Process cranes are crucial to many industries that need bulk handling of raw materials

Process cranes are crucial to many industries that need bulk handling of raw materials. Julian Champkin investigates.

Process cranes are those which feed raw material, generally in bulk and of low intrinsic value, into a factory or processing plant where it is processed into energy or some product of greater value. A characteristic of these cranes is that they need to be absolutely reliable: even a short-duration failure of raw material supply can close down a whole complex network of subsequent processes, for a much longer time, and cost huge amounts in lost output and in money.

A second characteristic is that they are often required to work continuously, 24 hours a day, seven days a week, and to do so in the open air, exposed to anything that climates can throw at them, or in harsh dustfilled environments such as steel mills or foundries. Robust construction is a necessity, and not just of the rigid and the electrical components. Continuous operation demands robustness in wire lifting ropes as well. FEM, the body that sets European standards for this, categorises load spectrums not only by the weights that are to be lifted but by the duration of operation required from them as well. Thus a crane with an average operating time of over 16 hours a day requires a heavier-duty rope than one that is in operation intermittently just a few hours daily.

By their nature, the tasks required of them are repetitive; they are therefore prime candidates for automation, though the installations featured here have the capacity for manual operation also. And, since process cranes are such crucial elements in bulk movement for largescale industries, economies of scale mean that bespoke solutions rather than off-the-shelf are often desirable. Here are some examples.

RECIPE FOR SUCCESS

Georg Fischer Automotive in Herzogenburg, Austria manufactures around 35,000 tonnes of castings every year. The castings, for crankshafts, exhaust manifolds and other vehicle parts, are made from nodular cast iron, which itself is produced on-site. The cast iron for the different parts is made from raw materials in exactly defined proportions.

To this end, the foundry sends ‘recipes’ to the raw material mixing plant, which is a large hall, shrouded in dark grey dust. Inside it, 25 separate bunker areas are each piled with heaps of metal and scrap that tower metres high. There the process crane fills containers with the right quantities of different materials and transports them via a rail system to the smelting furnace. The fast high-tech crane from Stahl CraneSystems’s Austrian cranebuilding partner works both manually and fully automatically.

Just as a chef mixes their ingredients, so the crane operator mixes the different types of metal according to the foundry’s recipe. The composition needed is sent to him by the control system and shown on the display in his crane cab. For this, the crane controller communicates with the foundry’s charging system by Profibus link. In manual operation the crane operator drives to one of the numbered bunkers and picks up the bulk material by magnet, to a maximum of five tonnes per lifting movement. The operator’s display shows information on the amount of material that has been picked up and the amount still required from that particular bunker. He can release excess material from the magnet in fine steps, touch-button controlled, to give the exact proportions for his ingredients.

The casting ingredients are unloaded into a material container. The two containers for this are located in a frame carried underneath the crane bridge, so they are always close to the magnetic grab, making for short distances travelled during loading. The large size of the hall, which is 60m long, the number of bunkers, and the required charging rate of 15t/h necessitate high crane and crab speeds and therefore a robust construction. The crane bridge can be accelerated smoothly up to 63m/min by four drives. The cross travel speed is 40m/ min and the lifting speed 20m/min. An SH wire rope hoist from Stahl CraneSystems with a high FEM classification of 3m (ISO M6) serves for these exacting demands.

Apart from manual operation during the day the crane is also equipped for automatic operation at night. In this mode the crane automatically drives to the raw material bunkers required by the foundry’s recipe, picks up the required quantities and fills the material containers as specified. Depending on the composition, it takes a maximum of 20 minutes to fill a container in continuous automatic mode.

The heart of the crane is a doublegirder trolley with special headroom equipped with Stahl CraneSystems’ SH 6025 wire rope hoist with true vertical lift. A calibratable weighing system on the trolley frame is used for exact determination of the weight that has been picked up. Stahl CraneSystems helped to develop and build this demanding crane technology.

The exact position of the crane and travel carriage must be known for automatic operation. The position is determined redundantly by multiple safe absolute angle encoders to monitor the long travel, cross travel and rope drum, and by an additional laser-based distance measuring system between the crane bridge and hall wall.

For safety reasons the hall must be completely empty of people when the crane is working in automatic mode. This must be confirmed at ground level with a switch before automatic mode can be activated. The doors are locked electrically. Automatic mode is started at the height of the crane operator’s entrance, about eight metres above the hall floor.

The bunkers can only be refilled in manual mode, but it is possible to fill the cycle boxes by lift truck to a limited extent in automatic mode as well.

GRABBING GRAVEL

A gravel production company installed a double girder ‘ZP’ electric overhead travelling crane of 3x16t capacity, manufactured by Kuli. With a maximum capacity of 40t—32t payload and 8t tare weight—and a relatively short 5m span, it has a lifting height of 19m and lifting speed variable from zero to 8m/min, using frequency inversion control. It has three lifting motors, each of 30kW, and a profile girder; the design is to DIN 15018, H2B3, while the hoist cable is to FEM 3m. Platforms and guard railings are installed throughout, with a safety door for access. In operation, trucks arrive carrying loads of gravel. They discharge their loads into a hopper which at that stage is at ground level. Under operator control the crane lifts the hopper first upwards, then laterally to a position above one of seven silos, each silo being for a different grain size.

