Design of Materials Handling Machines to AS

Transcription

1 Design of Materials Handling Machines to AS R. Morgan * * Director, Aspec Engineering Pty Ltd, 349 Coronation Drive, Milton, Queensland, 4064, Australia; rmorgan@aspec.com.au ABSTRACT The increasing level of mineral exports from Australia has resulted in the need for expansions to existing facilities and new export facilities. Rail-mounted materials handling machines such as shiploaders, stackers and reclaimers are significant investment items for the ports and mines involved in the supply chain for export of these commodities. Australia is one of the few countries to have its own standard for such equipment. AS Mobile equipment for continuous handling of bulk materials - General requirements for the design of steel structures (Standards Australia, 1995) was introduced in response to a number of machine failures within Australia. This Standard specifies general requirements, design loads and specific requirements for the structures of mobile equipment for continuous handling of bulk materials. The International Organization for Standardization (ISO) has a design standard for bulk materials handling machines ISO (ISO, 1994), which has been widely used internationally; however its use has been discontinued in Australia since the introduction of AS Australian Standard AS has been in use for 16 years now and major machine suppliers and design audit engineers operating in Australia are generally familiar with the document. The author describes the background to the Standard and experience in using AS for machine design. Suggestions are offered as to how the Standard might be improved in future revisions. KEYWORDS Australia; Bulk; Design; Handling; Machine; Standard. INTRODUCTION AS Mobile equipment for continuous handling of bulk materials - General requirements for the design of steel structures was introduced in The Standard had a long gestation period, with work commencing in 1978 (Morrison et al, 1996). Its release in 1995 was timely in response to a number of failures of bulk materials handling machines in Australia in the early 1990s. This Standard applies to mobile equipment for continuous handling of bulk materials, e.g. excavators, stackers, reclaimers, shiploaders, and ship unloaders. It was intended that AS4324: Part 1, which deals with steel structures, would be followed by other parts addressing mechanical, electrical and other aspects. However this has not occurred. The International Organization for Standardization (ISO) has published a design standard for bulk materials handling machines, ISO (ISO, 1994), which has been widely used internationally and was used in Australia prior to There are significant differences between AS and ISO5049.1; generally speaking, machines designed according to AS are heavier than similar machines designed to ISO German Standard DIN 22261(DIN, 2006) has been written specifically for machines working in large brown coal open cut mines, including bucket wheel excavators. AS has adopted material from DIN in addition to material from ISO Protective or load limiting devices in the electrical, control, mechanical and hydraulic systems are very important in determining the load imposed on a machine. This is an area which requires close attention both in the design phase and on site to ensure that the installed devices perform the correct

2 function. AS adopts a philosophy of not overly relying on electrical protection devices for structural integrity. APPLICATION OF AS TO TYPES OF MACHINES AS is intended to apply to both rail-mounted yard machines and continuous mining machines which are usually mounted on crawlers. Appendix E in the Standard gives illustrations of the types of machines for which the Standard is intended to apply. A description of some of the more common types of rail-mounted machines follows below. Stacker Reclaimers Figure 1 shows the components on an older style of stacker reclaimer. The machine has a long travel motion along tracks propelled by driven wheels on the bogie system. In the stacking mode, bulk material is fed onto the machine from the yard belt via a tripper which discharges onto the elevator. Material travels on a conveyor up the elevator and discharges through a chute onto the boom conveyor. The bulk material discharges onto the stockpile from the end of the boom. Figure 1. Stacker Reclaimer In reclaiming mode the boom conveyor reverses direction. Bulk material is reclaimed from the stockpile by the bucket wheel which rotates via a driven shaft. The buckets dig material from the stockpile and discharge them onto the boom conveyor. The boom conveyor discharges material through a central chute onto the yard conveyor. The boom can pivot in a vertical plane about a central bearing to follow the stockpile terrain. This motion is driven by hydraulic cylinders and is termed luffing. The boom can also rotate in the horizontal plane about a circular bearing. This motion is driven by a gear system and is termed slewing. Machines of this type are sensitive to changes in balance about the luffing pivot. Changes in weight and weight distribution need to be carefully monitored and controlled. The repetitive loading due to the bucket wheel motion requires consideration for metal fatigue of the structure and slew bearing. Other machine configurations are used, e.g. a C frame configuration can slew to both sides of the rail tracks without the need to extend the elevator away from the machine with the tow bridge.

