliesthat
the next size up of 160 kW was specified. Duringprocurement, 185 kW
motors were supplied due to theunavailability of the 160 kW motors.
This decision was madewithout consultation with the relevant design
engineers.
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180 MARCH 2015 VOLUME 115 The Journal of The Southern African
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Figure 2—Failure of the bogie on a portal reclaimer
Figure 1—Typical arrangement of a portal reclaimer
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Upon investigation, it was also found that the fluidcouplings
installed between the drive motors and reducerswere rated at
service factors such that a reclamation drivetorque could be
delivered that was only marginally below themaximum electric motor
torque. Torque transfer through fluidcouplings can be limited
according to the design requirementby reducing the percentage oil
fill, which is normal practice.The commissioning data revealed that
the fluid coupling wasoverfilled by approximately 15%. Small
amounts of oil athigh percentage fill levels will lead to a
significant increase intorque transfer capacity (Anon., 2007).
The machine could not withstand the motor startingtorque as
prescribed for the abnormal digging resistancecriteria as outlined
in ISO 5049-1 (1994). Depending onstart-up torque control, the
motor torque during start-upcould exceed twice the operating torque
on the motor,depending on the motor type selection, as shown in
Figure 3.(Curves B and C represent a typical conveyor drive
selection).
Multidisciplinary design integration – scraper drive systemThe
lack of proper design integration between mechanical,structural,
electrical, and control and instrumentationengineering disciplines
was revealed during the investigation(Anon., 2007). It is essential
that the structural designengineer understands the effect and
magnitude of forces thatcould be exerted on machine structures
under abnormalconditions. The mechanical, and likewise the
electrical,design engineer must understand how the selection
andcommissioning of equipment such as fluid couplings andelectric
motors could have an adverse effect on structuraldesign parameters.
The importance of interaction between thecontrol and
instrumentation and the structural andmechanical designers to
ensure that alarm levels and limitsare correctly designed and
commissioned cannot be overem-phasized.
Final acceptance and approval of the machine, and
morespecifically the validation of protection systems by the
OEM’sstructural design engineer or representative who
understandsthe structural limitations of the equipment, are
crucial. Thiscollapse highlighted the importance of understanding
theadditional risks associated with the production use of amachine
that has not been fully commissioned, and whereprotection systems
may be inoperative and stockpile volumeshave not yet been fully
calibrated. The operation of machinesthat have not been fully
commissioned must be prohibited,regardless of production
pressures.
Case study 2 - Collapse of a slewing stacker
BackgroundThe machine was successfully operated for
approximately a
year before collapsing completely. An incident in which theboom
conveyor belt was overloaded preceded the failureevent. The failure
of a critical tie-beam connection, which ishighlighted in Figure 4,
initiated the collapse of the boom andultimately ruined the entire
machine.
The extent of the damage can be seen in Figure 5.
Key findings from the investigationLoading conditions were
underestimated because an incorrectmaterial bulk density was used
in the design. The incorrectcommissioning of the speed switch
settings associated withthe boom belt contributed to the structural
overloading ofcritical tie-beam connections when slippage of the
boom beltoccurred.
Based on the design requirements of ISO 5049-1 (1994),critical
tie-beam connections were overloaded, although theultimate carrying
capacity exceeded the most severe designload combination. The
design of these connections istherefore considered to be marginal.
The tie-beamconnections utilized bolts in double shear in such a
way thatfastener threads intercepted a shear plane.
Furthermore,high-strength electro-galvanized fasteners, which
aresusceptible to hydrogen embrittlement (Erling, 2009), wereused
in this critical tie-beam connection. The topic ofcorrosion and
embrittlement is discussed at length in theAmerican Institute of
Steel Construction (AISC) Guide todesign criteria for bolted and
riveted joints (Kulak et al.,
Avoiding structural failures on mobile bulk materials handling
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181The Journal of The Southern African Institute of Mining and
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Figure 3—Characteristic start-up curves for different electrical
motors(Baldor, 2004)
Figure 4—General arrangement of slewing stacker. Critical
connection highlighted
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Avoiding structural failures on mobile bulk materials handling
equipment
1987). From laboratory tests referenced, Kulak et al. note ‘…it
became apparent that the higher the strength of the steel,the more
sensitive the material becomes to both stresscorrosion and hydrogen
stress cracking. The study indicateda high susceptibility of
galvanized A490 bolts to hydrogenstress cracking.’ It is ultimately
concluded that ‘galvanizedA490 bolts should not be used in
structures. The tests didindicate that black A490 bolts can be used
without problemsfrom brittle failures in most environments.’ (A490
bolts arethe direct equivalent of the Class 10.9 bolts used in
SouthAfrica). High hydrogen contents were confirmed by
themetallurgical examination of the fasteners, while surfacecracks
were noted at the thread roots of some specimens.Through the
application of fracture mechanics, it can bedemonstrated that the
load carrying capacity of the tie-beamconnection fasteners may have
been reduced by hydrogeneffects to a value far below what would be
required to sustaina boom load associated with the luffing
operation of anoverloaded stacker boom (Schmidt, 2014).
