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BULKtalk: Condition monitoring of belt conveyors
talk
Condition monitoring of belt conveyors
Steve Davis explains the ins and outs of conveyor condition monitoring and how it can be deployed to save operators time and money.
STEVE DAVIS In his regular BULKtalk column, Steve Davis considers the basics of bulk handling that sites often struggle with. Steve has worked in bulk handling for 30 years, for both resource companies and professional engineering firms, in Australia, South Africa, the Middle East and Canada. His experience encompasses such commodities as iron ore, coal, potash, phosphates, petcoke, sulphur, sands and grain.
A ONE-KILOMETRE-LONG
conveyor will have 4000 plus components: belt, pulleys, bearings, idlers, drives, cleaners, chutes etc. These components all wear out at different rates. Damage is common and component life becomes difficult to predict. What options are there for getting the best life from your conveyors?
Firstly, what should we be looking at on typical conveyors to maximise uptime and component life?
Belting and all aspects of the conveyor that cause damage to the belt, as this is the most expensive to replace: belt cost, labour cost and lost opportunity. In the worst case, a failed belt or splice will lead to potentially fatal outcomes as the belt and load run away and the take up falls. Belt tracking causes, and there are many, are usually the result of other condition issues. The result can be extreme belt damage, spillage, and structure damage.
Pulley condition, as poor lagging and entrapped material create uneven stress distribution across the belt and lead to carcass and splice failure and bottom cover wear, sometimes puncturing through the belt. Seized or collapsed pulley bearings, excessive shell wear, loose connections have many impacts and can lead to serious collateral damage. Allowing pulley bearings to run with incorrect lubrication or buried in spillage encourages early failure.
Loading zone problems will cause belt tracking through off centre loading, incorrect skirts set up and adjustment can wear through the carry cover or allow spillage and dust escape. Worn or maladjusted belt cleaner systems allow excess carry back and may damage splices.
Ploughs and safety attachments, as they prevent rocks getting trapped in the pulley nip point and damaging the belt and pulley; even worse if they come loose and run into the pulley.
Seized idlers can fail producing the ‘potato peeler’ and ‘pizza cutter’ effects, cutting through the bottom cover and can heat up and cause fires as they grind to a halt. Watch for idlers that have come out of the frame, as they can damage and injure. Buildup of carry back on idlers will cause belt cover damage and can cause belt tracking. Idlers that are buried in spillage will fail early as the bearing seals are penetrated. Spillage increases idler drag and energy consumption and is a cause of belt tracking. Spillage can initiate corrosion leading to shell failure.
Check drive components and maintain. Brakes and holdbacks are safety devices and must be functional. Couplings may need lubrication. Gearboxes need oil. Check the drive set up, as incorrect fluid coupling fill or variable frequency drive set up can limit starting ability or increase acceleration time such that dangerous problems results.
Guards should be visually assessed for condition and attachment. Any part of the guard that allows access to a danger zone or is a danger from corrosion or damage, snags or sharp edges must be fixed.
It is difficult to assess internal components visually, so often operators rely on noise, visible bearing collapse, and seizure to identify failure or near onset failure.
There is the run it until it breaks and then repair philosophy. The only condition monitoring is by inspection. Failure or wear is determined through walking the machine and checking for visible damage or unusual noise, temperature, operation. This option is viable for smaller installations where manpower with sufficient knowledge to determine condition is available.
Component life is maximised, but the downside is the random nature of failure causing outages and downtime. There is high potential for consequential damage, like allowing pulley lagging to deteriorate to where belt damage results, or high carry back because cleaners are not repaired. Component replacement is mostly planned based on an estimate of life. However, this likely sees some early unplanned failures and many components replaced earlier than necessary.
More common is an approach that combines inspection with a range of basic condition monitoring techniques. Typically, the installation will include belt tracking sensors, as belt replacement due to damage has the highest cost in materials, labour, and lost opportunity. In addition, we might monitor pulley and motor bearing temperatures, gearbox and hydraulic oil temperatures, lubricant reservoir levels, with an alarm or trip when a set point is exceeded. These indicators allow operators to inspect or stop conveyors before there is a catastrophic failure and limit resulting damage and outage. Where there are progressive changes in condition it makes sense to have multi-stage monitoring, with a warning before a stoppage for noncritical aspects. For example, a warning when gearbox oil temperature is higher than expected so that condition can be monitored and assessed, with automatic shutdown when the temperature exceeds the highest permissible level. Planned maintenance is still used.
