Learning from structural failures of materials handling equipment

Transcription

1 Learning from structural failures of materials handling equipment G. J. Krige* * WAH Engineering Consultants, Johannesburg, South Africa ( geoffk@waheng.co.za) ABSTRACT Bulk materials mines utilise a wide range of materials handling machines. Generally, the structural components of these machines are designed in compliance with ISO which was published in 1994, although it is widely recognised that there are serious shortcomings in this standard, which has led to the publication of AS in Australia. The standard is Part 1, the original intention being that further parts should cover mechanical and electrical components and the control systems of this equipment. However, no further parts were ever written, which is a pity because many of the shortcomings relate to the interaction between structural, mechanical and electrical components of the machines. Experience shows that in many cases this equipment is vulnerable to collapse or functional failure of the structural components. Several brief case studies are presented to give examples of typical problems. It is concluded that it is necessary to reconsider the design philosophy and commissioning precautions applied to these machines. In particular it is recommended that a comprehensive risk assessment should be used to determine the potential accidental conditions and the protective measures proposed, during commissioning and permanent operation. Design load cases should be based on the outcome of this risk assessment. KEYWORDS continuous materials handling; design; functional failure; risk assessment; structural failure. INTRODUCTION Bulk materials mines such as coal, copper and iron ore mines utilise a range of different materials handling machines. These include stackers, reclaimers, shiploaders, wagon tipplers, shuttle conveyors and others. Other mining operations may also utilise equipment such as waste spreaders. Generally, the structural components of these machines are designed in compliance with ISO which was published in 1994, or on Australian mines in compliance with AS which was published in 1995 and addressed what were then felt to be serious shortcomings in ISO These standards are both Part 1, the original intention being that further parts should cover mechanical and electrical components and the control systems of this equipment. However, no further parts were ever written. Experience shows that in too many cases this equipment is vulnerable to collapse or functional failure of the structural components, suggesting that it will be prudent to reconsider the design philosophy and commissioning precautions applied to these machines. BRIEF CASE STUDIES The mining industry unfortunately experiences a fairly high incidence of failures on bulk materials handling machines. In some cases, outright collapse of structural components is the result, whereas

2 in other cases there may be functional failures arising from excessive deformation or vibration. Several cases studies are briefly presented. Stacker boom collapse The luffing boom of a stacker collapsed as shown in Figure 1, prior to completion of commissioning, but after production use of the stacker had commenced. It appeared that overloading of the stockpile had inundated the discharge end of the boom in product and the operator had attempted to extract the boom by forcing it upwards using the luffing cylinder. Unfortunately the upper limit of oil pressure measurement was exceeded in this incident so it was not possible to establish what force had actually been applied in the cylinder. However, based on the maximum possible oil pressure and the area of the cylinder, it was established that the luffing cylinder was capable of breaking the boom. Figure 1. Stacker with collapsed boom. Stacker tripper boom collapse The boom of the tripper section of a stacker collapsed as shown in Figure 2, prior to completion of commissioning, but after production use of the stacker had commenced. The tie that failed appeared to have poor fit-up to the extent that initially only one of the two bolts carried the entire load and broke in shear, after which the second bolt attempted to carry the entire load and it also broke in shear. Figure 2. Stacker tripper with collapsed boom tie.

3 A frame reclaimer collapse The bogey hinge pin on an A-frame reclaimer failed during reclaim to load a train as shown in Figure 3, leading to collapse of the reclaimer. The reclaimer had been in production use for several months, although final commissioning had not yet been completed. At the time of the collapse the production rate (approximately tons per hour) exceeded the design rate (2 750 tons per hour) and the stockpile proximity probes appeared to not be working, allowing the rakes to dig at an excessive depth as shown in Figure 4. This led to an unexpectedly high lateral digging force being exerted on the structure. Figure 3. Collapsed A frame reclaimer. Figure 4. Proximity probe not working and excessively deep digging. Shiploader collapse A shiploader that had been in service for some 14 years collapsed under overload from spillage during cleaning of spillage. Environmental requirements demanded that the shiploader had to be fully enclosed, so at the design stage it was recognised that spillage could significantly exceed the ISO 5049 encrustation load. Some extra provision was made for extra spillage loads, and a vacuum system was installed to remove spillage. When the vacuum system became unreliable and was decommissioned, spillage accumulated to the extent that the shiploader boom collapsed. At the time of collapse it was estimated that the actual mass of spillage (approximately 1300 kg/m) exceeded the design provision (70 kg/m) by a factor of almost 20. It appeared that the mine site personnel were not aware of the high level of risk posed by decommissioning of the vacuum cleaning system.

4 Waste spreader collapse A recently commissioned waste spreader collapsed during high winds in a thunderstorm as shown in Figure 5. The waste spreader had been designed to the ISO 5049 wind loading requirements based on a wind speed of 20 m/s, although the project specification called for the use of the local national wind loading standard, which required design to resist a wind speed of 45 m/s. A concession for this change was allowed by the project clerk of works, who was a carpenter by trade, and had no idea of the implications of allowing the concession request. Figure 5. Collapsed boom of waste spreader. Bucket wheel reclaimer collapse A bucket wheel reclaimer was being used to remove overburden when the high wall collapsed, causing severe damage to the bucket wheel and its drive shaft. Although the reclaimer had been purchased for the purpose of moving overburden, collapse of the highwall had never been identified as a risk, so no provision had been made in the design procedure for this extreme load condition. Stacker-reclaimer blown off rails A stacker-reclaimer was blown along its rails, the end stops sheared off and the machine eventually came to rest in soft ground between the end of the rails and the feed conveyor transfer tower as shown in Figure 6. Unfortunately the anemometer was damaged in the incident so the actual wind speeds were not measured, but several trees in the vicinity were also blown over and there was other wind damage in the area. The SCADA system showed that the machine was about 5 minutes into its automatic shut-down procedure when it was blown along the rails. Figure 6. Stacker-reclaimer blown off its rails.