3 Stackers Stackers predominately long travel with limited slewing motions in order to lay the stockpiles for subsequent reclaiming by a slewing or bridge type reclaimers. On modern designs luffing is carried out by means of hydraulic cylinders. Stackers with longer spans are often articulated to provide less variation in load during the luffing motion. Reclaimers Bucket wheel boom type reclaimers are similar to a bucket wheel stacker reclaimer (see Figure 1) but without the stacking function, so they do not include a tripper and elevator. Bridge reclaimers of the bucket wheel type are often used for reclaiming on the face of a blended stockpile. Such machines have rakes which are used to loosen material on the active face. Shiploaders Long travelling shiploaders with a wheelbase up to approximately 20 metres commonly have a portal structure spanning the rails and a fixed boom gantry set at 90 degrees to the rail track. The boom conveyor and shiploading chute shuttle in and out to load the hatches and due to geometry, there are limitations on the length of in-board travel of the shuttle. The shuttle mechanism may vary the length of the boom or the boom may be of fixed length with the shuttle within the boom. Another configuration is the bridge type which has a large travelling bridge spanning from the seaward rail to a second rail or pivot point some distance behind the berth. A shuttling trolley system, which supports the boom, tower, and luffing winch system, travels along the bridge. The portal slewing type shiploader is suitable for ships without masts and cargo gear. Trimming of hatches is accomplished by a combination of slewing and long travel motions. The portal slewing and shuttling type shiploader allows for greater flexibility in loading different ship types than the portal slewing type. LOADS The following section describes some of the important load conditions which have been expanded or covered in more detail in AS than in the ISO or DIN Standards. Digging Resistance U and Abnormal Digging Resistance UU The calculation procedure for digging resistance is similar to DIN 22261, being based on the drive motor torque limit. Whereas ISO5049 allows digging resistance for storage yard machines to be based entirely on the cutting characteristics of the material being reclaimed, AS requires this load to always be based on drive motor torque. Site measurements generally show that most machines are driven as hard as the drive system will allow. Lateral Digging Resistance S and Abnormal Lateral Digging Resistance SS AS requires lateral digging resistance to be based on available slew or long travel drive capacity, an approach which is similar to that used in DIN The reason for adopting this approach, even for bucket wheel reclaimers or other yard machines is based on site measurements. Permanent Dynamics D The treatment of permanent dynamics is similar to the DIN approach, using dynamic effects factors which multiply the appropriate dead and live loads. The factors used in AS are based on DIN 22261, but include additional values to cover rail-mounted machines. With ISO5049.1, dynamics can be neglected if acceleration or deceleration is less than 0.2 m/s 2. Wind Loads Operating W and Wind While Idle WW Wind loads, either for operating conditions or for extreme winds with the machine out of operation, are referred to the Australian Standard AS Additional requirements are included for gust effects on the superstructure and for wind at a 45 degree angle to the main structure axes.

