Sunday 22nd Sep, 2019

BULKtalk: Best practice in chute design – Part two

In his regular column, Steve Davis explores how the multiple functions of chutes can result in designs that leave safety, operation and maintenance legacies.

In his regular column, Steve Davis explores how the multiple functions of chutes can result in designs that leave safety, operation and maintenance legacies.

Accessories

Chute systems include a wide range of different accessories, including pulleys, drives, doors, samplers, magnets, sprays, dust collection, lights and sensors. These components, and their design and location, all contribute significantly to how well the chute system functions. The interfaces between chute and components can make access and maintenance difficult and unsafe.

Conveyor Drives

Drives are often part of a chute system, and usually have external features such as oil circulation and cooling, brakes, electric motor cooling and external runback devices. When designing a chute, it is important to use proper interface management to ensure these external features to not limit access to chute flange bolts bearing lubrication, and even some integral drive components such as gearbox level and temperature sensors.

Arrange these components with space and maintenance access and avoid compromising access to other parts of the chute system, and to personnel egress routes. Give sufficient walkway access around drives for maintenance. Overhead monorails may facilitate removal of components. Consider the height of the lifting equipment and slings, the weight of components, and location and direction of travel for the components to a landing zone. Incorrectly designed monorails lead to unsafe use.

Components such as walkways, handrails and structure may hinder removal of drives for repair, or access to the chute or pulley. When removing additional parts and structures, the process can be time-consuming and complex. Consider the location of electric cables, overhead structures, dust extract, pipework and the like in design. If electrical and control components allow, consider quick-disconnect fittings instead of direct wiring.

Pulleys

Head chutes incorporate pulleys. Pulley bearings should be outside of the chute and supported from a separate structure with alignment provision. The bearing location and support base should ensure force transmission into the base of the bearing housing. Bearing housing bolts should not be subject to shear or tension loads. Pulley alignment is critical for many reasons and a sufficient three-plane adjustment should be provided in the design.

Pulleys wear out; lagging, bearing, shaft locking element or shell. Design the chute system to facilitate pulley removal from the chute quickly and safely at some stage in the life of project. Provide a method that does not require belt cutting and if a special pulley-lifting device is required, provide methodology.

Snub and launch pulleys on head end drives present an additional maintenance problem due to limited access, and can compromise chute design. Design without a snub pulley is feasible in most situations. If included, the design should provide for a safe methodology for maintenance and removal of pulleys.

Chute Penetrations

Pulley shafts penetrate the chute, as do belt cleaners and other components. Any penetration must consider the chute as a complying safety guard. This usually results in bolt on covers, plates or other devices that are adjustable and removable to provide for alignment and maintenance. Sometimes the design omits this need, and unsafe chute systems result.

Many chute penetration covers are light duty, easily damaged and difficult to remove and replace. Consequently, there are safety violations on chutes from inability to refit covers. There are proprietary covers available from belt cleaner and other suppliers. However, some rubber covers may not comply with guard standards, so be sure to ask for covers that are safe and functional.

Doors and Hatches

There are many examples of chute access doors that may have been acceptable 30 years ago, but do not meet minimum safety and practicality requirements today.

Chute systems incorporate doors and hatches for inspection and access. Each door is a breach in the conveyor safety system, and the design must consider the impacts of this. Tool removable internal protective mesh or bars allow opening of inspection doors while the plant is operating at minimal personnel risk (although an assessment is required). Mesh and bars are a guard and must comply with safety standards. If there is no mesh, then there must be a sign indicating machine isolation is required before opening doors.

Doors and hatches must be in a suitable location, be the appropriate size and perform the correct function. Many doors and hatches open into an area of the chute that has no viewpoint or has no maintenance function, making them less useful. Doors and hatches without permanent personnel access are common and are occasionally located metres up in the air or on top of chutes, which is a poor ergonomic choice. Working at heights is a major safety concern, and if doors must be there for a reason (such as inspections), then permanent access to them is preferable.

Many chute doors and hatches do not have usable or safe access, with pinch points often common in the door itself, and between door and adjacent chute and structure. Doors that block walkways when open, cut off egress in emergencies, or can trap personnel between the door and handrail or structure, are also common. Doors can become sails in windy conditions so may need to be latched open. Doors on inclined chute faces require more force to open than on vertical faces and go “over centre” past the half-open point. This creates a safety consideration. 

Access door, 1, with safety bars (Bendtech) note: 2, access platform to door; 3, inclined face adds gravity force to door operation; 4, good pulley shaft guard; 5, few bolts on chute flange; 6, dust extract (poor design for flow) and conduit have to be removed to split chute.
Access door, 1, with safety bars (Bendtech) note: 2, access platform to door; 3, inclined face adds gravity force to door operation; 4, good pulley shaft guard; 5, few bolts on chute flange; 6, dust extract (poor design for flow) and conduit have to be removed to split chute.