More advanced operations will spend a little more on prevention and include manual or on-line vibration monitoring, monitoring of lubricant flows, pressures and feeds, lubricant sample analysis. There are several methods for checking belt rips in use, but all require a rip to occur before indication. Some will regularly manually measure residual belt cover and pulley lagging thickness and visually inspect wear liners in chutes. These require shutdown. Motor power trend monitoring can indicate potential problems from bearings in pulleys and idlers, poorly adjusted skirts and other causes of belt drag. Trending of data is then used to predict condition, likely time to failure and to schedule shutdowns and change out.
A ‘portable’ installation can assess and record the condition of a belt over several cycles, detecting broken or damaged cords.
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Ploughs and safety attachments prevent rocks getting trapped in the pulley nip point and damaging the belt.
Our industry has developed many remote condition monitoring systems that give online feedback and warning. Others are being developed, and the Australian industry has many good suppliers. Most of these systems provide detailed software for analysing output.
We can monitor belt condition online via several methods. The most economical method requires a ‘portable’ installation that will assess and record the condition of a belt over several cycles. The analysis is repeated at regular intervals and builds a trend of the condition. This is useful for installations with multiple conveyors. Types available include magnetic and X-ray and they can identify missing, broken, and damaged cords, corroded cords, misaligned or kinked cords, changes in splice condition and belt cover thickness.
Other systems are available for permanent installation, and with continuous monitoring. These can identify all above issues and belt wander, cover condition, belt rip initiation and fabric carcass condition. The cost of these systems is easily amortised on longer belts against the savings from better understanding and life obtained if used correctly. Major belt manufacturers have their own systems and there are several independent suppliers.
Shock pulse monitoring is available for a more sophisticated analysis over vibration analysis for slow speed bearings as installed on pulleys. Bearing manufacturers will advise the best lubrication regime for these bearings based on operating conditions, and the application can be monitored on-line to confirm. For intermittently operated conveyors we have systems that can be programmed to supply grease only when required.
We have remote wear liner monitoring systems that allow wear trends to be developed without shutting down. These allow for maximised wear liner life and the possibility of implementing liner regimes to even out wear. How much quicker would it be to know which liners to change before shutting down and hosing out? Could high wear panels be made rotable?
Idlers are a major problem for assessment. Our one-kilometre-long conveyor has approximately 4000 idlers in carry and return frames. It is difficult to assess internal components visually and we usually rely on noise, visible bearing collapse and seizure to identify failure or near onset failure. It is sometimes difficult to hear unusual noises over background even with headphones and microphone. It is often near impossible to see centre roll idlers and sometimes there is only access to one side of the conveyor. Lighting is often poor. What can we do?
We have systems that can identify changes in idler baseline condition information through the installation of a fibre-optic cable on the conveyor stringers. This will generally indicate the gradual deterioration of idlers in a localised area such that inspection and change can be targeted at the next opportunity. It will indicate the change that comes with catastrophic failure also.
We have systems that monitor each idler frame and identify changes in baseline conditions for assessment. The sensors can be retrofitted to existing idler frames.
We have systems that are installed inside individual idler rolls and provide roll by roll condition on a continuous basis, and include data such as vibration, shell condition, bearing temperature and speed. The monitors are built into idlers during manufacture and supplied as complete units.
Many of the technologies available to monitor conveyors are now available with wireless connection, avoiding cabling and multiple I/O connections. I have recently seen a system that is fully plug and play, with ability to receive signals from hundreds of locations by piggybacking into a single gateway and that completes all integration and assessment in the cloud before directing output back to the plant control system.
Is it a value proposition to invest in remote condition monitoring? This will be system specific and depends on financial calculation such as net present value (NPV). NPV does not consider the potential to remove people from the monitoring equation, thereby reducing contact with conveyors and risk of injury. NPV also does not generally consider the possibility to anticipate and manage failure to best financial and operating advantage.
Build-up of carry back on idlers will cause belt cover damage and can cause belt tracking.
