Sunday 26th May, 2019

BULKtalk: Best practice in chute design – Part one

In his regular BULKtalk column, Steve Davis of Rio Tinto considers the basics of bulk handling that sites often struggle with. This is the first in a series of articles addressing systems involving chutes, which explores how the multiple functions of such systems can result in designs that leave safety, operations and maintenance legacies.


Systems involving chutes often have multiple functions. Here, we explore how these functions can result in conflicts and compromise in the design, which leave safety, operation and maintenance legacies.

Chute system functions

Transfers from conveyor to conveyor, chutes feeding to conveyors and chutes discharging from conveyors are all systems that have multiple functions.

The primary chute function is to contain, direct and deliver bulk material at a nominated rate to a receiving conveyor, bin or stockpile. Containment includes dust and cleaner discharge. Delivery should be at a speed and direction matching the receiving belt or to a bin or stockpile at a point appropriate to correct filling. Incorrect delivery can lead to accelerated conveyor belt wear, skirt blowout, belt drift and other loading problems. Eccentric loading to bins and stockpiles can affect structures, accelerate wear, cause offset stacking and other operating problems.

Most parts of a chute system will need adjustment, or will fail or wear out with time. Well-designed chute systems anticipate and minimise the failure and wear mode impacts. Well-designed chute systems provide safe and considered approaches to all aspects of maintenance required. In many operations, the cost of labour and production downtime required for a repair far exceeds the cost of the components. There are excellent designs including features to reduce the outage time and site labour content for repair work. The combination of extended life and ease of repair will add significant uptime to chute operations.

Chutes are usually a guard. Chute systems must comply with the requirements of the AS/ANZ 4024 Safeguarding of Machinery series and other applicable legislation in Australia. Similar legislation exists in many other countries.

Chutes protect personnel from contact with ore moving at high velocity, from rotating components such as pulleys and drives, and provide safe access for operation and maintenance activities. Integrate chutes into adjacent guards to cover nip points on pulleys and belt transitions, shuttles and trippers. Chutes may support electrical and control components, and must comply with AS/ANZ 3000.

Chute systems can include drive components, tramp magnets, sampling systems, dust extraction and/or suppression systems, flow diverting and other functions. Parts of the chute can move in shuttles or diverters, yet the chute should still maintain the key functionalities in all positions.

Plant layout and design

Chute systems are easier to design if the geometry of the plant provides in-line or right-angled transfers. Other alignments are feasible but may require more space or height for an effective design.

Chute systems are always a compromise if there is insufficient space and height in the layout for all required functions at the detail design stage.

Early definition in plant layout should provide sufficient space for required chute functions, and gives flexibility for an effective design. It is always possible to use less space as the design is refined, but difficult to add this back.

Chute does not need to be this high

Unfortunately, we see chutes installed with the original configuration, having survived transition from concept through various feasibilities and the design process. These give large conveyor lifts and product drops for no reason; conveyors cost more to build and operate and additional liner wear and attrition of bulk materials result.

Many chute system designs prioritise ease of installation. Installation occurs once only; operations have to work and maintain the systems daily for over 20 years. Design should provide and document the safest and simplest access and procedures to remove and replace maintainable items.

Chutes flange connections, access doors and panels may have “M20 bolts at 100 mm centres” or similar Many of these connections have little or no load, and if made well will seal sufficient for purpose. Use locating pins and minimal bolts to expedite installation and removal. Check to see that it is possible to remove every nut and bolt using standard tools.

Does the chute flange joint need so many bolts?

Cleaners

Belt cleaners are integral with a discharge chute, and define several aspects of the system. Location of supports, maintenance method, and discharge trajectory are all fundamental to best cleaning.

Cleaner selection is a priority, and the chute design should match cleaner supplier recommendations from the start. Changing the cleaner supplier late in the development of the chute system to save procurement dollars usually results in a clash of intent. The cost to modify the chutes to accommodate a late change, or ongoing cleaner problems, is higher than any saving.

Cleaners wear out. Installed in the correct location relative to the belt and maintained, often monthly, most cleaners work well. Access to adjust and to remove cleaners should consider elevation and width of walkways, position of handrail stanchions and other obstacles such as drives. Consider cleaner designs that give easier maintenance from outside of the chute. These also need sufficient access.

Cleaners discharge material in separate trajectories to the main discharge. Consider this early in the design, as higher transfer may be necessary.