The hopper will be unevenly loaded— the discharge from the truck will tend to accumulate gravel on the nearer side of the hopper. The three very strong hoist systems, each with a 16t weight limit, compensate for this. The rope drum has a left/right thread, with a 4-2/1 reeving arrangement to give a true vertical lift and there are adjustable rope anchors on both sides of all three hoists. A load summing device provides lifting force limitation and a thermistor in the hoisting motor controls a tripping device. An additional chain hoist is provided to lift ancillary and maintenance equipment such as wire ropes and tools.

The hopper is moved over the appropriate silo and deposits the gravel into it. Guide bars prevent rotation of the hopper during discharge and in its final position, and a vibrating unit ensures that all the gravel is discharged. A monitoring device on the crane ensures that the hopper is empty. The hopper is then returned to the starting position, a slack rope detection device showing when it is in its correct place.

The whole process is automatically controlled via SPS program. Unlike some other systems, however, the installation is able to work manually as well, should the automatic system fail. Besides giving total process safety this ensures that truck drivers are not required to wait before discharging their loads.

REDUCING WASTE

A new Mechanical Biological Treatment Facility in West Sussex has been designed to divert a large proportion of the county’s waste away from landfill, extracting recyclable materials and using the remainder to produce renewable energy. At full capacity, the facility can produce up to 3.5 megawatts of energy, enough to power around 7,000 homes.

Working on behalf of M+W Group and Biffa, SCX Special Projects designed and installed two semi-automatic 15t cranes which work in tandem to collect the waste and feed it into two shredders for processing.

Energy-from-waste cranes are usually fully-automated, but this system handles untreated black bag waste, which means that a level of manual sorting is necessary to prevent the processing of dangerous materials. The cranes are therefore semi-automated, with control booths for full manual capability. Another feature is regenerative braking: instead of being lost as heat kinetic energy from braking is fed back into the system to produce electricity, in keeping with the ethos of the plant. The high-speed cranes travel at 90m/ min and lift at 45m/min, and process up to 45t/hr. The cranes have an automatic ‘go-to-shredder’ function with two-button activation and a hold-to-run feature for safety. The 4.7m-diameter orange-peel grabs hold up to 10m3 of waste.

Booths set into the wall below the gantry house the operators’ control chairs, giving a view of both cranes. Running on a single gantry, normal operation restricts each crane to half of the pit area, either side of the shredders, to prevent a collision. Full encoder mapping provides precise positional feedback, and proximity sensors restrict each crane to its own working area. In the event of a breakdown the working crane can recover the other by pushing it into the maintenance zone; the control booths can select either crane at the push of a button but allow only one working crane in each half at any time. An anti-sway function protects against collision with the walls, as do hard-wired limit switches, and on-crane hydraulic dampers act as a failsafe backup should there be an, albeit unlikely, failure of a limit switch. Sensors on the truck inlet barriers automatically restrict the cranes from entering any area occupied by an unloading lorry. Load cells on the grabs provide a log of the plant throughput, viewable via the interface at the control chair.

One crane also features a 5t auxiliary hoist for maintenance or to help clear a shredder blockage.

The control cabinets are remotely located to avoid contamination from the waste. This area also permits access to the cranes’ platforms for maintenance. A trapped key interlock system ensures that the cranes are shut down before opening the safety gates. A maintenance key then allows restricted crane speeds purely for maintenance purposes.

BEST ON PAPER

In the Swedish town of Sundsvall, SCA Paper operates a plant with four machines to produce paper for printing newspapers and magazines. Two furnaces burning a mix of woodchip and slurry generate the steam needed in the production process. An automated Demag process crane feeds the furnaces and must operate 24 hours a day, even at temperatures of 30° below zero and in extreme wind conditions.

The double-girder overhead travelling crane has a load capacity of 12t and travels on a runway at a height of approximately 30m. It clears shredded wood chips from two tipping positions and first heaps them in the storage area. Using a motor-powered multi-jaw grab with a capacity of up to 8m3 the crane also picks up deliveries of slurry and distributes it over the entire storage area. The calorific value is homogenised by blending these two materials.

Since the crane is required to work 24/7 outdoors in extreme weather conditions the entire crab is provided with a heated, insulated and sound-proofed enclosure, which protects all components from the weather. The electrical equipment is additionally heated: the electric components are installed in an insulated and heated container on the crane bridge. Eight motors for the longtravel motions and four for cross-travel ensure that the crane positions exactly, even in strong wind conditions. Only at wind force 10 is the crane switched off and mechanically locked.

Noise reduction measures have to be taken because of a residential area on the opposite shore of the harbour. A complete package of measures enabled the noise level to be reduced significantly below the specified level of 80 dB. Besides the sound-proofed crab enclosure, this also included speed reduction on the open winch unit and the low-noise grab.

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