5 Wagon tippler functional failure A client utilising wagon tipplers for offloading trains had normally used very heavy plated tipplers. A new project adopted a much lighter lattice structure, primarily because of cost and project time savings, as well as increased local manufacturing input. When the new tippler was commissioned, there were complaints that its structural design was inadequate. It was clear that no limits on deflection had been identified, and the main reason for the complaints was differing expectations between different parties. Further investigation suggested that quite high deflections could be accommodated by the equipment, but the owner and operators were unhappy because they did not expect the level of motion that was evident during wagon tipping. In addition, it was not uncommon that wagon doors would be inadvertently left open after tipping, and foul on the tippler structure. The heavy plated tipplers were sufficiently strong to resist this force, and the doors would be damaged, identifying the problem as the person who left the doors open. The lighter lattice structure was not strong enough to resist fouling of open doors, and was damaged by the doors, leading to identification of the problem as an inadequate structure! Grab reclaimer collapse After more than 25 years of successful operation, a new operator complained of excessive lateral motion of an unsymmetrical goliath grab reclaimer. Structural engineers were appointed to investigate the problem and incorrectly concluded that the 3-pinned-arch type goliath grab structure was unstable. They recommended the installation of a knee brace across the apex pin to stiffen the structure. The knee brace induced high bending stresses into a short vertical leg, causing a fatigue failure within 3 months after the modification was completed as shown in Figure 7. Operators cabin with grab New knee brace inserted Ore bin Stockpile East Figure 7. Collapsed structure of goliath grab reclaimer. North Luffing stacker collapse A new luffing stacker collapsed during commissioning of the plant, as shown in Figure 8. A detailing error had placed a light lacing member in compression, whereas the design assumed it carried tension only, as shown in Figure 9. The occurrence of this error implied that the detail drawings had not been adequately reviewed by the structural engineer, if they had been reviewed at all.

6 Figure 8. Collapsed luffing stacker. Tension cable to support Luffing Stacker Incorrectly orientated diagonal member Tower omitted here for clarity Stockpile Luffing Stacker support tower Figure 9. Sketch of collapsed luffing stacker. Shuttle conveyor fatigue cracking Several shuttle conveyors operating in various portions of plant on the same mine experienced fatigue cracks in their supporting steelwork, as shown in Figures 10 and 11. The design had not considered the shuttle conveyors as inducing loads that would render the structure vulnerable to fatigue problems. However, observation of the shuttle conveyors during operation suggested that the applied loads were certainly fluctuating to the extent that fatigue should be a design consideration, and there were a number of factors that would exacerbate the fatigue condition. In one case, the storage bins fed by the shuttle conveyors were constructed of concrete, and constituted a very stiff support, whereas the run-off structure was a relatively light steel structure. The differential deflections led to unexpected longitudinal loads in the rails, and resulted in fatigue in the immediate vicinity of the rail supports. Another shuttle conveyor had a chassis that had a high torsional stiffness, leading to unequal distribution of loads along the uneven rails and thus higher load fluctuations than had been anticipated.

7 Figure 10. Shuttle conveyor support structure. Figure 11. Shuttle conveyor with torsionally stiff chassis. Oscillation of rake reclaimer A rake reclaimer, shown in Figure 12, that generally operated well, had lateral oscillations that resulted in complaints from the operators. Measurements showed that the vibration level was such that the complaints were realistic in terms of human discomfort, although there was certainly no structural distress. Figure 12. Rake reclaimer with operator s cabin on top.

8 PROPOSED DESIGN AND OPERATIONAL PROCEDURES It is proposed that these shortcomings in the international standards and design procedures for continuous bulk materials handling equipment can be overcome by utilising a modified design procedure. It is believed that following this proposed procedure will go a long way towards eliminating failures such as portrayed in these case studies. Risk assessment Determine risks. The design procedure should commence with a comprehensive risk assessment, aimed at determining the accidental events and unexpected conditions to which the equipment may be subjected. This should include consideration of impacts, the effects of differential deflections, equipment failure, and others. A full risk register should be prepared, so that future site personnel are informed of the risks. Assess protection. The risk assessment should also consider the protection measures in place, including soft measures within the control system and hard measures such as safety couplings and proximity probes. Design loads Specify the design loads. The design loads should then be specified to fill any gaps between identified risks and the protection measures in place. Alternatively, it may be determined that additional protective measures form a more realistic approach to dealing with the risks. Actions noted in risk register. The design loads, or additional protection measures, should be listed in the risk register for future reference. Change management Construction and commissioning changes. Where any changes are required, in particular when commissioning overlaps with production, the risks should be re-assessed, and appropriate measures taken. REFERENCES AS (1995). Mobile Equipment for Continuous Handling of Bulk Materials. Standards Australian, Sydney, Australia. ISO (1994). Mobile Equipment for Continuous Handling of Bulk Materials. International Standards Organisation, Geneva, Switzerland.