BULKtalk: Choosing the right conveyor belt

Steve Davis, senior bulk handling expert at Advisian, explains the ins and outs of buying the right conveyor belt and how choosing the wrong one can cost millions.

The conveyor belt is a significant component of the capital and operating cost of a conveyor. We should be looking for the best possible life from the belt and its splices, and taking every opportunity to minimise damage and wear, while meeting target throughput.

A conveyor belt that is volumetrically undersized for the duty will result in not meeting nameplate or produce spillage, both of which are ongoing losses to the operation, and costs in clean up and collateral damage. Many belts are designed around optimistic surcharge angles and reduced edge clearances or operated over original design parameters.

Poor loading onto conveyor belts leads to accelerated wear, carcass damage, belt tracking and reduced life.

At $200 per metre, a 20-kilometre conveyor (40-kilometre belting) is an investment of $8 million. For iron ore, the current value is currently about $80 per tonne and for a 5000 tonnes per hour conveyor, each hour lost is an opportunity cost of $400,000. The cost for an unplanned change-out of one reel of belt, assuming the belt is available, for two days, including labour and lost opportunity, would exceed $25 million.

What life to expect?

Conveyor belt life expectancy should be several years and perhaps more than 10 years before fatigue failure. Often in the mining industry, we see shorter lives. Three failure modes are common to all belts, cover wear, fatigue and overstress, and damage.

Cover wear is less of an issue when conveying low abrasive ores. Cover thickness, good load chute design with attention to loading speed and direction will result in good belt life.

Abrasive ores such as iron ore can cause one millimetre per month wear on short cycle conveyors. A three-year cover life for longer conveyors is good. Focus should be on load chute and skirt designs that minimise wear at the load point, and cover material and thickness that gives maximum practical life. The change in belt section from wear changes the tension distribution when wrapped around pulleys.

The selection of conveyor belting must be justified through comparative testing by TUNRA or similar, or some industry benchmarking to obtain the best cover material and longest life. Testing and experience indicate a wide range of wear rates between nominally similar compounds from respected suppliers, and for different ores. Particle size (coarse, lump, fine), moisture content, drop height, type of impact bed, belt speed and chute design all influence cover wear.

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Fatigue failure is the result of many cycles of bending and stressing and occurs in covers and in the belt carcass. A conveyor that is well designed, installed and operated should see even fatigue across the width of the belt. Poor design of transitions and turn overs, curves, tripper and shuttle approaches are common causes of tension and fatigue bias across belt width. Build-up of ore on pullies and significant cover wear can result in localised high stresses in the belt that lead to early fatigue, typically on the edges or in the centre. Fatigue results in cover compound cracking, splice failure initiation, and carcass damage such as broken wires. Misalignments and over-tensioning can also bias tensions across the belt. If part of the width has failed, even in one location, the strength of the belt has been reduced, and progressive failure is probable. If not monitored, this can lead to unexpected belt failure.

Design of the conveyor should consider the minimum possible number of pulleys, minimum allowable diameters, correct pulley spacing to reduce reverse bending (the one second rule), and correct design of transitions and curves. Operations should consider the result of any action that changes the tensions and transitions.

Damage can result from many sources, and most can be anticipated.

• Ultraviolet (UV) cover damage seen as cracking and spalling. This can be reduced by covering belts and UV-resistant covers.

• Chemical and heat damage can be reduced by selecting the correct cover compound. Some dry fine materials such as cement and alumina, not normally considered chemicals, attack some cover compounds and cause cracking.

• Edge damage from tracking into structures and equipment can be reduced by installing belt drift monitors and aligning the conveyor.

• Damage from carry back and spillage build up on pulleys and idlers can be reduced by sizing the belt for the maximum throughput and installing quality belt cleaning and spill protection devices.

• Wind can turn the conveyor belt over or push it into structures resulting in spillage and damage. Wind barriers will prevent this.

• Damage to and from pulley lagging can be reduced by having correct lagging, minimising spillage, installing pulley cleaners, shedders and belt ploughs to keep spillage from entrapment between the pulley and belt.