Wear and material flow

Chute designs must provide wear resistance, but not at the expense of controlling flow. Some bulk materials do not generate significant wear and flow easily, eg. salt, where stainless plate may still show printed cast marks after several years use. Other bulk materials, such as iron ore, can be extremely abrasive and have poor flow properties when wet. Adopt a good combination of chute wear liner and flow control to match the bulk material.

Material flow data for the bulk material gives guidance on the key flow parameters. Assumptions based on textbook data or “similar” materials are suspect.

There are specialised material test laboratories in Australia and globally, and the costs are small relative to the potential loss from non-performing chutes (and bins, stockpiles etc.). The tests should be performed using representative bulk material samples and on the wear liner(s) that could be used in chutes. Interpreting the output from tests requires some knowledge. Test the bulk material and use the results for chute design.

Simulate both wear and flow in a Discrete Element Model (DEM) during chute design. The chute DEM should represent the build, using the fabricated shape rather than a smooth or contrived surface. The DEM should use representative, characterised bulk material particles. DEM modelled flow of identical spheres through a perfectly smooth curved chute is not representative; even spherical sulphur granules vary in diameter. Calibrating DEM models from an existing chute flow is beneficial for upgrades but difficult for new projects. Add Computational Flow Dynamics (CFD) for dust and airflow simulation to give a complete picture, and for best dust management.

Chute liners intended to minimise wear; poor flow results in build-up and blockage.
Chute liners intended to minimise wear; poor flow results in build-up and blockage.

Other options for modelling bulk flow in chutes are the scale modelling facilities that are available in Australia. These models generate considerable discussion on the use of scale bulk materials and scale up of the final design. They are mature techniques after many years of refinement.

DEM and other models do not tell the whole story. They reflect a moment in time with specific bulk material properties derived from a small test sample. DEM cannot compensate for manufacturing and installation deviations, change in bulk material properties, surging, water sprays, damage and wear unless developed to do so at extra cost. Generally, models give a clear indication of something in a design that is unlikely to be functional, but not a precise indication of a faultless outcome.

Bulk material flow is the main function of a chute. The design should consider the discharge from the chute to the next conveyor or to a bin or stockpile. The chute design might also consider whether airflow created by the movement of the bulk material creates or manages dust emissions. Use the model to design dust extract locations and shapes.

Wear liners wear out. Better flow control and better wear materials allow liners to last longer, but they will need replacement.

Chutes do not contribute to the bottom line of an operation, but the loss of a chute reduces overall availability in most operations. The best chute designs consider how best to safely rotate wear components in the shortest practical time. Designs should be modular, with rotable suitably sized components and consideration of access with minimal collateral component removal. Do not forget lifting lugs and WLL for each rotable piece and the manual that tells how to do the change out.

Wear liner materials and installation practices have developed significantly in recent years. Design of chutes to protect the liner using ledges and boxes may not be the best flow solution when smoother surfaces with better wear materials are available. Use appropriate wear materials and thicknesses to match the expected wear rates at each location; chutes can, and in many cases should have multiple different liners to suit. Any gap, ledge, bolt recess, valley, protrusion in the flow may initiate chute wear and blockage.

There are several innovations in lining practice available in Australia, including ongoing material development, various single sided liner replacement processes, single sided skirt liner access, adjustment and replacement, remote liner wear monitoring and life prediction. Consider best options in design and selection.

Fabricate part or complete chutes solely from wear material when possible. Rotation of chute modules for off site repair is much quicker and safer, and generally better quality than insitu liner change.

Consider proprietary chute systems that are available from various suppliers in Australia. In the right application, there have been some excellent success stories for these chutes, combining wear resistance, excellent flow and a systematic approach to a maintainable chute system.

Chute adjustment

Generally, chutes are designed for flow based on a small sample of bulk material. This cannot give precise designs. Chute components are often adjustable for repositioning during commissioning, and if conditions change. Poor performing chutes can result if not adjusted after installation.

In some cases, design makes it difficult to adjust position easily, in others it seems that the operators had no guidance, and did not recognise the benefit of adjustability.

Provide good access and simple methodology to adjust chute components. Adjusters that extend into walkway space are an HSE hazard. Access to adjust chutes can be too difficult, such as chute cover removal to reposition the hood / deflector. Chute adjustment facilities should consider height adjustment as well as horizontal and tilt adjustment. Removable sections are part of the adjustment facility in some chutes.