Latches keep doors closed in operation, and release to open. Several styles of door latch are in use. Sea container style double latching systems function well and remain closed. Rotating latches can vibrate open, or need locking so tightly it is difficult to open them manually. Wedge locks lead to unsafe use of trash as hammers, levers and similar. 

In Chute Sample Cutters

Falling stream sample cutting in chutes gives the most representative samples. These sampling systems add complexity in almost all aspects of chute system design. They affect ore flow design, need extra space and chute height, need maintenance access and add a sample discharge in an already congested area. Select a supplier early and integrate the sampling system into the chute for a better design, avoiding compromises in operation and maintenance and potential health, safety and environment shortfalls. Consider different sampling systems if the reduced sample accuracy is acceptable

In Chute Tramp Magnets

The best tramp separation is from suspended magnets over the conveyor free fall discharge. To be effective, the magnet must be sized for the duty, located close to the trajectory and at a matching angle. Magnet support, maintenance access, electric cable support, tramp metal discharge arrangements (manual, cyclic or self-cleaning and likely a tramp discharge chute) all affect the operation and maintenance of many other chute components. Non-magnetic materials may also be required for pulley, cleaner and chute. Magnets also need extra guarding and signage due to the magnetic field.

Over-belt suspended magnets are an option if some reduced functionality is acceptable. Consider non-magnetic materials for stringers, idler frames and idler rolls.

Moving Chutes

Diverters, splitters, shuttles, ploughs and trippers all provide means to divert a chute discharge to two or more positions. These chutes add complexity and additional height to the design. There are several more components to consider and most will need maintenance. Safety considerations are more complex, as parts of the system move from place to place, with multiple discharge points, and everything still requires safe access to operate and maintain.

Common operational shortcomings with moving chute systems, apart from lack of safe maintenance consideration, result from change of position on the run. It is relatively easy to accommodate stop-start flow changes where no chute discharge occurs in the interim locations. Where material flow is continuous during chute movements, consider ore flow paths during movement. Spills, build up on structure and rails, and damage and entrapment of components are common. Fixed and moving chute component interfaces often need added flexible seals to contain ore and dust flows.

Flop gate diverters or splitter chutes are complex, often having operational and maintenance problems such as jamming, partial bypass, accelerated wear, or unguarded actuators. It is difficult to get accurate flow splitting with a bottom-hinged gate. Consider top swung diverters or shuttling chute components to overcome these problems.

Capacity and Blockage

All chute systems can become bogged or blocked, meaning each should have blocked chute detection installed. The detector shuts down the upstream feed to the chute before any serious damage results, but also stops potential injury and mess resulting when the chute spills over. The detector is not expected to generate spurious upstream shut downs through incorrect operation.

Chute capacity above the blocked chute detection point should contain any run on from the upstream feed, such as run-down of the feed conveyor, without damage or uncontrolled overflow. For large run-down capacities, such as from an overland conveyor, use either a large chute, even a bin, or a chute with a controlled overflow facility to a safe collection location.

Select the block chute detection point and detector type so that normal flow through the chute, in dry dusty, wet or other conditions does not trigger shut down. A short delay between detection and shutdown may prevent spurious trips from fly rock.

Multiple blocked chute detectors may be beneficial in large chutes. Generally, the speed at which a chute fills from onset of blockage is too fast to monitor level in the chute. There are some developments that may give early warning of ore build up through change in vibration signature and allow for a controlled shut down.

Detectors should always fail to the blocked condition. Install blocked chute detectors so that they can be maintained with the chute still operating.

Containment

In addition to containing ore flow, chute systems have to manage dust and airflow generated by ore flow. There are many types of dust extraction and suppression systems available. They are only effective if designed into the chute system where the dust is generated. A convenient stub in air duct connection into the side of the chute is unlikely to provide sufficient control of the airflow in the chute to extract much dust. Model dust flow as an extension to discrete element modelling to confirm dust extract locations. Dust extraction and wet suppression combined will result in blocked air ducts from wet dust.

This chute could be better: 1. Inaccessible heavy access door; 2. Too many bolts for top cover; 3. Trip hazard from adjuster (top adjustment inside chute); 4. Unsafe gap; 5. Cleaner cannot be inspected or removed; 6 Welded to structure prevents removal; 7. Pulley inaccessible, dismantle structure and chute to remove.
This chute could be better: 1. Inaccessible heavy access door; 2. Too many bolts for top cover; 3. Trip hazard from adjuster (top adjustment inside chute); 4. Unsafe gap; 5. Cleaner cannot be inspected or removed; 6 Welded to structure prevents removal; 7. Pulley inaccessible, dismantle structure and chute to remove.

Skirt systems are installed at the inlet to the chute, and at the discharge to a receiving conveyor. Skirt systems can cause significant belt damage. There are many different styles of skirt available, and no universal solution to containment. Select a skirt system proven for the duty, and one that is easy and safe to maintain. Provide a method to safely access skirts for adjustment and maintenance, including removal of mandatory guarding. Consider a slider bed under the skirts to provide better wear and sealing. These are available in many forms, with and without centre rolls, and with impact absorbing devices.