• Damage from tramp metal is less easy to prevent but damage can be minimised by removal of tramp using an over belt magnet and by metal and rip detector systems at critical locations.

• Groove damage from a dragging belt skirt that removes cover rubber directly or by ore entrapment should be designed out.

• Idlers that fail into ‘cookie cutters’ and ‘potato peelers’ should not be used, and quality idler seals and bearing arrangements are preferred. Identify and change seized idlers before damage results.

• Limit belt damage by installing and maintaining quality cleaners and ploughs. Splice interface with cleaners and ploughs should minimise risk of ‘digging in’ under the front edge of the splice.

The above list is not exhaustive. Providing well designed and properly installed components with good access to maintain will not add much to the cost of a conveyor. The life of the belt will benefit, and most of the components will last longer. Plant safety will be better. A 20-kilometre belt will have approximately 75,000 idler rolls. Even if a cheaper roll is $20 less than a better-quality roll, does the potential $1.5 million saving balance out the $25 million potential loss from a small rip, not to mention the greater number of roll failures?

Belt strength selection

Computer design programs help select belts using user defined inputs like minimum bulk density, surcharge angle, edge clearance, design tonnage, belt speed and belt safety factor. They then define an acceptable belt strength based on the inputs and other components to suit. It is relatively easy to iterate the calculations and check options. Absolutes used in conveyor specifications should be viewed with care. For example, specifying a maximum speed of six metres per second may result in a wider or higher strength belt than 6.4 metres per second. There is no practical difference between these two speeds. Be flexible.

There are several load factors in use, nominal and design and other values for capacity are used. Interpretation varies, but it is vital the designer understands user expectations. Assuming a surcharge angle from a text often results in incorrect belt selection as, for various reasons, loading does not produce this angle. Low, zero, and even negative surcharges are common in practice, due to ore variation and chute configuration. Surcharge angle can reduce along long conveyors reducing edge clearance. Conveyor loading is rarely a steady state, as in belt design, and therefore an allowance for surging is appropriate. Feed from an apron feeder or bucket wheel can result in 25 per cent surging. Ore properties can be extremely variable. The best source of data is from a site visit to a similar conveyor.

Safety factors allow for inefficiencies in splicing, and account for many variables in the life of the belt, especially in the field where conditions are rarely ideal. The safety factor derates the nominal strength of the belt carcass to allow for these variables. Safety factors were first stated in DIN 22 101, 1982, and there is considerable discussion around what to use in each application.

1. For fabric belts, belt safety factor is often a nominal 10:1. Little testing of fabric splices has been completed thus far. Many fabric belts have higher design safety factors if operating conditions are considered poor. If clip splices are used, consult the supplier.

2. Steel cord safety factors were a nominal 6.7:1, but advances in splice design and laboratory testing have led to consideration of lower factors, down to 4:1 or lower. There are several splice test facilities, so for a long or expensive belt it is worth testing the splice to confirm efficiency. Lower factors are acceptable and reduce belt strength requirement and cost, however splice quality in the field must be good quality, and other aspects of belt change during life should be considered.

3. Increasing the tonnage throughput on the conveyor reduces the safety factor when based on using ‘spare’ power and capacity. If it is likely this will occur, design initially with a margin or be prepared to change speeds and power.

4. Using smaller than recommended diameter pulleys or locating reverse bending pulleys too close, or any of the issues in the previous section, increase splice fatigue rate and further reduce the safety factor.

5. For longer life belts, the strength of carcass and splice will diminish with time.

6. Changing the belt source at a belt change-out could result in a different splice efficiency.

7. For conveyors with high dynamic belt tensions from stopping and starting, generally overland, the selection of take up type and other factors change belt tension requirements.

Each conveyor is different, as is each design team, so different solutions are possible. There is no single correct design or selection. I recommend clear user definition of expectations and minimum design inputs, and that the final design be re-evaluated when all details are firm. Due to differences in interpretation, I recommend an independent check of complex conveyor design at this stage.