Supervisory control and data acquisition (SCADA)recordings
revealed that the boom loading significantlyexceeded the intended
design parameters prior to thecollapse. The alarm set-point to
alert a boom overloadcondition was specified at a level that was
too high to preventstructural overload. The machine could therefore
be exposedto severe loading conditions without any operator abuse.
Theprobability that operator abuse contributed to the failurecould
not, however, be ruled out altogether.
The protection systems on the machine were found to beinadequate
to ensure that structural loading remained withinthe intended
design parameters. At the time of the collapse,the machine had not
been formally handed over to theoperations team.
The root cause of the stacker collapse can therefore
besummarized as follows.
Design deficiencies contributed to a marginal design ofcritical
connections, which was further exacerbated bydefective bolts,
adversely affecting the carrying capacity. Theabsence,
malfunctioning, and incorrect commissioning ofmachine protection
systems allowed an overload condition todevelop, which led to the
catastrophic collapse of the stacker.
Case study 3 – structural damage to a drum reclaimer
BackgroundAlthough no failure occurred as such, significant
damage wasdone to the support legs of a drum reclaimer when the
controlsystem of one of the long travel drives
malfunctioned,resulting in a skewing action that imposed excessive
loadingwhich was not considered in the original design. A
typicalarrangement of the machine is shown in Figure 6.
Key findings from the investigationThe overall machine control
system was originally configuredwithout interlocks between the
independent long travel drivesystems located on adjacent bogie
wheel sets. When thecontrol system for the drives at the one end
malfunctioned,the drives on the opposite end continued with the
longtravelling sequence until the drives tripped on overload as
aconsequence of the skewing of the machine. Severe localdamage and
permanent deformation were caused to theboxed plate structural
section of the fixed legs. The machine,as shown in Figure 7, had
been in service for decades.
DiscussionSkew control can be achieved by comparing signals
fromincremental encoders on both sides of the machine
(McTurk,1995). Skew should occur only if one side of the
machinecannot travel for accidental reasons, e.g. an obstacle on
therails, and if this happens a signal must trigger the
immediateshutdown of the machine. The control systems
associatedwith the long travel of the machine were not
fail-safe.Abnormal loads, not anticipated in the original
structuraldesign, were subsequently exerted on major
structuralmembers. The equipment was nevertheless
operatedsuccessfully for many years prior to the skewing
incident.Insufficient design integration existed between the
OEM’sstructural, mechanical, electrical, and control and
instrumen-tation engineering disciplines during the detail design
phaseof the original project. The damage could have been avoidedby
the incorporation of additional protection instrumentationfor
negligible additional capital cost.
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182 MARCH 2015 VOLUME 115 The Journal of The Southern African
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Figure 6—Typical arrangement of a drum reclaimer
Figure 5—Collapsed stacker
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Integrated design approach
The lack of interdisciplinary design integration, as discussedin
the above case studies, is of concern. This is probably ahighly
controversial topic which design engineers wouldgenerally not want
to embark upon. Of course, some BMHequipment OEMs will address this
engineering challengebetter than others. Unfortunately, the facts
presented in theabove case studies demonstrate that design
engineers oftendesign with an engineering discipline-specific
approach,without the required understanding of design details
fromcounterparts representing other engineering disciplines.
Thismay have a direct influence on the overall performance of
theequipment. The author acknowledges that
discipline-specificspecialists are nevertheless required for the
successful designof mobile BMH equipment. The appeal is merely for
betterdesign integration, which is not based on perception
butrather on a thorough understanding of interdependencebetween
engineering disciplines. Although the competitivenature of the
mobile BMH industry generally leads to atendency amongst OEMs not
to openly share design contentwith their client representatives, it
would be advantageous toboth parties, especially where the client
appoints a third-partydesign auditor. While it is more common for
larger corporateclients to have skilled engineering staff assigned
to capitalprojects for the purposes of engineering oversight,
smallerenterprises generally rely entirely on the OEMs for
thesuccessful delivery of functional mobile BMH equipment
asspecified in the supply contract. Liaison between the OEM’sdesign
engineers and the client’s engineering discipline leadsis
invaluable for ensuring successful project delivery.Furthermore,
larger corporate clients often have a number ofoperations where the
same or similar mobile BMH equipmentmay be utilized in ways other
than was envisaged under thesupply contract. The input from
operational personnel, whoare responsible for the daily operation
and generalmaintenance of existing equipment, must not be
underes-timated, but the ability of such individuals to influence
newdesigns remains largely dependent on their skill
andexperience.