4 Travel Skew Forces LS For rail-mounted machines, AS nominates a calculation procedure for travel skew forces which is similar to that in the Crane Standard AS1418. This includes consideration of asymmetrical traction forces on each rail, commonly encountered on bridge-type shiploaders. Travel Device Obstructed LL In AS4324.1, the approach adopted is to assume that these loads are generated where one side of a travelling machine is suddenly obstructed and brought to rest in 300 mm. Consideration of the dynamics of the event, including inertial effects, is required. ISO5049, by comparison, considers only a quasi-static case with friction limited drive forces applied asymmetrically. Boom Collision Loads FS, FT AS considers the combined effect of both inertia and drive torques. By comparison, ISO5049 requires consideration of either inertial effects or drive torques but not both. DIN does not include an impact load case of any kind. The choice of stopping distance of 300 mm is subjective, however in a real boom impact, both inertia and drive torque/force effects will be additive. AS includes load cases to address such accidents for both slewing and non-slewing machines. In the latter case, the limiting long travel drive force rather than slew torque will determine impact loads. By comparison, ISO does not define an impact load case for nonslewing booms. AS includes a longitudinal boom impact case, representing the situation for a slewable boom machine, where the impact might occur while the boom is facing forward at a shallow angle to the long travel direction. Buffer Loads OO The principle adopted is that rail-mounted machines should be equipped with buffers, and that the machine structure should be capable of surviving a buffer impact situation where the machine is driven into the buffers at full long travel speed. With this impact case as well, both inertia and drive forces are required to be considered concurrently. Bucket Wheel Loss BL AS also includes a requirement to design bucket wheel machines for the situation following loss of the bucket wheel, shaft and associated gearbox from the boom, as a result of a bucket wheel shaft failure or similar accident. Inclusion of this load case was a result of several failures of this kind, having serious implications for operator safety. Non-Permanent Dynamic Effects DD This applies to inertia forces due to dynamic load effects, such as abnormal acceleration and braking of moving parts occurring less than 20,000 times during the life of the machine (e.g. emergency braking). Allowance needs to be made during the design phase for these effects. During the commissioning phase, care needs to be taken in testing the emergency stops on the machine. When emergency stopping is via the braking system, rather than via controlled electrical stopping, severe forces can be imparted into the structure if the brakes or rail clamps are not adjusted correctly and are applied too quickly following power deactivation of the electrical system. Burying (ZZ) This load case is for where collapse of a stockpile or slippage of the bank could cause the reclaiming or excavating component of an operating machine to become partially or fully buried. The Standard allows for the need for any such appropriate loading to be addressed in the purchase specification and gives suggestions on how this may be covered. LOAD COMBINATIONS AS shows how different load components should be considered in combination. These are summarised in Table 3.7 of AS4324.1, together with safety factors and stability margins. Because of the greater number of load components in AS4324.1, this table is larger and more complex than

5 the corresponding table in ISO Loads are grouped according to their frequency of occurrence, i.e. main loads, additional loads and special loads. The frequency of occurrence of these load groups are similar to those stated in the crane standard, AS (Standards Australia, 2002). STABILITY AGAINST OVERTURNING In order to check safety against overturning, AS requires the stability ratio, (M s /M o ) to be calculated for the prescribed load case combinations. M s is the minimum stabilising moment due to the total permanent load referred to a possible axis of tipping and M o is the maximum overturning moment due to the prescribed load case combination of vertical and horizontal non-permanent overturning forces referred to the same axis of tipping. The Standard nominates minimum stability ratios against overturning to be applied to load case combinations varying between 1.5 and 1.1, being higher (1.5) for more frequent operational loading conditions, lower (1.33) for less frequent operational loading conditions, and lower again (1.2 or 1.1) for much less frequent abnormal loading conditions. FATIGUE AS refers to AS4100 for fatigue design. AS4100 reflects current practice for the design of welded steel structures subject to fatigue loads. There are important differences between AS and ISO AS is prescriptive and gives detailed guidance on how to calculate the load cases and which load combinations to consider. ISO uses an outdated mean stress approach to fatigue design which is not adapted to modern standards. It should also be noted that structures affected by fatigue must be regularly inspected for fatigue damage for design code rules such as AS4100 to apply. For combining the effects of cyclical load components, the AS approach is to consider the fatigue damage resulting from the cyclical stresses produced by each component of the loading spectrum and carrying out a cumulative damage assessment by Miner's rule summation. AS4100 requires a capacity factor of 0.7 to be applied for non-redundant load paths and inaccessible areas for inspection (Standards Australia, 2008). STRENGTH ASSESSMENT AS allows for the use of either the permissible stress method (also termed working stress ) to AS3990 (Standards Australia, 1993) or limit states method to AS4100 to be used for strength design. BUCKLING ASSESSMENT AS permits buckling assessment, either to the limit state procedures of AS4100 or to the permissible stress procedures of AS3990. This is directly applicable to design of beams and columns. Design of plate work structures for the base and other major components to resist buckling and to accommodate shear lag effects is not well covered in the steel design standards AS4100 or AS3990. AS covers this to some extent in section 5.4 and Appendix J. MACHINE PURCHASE SPECIFICATION The standard method for procuring bulk materials handling machines is a design and construct contract with the contractor having responsibility for design, manufacture, supply and installation. The purchase specification needs to be written to ensure that the configuration and performance parameters upon which the requirements for the machine were determined can be met realistically in practice. Appendix B in AS gives guidance on issues which should be covered specifically in the purchase specification. DESIGN AUDIT Appendix K in AS gives guidance for design auditing and certification by an independent third party engineering consultant. This may be by means of independent calculations or by checking and reviewing the original design calculations and by computer analysis.