The other parts of the belt

Having considered all the above, we now have belt speed, width and strength for the carcass of the belt. This is based on agreed design inputs.

There is no global standardisation on the make-up of the carcasses, so there may be several combinations of steel cord diameter and spacing that make up a particular strength. Fabric carcasses have more potential variations that achieve the same strength. Selection may be dictated by standardisation, otherwise allow suppliers some flexibility in proposals.

We have to select the compound for covers. There are many cover compounds available, and each manufacturer has their own proprietary mixes. Select covers that meet requirements. Options include low rolling resistance for bottom covers, gouge or abrasion resistant top covers, special covers for temperature or for alumina and cement, oil and chemical resistance colour and food grade. Grade M or N and other generics may be appropriate, but is it best for life costing? Are all similar grade covers the same? As noted earlier these compounds can be compared at a test facility.

Cover thickness is a trade-off of cost versus wear rate. Thicker covers may give a longer life but consider pulley diameters and belt cost. Most belt suppliers will provide guidance on selection of their belts. Most can meet the detailed manufacturing quality requirements of regional standards. Australian Standards require QC testing; do you need other tests?

Splices

Whether steel cord or fabric, all belts must be spliced to form an endless loop. The splice is the weak point in the belt and is the main reason we have a safety factor. Vulcanised splices rely on the spliced cover and filler compounds to carry belt tension in shear. There is no direct joining of the carcass materials. Splices require good design, and the quality of the equipment and process to make them in the field. Good conditions give best quality, so include splice facilities in the conveyor design.

Steel cord belts must be hot spliced. Fabric belts have the option of hot or cold splicing and ‘clipping’. Hot splices must be between two pieces of belt with the same cord or carcass configuration and cover compounds. Cold splices for smaller belts may be able to join two similar pieces of belt. Belt clips are available in many different formats, and guidance from vendors is recommended. A detailed record of all splices should be maintained.

If site conditions are dirty, dusty or cramped, or if quality splices can’t be guaranteed, increase the belt safety factor to compensate. Obtain a splice design from the belt supplier and ensure compliance.

All hot or cold splices require a splice kit. These contain glue and filler pieces, which are specific to the belt. Use of incorrect kits may reduce splice life. These kits have a shelf life and storage requirements, usually cool or cold. Once past expiry date they should not be used. Incorrect splice kit transport to site or cooler failure will affect splice quality. Some suppliers will confirm suitability of splice kits, which is beneficial for replacement of critical belt splices.

Splits, joints and edge sealing

Split belts, mostly fabric carcass, are available from many suppliers. Suppliers carry stock of a wide belt and slit it to width. This is useful in an emergency, but there are some risks in making sure the conveyor belt is compatible in a repair, and in some cases a resulting belt drift problem. Tracking is due to unbalanced tensions from splitting a symmetrical fabrication and generally cannot be fixed. Centre splits are better than edge splits as the tensions are likely to be more balanced. For fabric belts, clipping instead of splicing will avoid cover incompatibility. Split steel cord is rare, but I have seen different width belts with similar specification spliced for emergency repair.

Centre joints are when a wider than available belt is required, and results from two pieces of belt being longitudinally spliced together in the factory. The current maximum belt width is 3.2 metres, any wider requires a joined belt. I would not use these in a critical application.

Steel cord belts are generally made in the specified width with sealed edges. Fabric belts can be requested with or without sealed edges. Sealed edges protect the carcass from chemical or moisture ingress. Split belts always have one edge that is not sealed. Observation indicates that open edge fabric belts can start fraying after some time in service. Sealed edges do not protect against contact with structure but are better than unsealed.

Specifications

If the specification used to purchase belt and splice kits is not clear and detailed, there is a considerable risk of misinterpretation and incorrect supply. As belts are mostly made to specification, it can include valuable information such as the lead time for supply. Avoid being forced into using incompatible belts. A marked change in belt cover, cord or splice life after a repair can indicate that an incompatible belt was procured. Mechanical damage is generally independent of belt specification.

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