A typical integrated design team organization structurethat is
conducive to a high level of design integration with asystems
design approach is depicted in Figure 8. The
following aspects characterize such a team structure:
� Within the OEM’s design team organizational structure,there is
a free flow of information directly related todesign interfaces
between engineering disciplineswithout interference in
discipline-specific matters
� Design interfaces are approached as an integratedsystem with
input from relevant role-players as a teameffort across engineering
disciplines
� The respective engineering disciplines have a
soundunderstanding of how equipment selection and systemsdictated
by engineering counterparts influence theirindividual designs
� The client owner’s team participates in the design
scopedefinition and design risk assessment with specificreference
to machine protection and controls.Engineering input, oversight,
liaison, and progressivereview are provided by relevant
representation from theclient
� Specific design requirements are agreed between theOEM and
client owner’s team within the agreedcontractual arrangement
� There is a free flow of information between
thediscipline-specific engineers from the owner’s team andtheir OEM
counterparts responsible for the design,without compromising the
latter party’s intellectualproperty rights.
Although it is expected that most OEMs will embrace andadvocate
the integrated model, case studies unfortunatelysuggest that a low
level of design integration is oftenencountered within the
industry.
ConclusionThe brief case studies as discussed have highlighted
pastincidents where incorrect commissioning or inadequateprotection
systems and controls contributed significantly tothe collapse or
severe damage of mobile BMH equipment.Deficient protection systems
can often be linked back to the
Avoiding structural failures on mobile bulk materials handling
equipment
The Journal of The Southern African Institute of Mining and
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Figure 8 – Ideal design team organisational structure for
facilitatingdesign integration
Figure 7—Side elevation of drum reclaimer
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Avoiding structural failures on mobile bulk materials handling
equipment
lack of design integration across engineering disciplines.
Although the end-user may be inclined to assume that a
high level of interdisciplinary engineering integration
isexercised, the studies demonstrate that this is not
necessarilythe case, which subsequently necessitates that the
matterreceives greater focus during current and future
machinedesigns.
While a design standard can never be a substitute for apragmatic
design approach, the only international designstandard available
for the design of mobile bulk handlingequipment, ISO 5049-1 (1994),
does not address rulespertaining to machine protection systems. The
case studiesdiscussed demonstrate the industry’s need for an
updatedstandard to facilitate safe BMH designs in this regard.
A design team organization structure is proposed tofacilitate an
integrated systems design approach.
RecommendationsA technical committee should be appointed to
review the ISO5049-1 (1994) standard to include rules and
guidelinesregarding machine protection systems. Consideration
shouldbe given to the revisions envisaged to the AS 4324-1
(1995)standard facilitated by the Australian Standards
CommitteeME43 in this regard. Although this paper focuses on
machineprotection systems, there is an opportunity to consider
theuse of alternative lightweight and compound
constructionmaterials, as well as new rope technology for tie
systems,while revising the ISO 5049-1 (1994) standard. It
isfurthermore recommended that guidelines are provided fordesigners
who wish to follow a limit-state design approach,since there are a
number of reputable OEMs in the mobileBMH industry who do not
follow allowable stress principles.
It is recommended that the structural design engineer beclosely
involved with the verification of alarms and set-pointsassociated
with machine protection systems, in conjunctionwith other
specialists responsible for the design and commis-sioning thereof,
to make absolutely certain that these systemsand controls comply
with the design intent before finalhandover.
A high level of interdisciplinary design integration mustbe
pursued with specific reference to machine protectionsystems and
controls. A risk-based design approach shouldbe mandatory.
AcknowledgementsAnglo American plc for permission to use the
materialpublished. The opinions expressed are those of the
authorand do not necessarily represent the policy of AngloAmerican
plc
Dr G.J. Krige for input into this study topic and my career
Sandvik Mining and Construction for valuable input into
thestudy
OEMs who participated in the study survey.
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