6 WEIGHING AS requires that after a machine has been constructed, the mass and centre of gravity of the machine as built should be accurately determined. The final weight of a machine is often greater than that advised at the time of tender even when the supplier has carried out an upfront concept design phase. AS stipulates that if the construction mass exceeds the mass used in the calculation of static loads by more than five percent, the stresses in the machine should be rechecked. EXPERIENCE WITH AS AND SUGGESTED CHANGES The following section covers some of the areas where the author has found issues in the application of AS that required resolution. Areas that may need to be addressed in revisions to AS are also identified. Fatigue non-redundant load paths The requirement in AS4100 to allow for a capacity factor of 0.7 for non-redundant load paths or for areas which cannot be readily inspected has caused difficulties and can be subject to quite different interpretation by different parties. Purchase specifications should be specific in this respect to avoid differing interpretations. Treatment of burying load case The burying load case is applicable to reclaimers and particularly covers the situation where material from the bank or stockpile slumps onto the end of the boom. One way of handling this, as suggested in AS4324.1, is to assume that the boom can support the full weight of this material. Another way this has been handled is to allow the luffing hydraulic system to relieve and the boom to be partially supported by the stockpile. Purchase specifications should be specific in this respect. Blocked chute flooded belt The case of blocked chutes and conveyor-flooded belts happening concurrently can be onerous and there is a temptation by designers to try to relax requirements to design for a flooded belt by measures such as installing a profile plate in the feed chute. This approach should be treated with caution as the machine can easily be modified in service to remove the profile plate without reference to the designer. Similarly, designers may tend to rely on blocked chute cut-off switches to limit the load from material overflowing from the top of a clogged chute. In practice, these cut-offs are not instantaneous or may malfunction, causing greater loadings than assumed. Design audit Qualifications of the design audit engineer and the need to audit mechanical and electrical items are not covered in AS Where the audit engineer s function is provided as part of the contract for the machine, the purchase specifications should be specific in this respect to avoid differing interpretations. Boom collision on non-slewing machines On non-slewing machines such as shiploaders and stackers where the boom is fixed at right angles to the long travel tracks, it is usually not practical to design for the boom collision load case and it may be necessary to rely on anti-collision systems. It is suggested that further guidance be given in revisions to AS Travel device obstructed on bridge machines On long span bridge machines such as shiploaders and reclaimers it is usually not practical to design for the travel device-obstructed load case and it is necessary to rely on anti-collision and skew control systems. It is suggested that further guidance be given in revisions to AS Digging cut-off settings and protection systems Typically, bucket wheel drives are sized to provide sufficient power to dig and lift the stockpile material. The drive is sized to operate at about 100% of motor-rated torque for the majority of its

7 operating time. During operation the digging torque will vary based on a number of factors, such as the type of material being reclaimed and stockpile slumps. The primary and secondary protection settings are provided so that the load on the machine is not excessive and the machine can continue to operate without too many overload stoppages. The normal digging resistance (U) is calculated based on 1.1 times the first protection setting but not less than 1.1 times the motor-rated torque. The abnormal digging resistance (UU) is calculated based on 1.1 times the greatest protection setting but not less than 1.5 times the motor-rate torque. Diversity for protection systems is particularly beneficial. For example, a protection system implemented using one mechanical protective device and one electrical protective device has a high diversity component. Fluid couplings of any type on the bucket wheel drive train are not generally suitable as a torque limiting device for normal digging or associated lateral digging and it is suggested that the Standard be revised so as not to refer to fluid couplings as a load limiting device. With the modern drives incorporating flux-vector electric or hydraulic systems, this device would not be utilised. Permanent Dynamics D For stability calculations, a uniform dynamic multiplier as adopted in AS can produce nonconservative results. Therefore, a triangular-distributed acceleration as shown in Figure 2 is more appropriate. It is suggested that this be included in revisions to AS Figure 2. Triangular Distribution of Acceleration Redundancy of stays, ropes and hydraulic cylinders In cases where an operator's cab is located on a boom, is the requirement for the boom support to be redundant, with two totally independent support systems. The design of ropes or stays is required to consider the dynamic loading which would occur following failure of one of the support systems. The magnitude of the dynamic load multiplier and the need to have this on top of the safety factors is an area which can be subject to quite different interpretation by different parties. It is suggested that further guidance be given in revisions to AS Loss of bucket wheel This load case was introduced primarily for situations where the bucket wheel is on a cantilevered section of shaft. Where the bucket wheel is not cantilevered but held captive by the boom support structure and the discharge/circular chute in case of shaft failure, this load event may not be applicable. It would be appropriate to revise the Standard to reflect this. Wind loads The Standard is written with reference to the 1989 edition of AS and uses wind forces for permissible stress design, V p rather than wind forces for ultimate design V u. Subsequent editions of the wind load standard, AS published in 2002 and 2011 only include V u and do not include V p. This is an area which needs revision. It may be necessary to define an intermediate wind speed for relocation to the storm park position and for parking with rail clamps. Care needs to be taken in cyclonic areas where wind is a controlling load case, as use of permissible stress design wind speed may be non-conservative.

8 Plate buckling The AS assessment method for plate bucking can be difficult to apply. In practice, plate bucking is generally handled by finite element analysis and alternative design standards such as BS5400 (BSI, 1982) or the Merrison Committee recommendations (Merrison et al, 1973) are used. It is suggested that revisions to AS should address this. Limit states code calibration Table 3.7 of AS gives load multiplying factors to be used with the limit states method to AS4100. Normally, the limit states design method uses partial load factors γp, which differ for each type of load and range generally between 1.2 and 1.5 depending on the statistical variability of the load type. However, for AS this factor is taken as the same for all load components. In AS for cranes, the standard notes that where the limit states design method is used, care needs to be taken so that the design gives a degree of safety not less than that for the permissible stress design method to AS3990. Such a cautionary note should be included in a revision to AS in the short term. Ultimately a full code calibration of the load multipliers should be carried out (Warren et al, 2005). CONCLUSIONS AS has been in use for over 16 years and major machine suppliers and design audit engineers operating in Australia are now familiar with the document and are making due allowance for the differences between AS and the International Standard ISO Since the introduction of AS4324.1, the majority of new machines in Australia have been subject to a third party design audit. Its application in the purchase of bulk handling equipment for Australian ports and mines has generally resulted in robust and reliable machines which are expected to offer long-term benefits. Some areas in the Standard have caused issues and pending revisions to the Standard, this has generally been covered in purchase specifications. Now that most industry participants are familiar with the Standard, revisions would be appropriate as part of continuous improvements in the industry. REFERENCES 1. British Standards Institute (1982). BS5400: Steel, Concrete and Composite Bridges. 2. German Institute for Standardization (2006), DIN Excavators, Stackers and Ancillary Equipment in Brown Coal Open Cut Mines Part 2 Calculation Principals 3. International Organization for Standardization (1994). ISO5049.1: Mobile Equipment for the Continuous Handling of Bulk Materials Part 1 Rules for the Design of Steel Structures. 4. Merrison, A. W., Flint, A. R., Harper, W. J., Horne, M. R. and Scruby, G. F. B. (1973). HMSO Merrison Committee Report on the Design and Erection of Steel Box Girder Bridges, Part 1 to Part Morrison, W. R. B. et al. (1996). A New Australian Standard for Continuous Bulk Materials Handling Machines, National Conference on Bulk Materials Handling 30 September - 2 October 1996, Melbourne. 6. Standards Australia (2008). AS4100: Steel Structures. 7. Standards Australia (2002). AS1418.1: Cranes, hoists and winches - General requirements. 8. Standards Australia (1993). AS3990: Mechanical Equipment Steelwork. 9. Standards Australia (1995). AS4324.1: Mobile equipment for continuous handling of bulk materials - General requirements for the design of steel structures. 10. Warren, J. S. et al. (2005). Reliability models of overhead traveling crane loading for code calibration, ICOSSAR, Millpress, Rotterdam.