Industrial mining machinery predominantly operates in moderate to extremely harsh conditions and often experiences failure as a result – but this doesn’t have to be the case. Chris Hansford, Managing Director at Hansford Sensors, explains how vibration monitoring can improve the reliability of machinery and minimise costly unscheduled downtime.
It’s been a tough year for companies operating in the African mining industry, with fluctuating commodity prices have placed significant pressure on profit margins. As a result, mining companies are placing greater emphasis than ever before on maximising performance and productivity. However, machine failure continues to be harmful to profitability and hinders high levels of productivity, despite there being readily available and cost-effective options to minimise unscheduled downtime.
Many of the causes of component wear that leads to machinery failure can be controlled by operating a fixed maintenance programme. In this programme of maintenance, mining companies wait for scheduled shutdown periods to check the health of machinery and complete repairs. These procedures are, however, only effective to a certain extent and can be costly, with unscheduled downtime often an inevitability. In addition, the unnecessary replacement of components that do not need changing but are replaced anyway because the fixed schedule demands it further reduces profitability because of expensive component costs – the time the machine is not operating is also unnecessarily extended. Therefore, a correctly specified and effective maintenance strategy is essential.
Although there are a number of options for managing component wear, predictive maintenance in the form of condition monitoring has proven to be one of the most effective and a preferred technique for mining companies operating around the world. Through this approach operators proactively monitor the condition of mining machinery on a routine basis, instead of reacting to a problem once it occurs. Maintenance and repair duties are completed based on the condition of components prior to failure and, as a result, the efficiency of the equipment can be optimised, maintenance costs can be reduced and productivity can be improved.
Vibration monitoring has become a key asset in the tool box of those responsible for running a predictive maintenance programme in African mining environments. It offers a range of valuable benefits that is helping mining companies to maximise machine uptime. By monitoring vibration mining companies can identify component wear prior to machine failure and therefore schedule component repair or replacement for minimal disruption and downtime – this is especially important where unforeseen rapid increases in wear could cause catastrophic and dangerous machine failure. Furthermore, in some circumstances the careful monitoring of vibration can enable the operating life of machinery to be extended beyond recommended maintenance intervals.
In vibration monitoring, the vibration fingerprint from a piece of machinery is assessed and subtle changes in operating conditions are detected and recorded. Examples of the types of machinery in African mining where vibration monitoring is common include: vibrating screens, crushers, pumping systems, gearboxes, load-out facilities and truck fleets. These pieces of machinery feature rotating components such as bearings, shafts and pulleys, which produce a vibration ‘signature’ in operation. If the rotating component is not operating properly – such as a bearing running with insufficient lubrication – the signature will change, alerting maintenance teams of an issue. Considering the total cost of unscheduled mining machinery downtime is estimated to be as much as 15 times that of scheduled downtime, resolving any issues prior to failure is clearly an effective way of protecting productivity, and therefore profitability.
Effective vibration monitoring requires a vibration sensor (also known as an accelerometer), which typically contains a piezoelectric crystal that is bonded to a mass. Under an accelerating force, such as vibration, the mass compresses the crystal and generates an electrical signal that is proportional to the force. This signal, which is amplified and conditioned, can then be used by data acquisition or control systems. Output data from sensors mounted at key locations can be read using hand-held devices for immediate analysis, or routed to a centralised system for continuous monitoring.
Accelerometers fall into two categories: AC and 4-20mA. While both can detect imbalance, bearing condition and misalignment, there are subtle differences between the two. AC accelerometers are typically used with a data collector for monitoring higher value assets such as wind turbines, whereas 4-20mA devices are used with a PLC to measure assets such as fans and pumps. Although both options are similar, AC accelerometers are able to identify cavitation, looseness, gear defects and belt problems.
Specifying vibration monitoring equipment for use in the mining sector is often more complex than applications in other industries. While the vibration measurements in each case will work on the same principle, conditions are likely to vary widely. For this reason, maintenance engineers are advised to look beyond only vibration levels and frequency ranges; they should also consider environmental factors such as humidity and temperature. Factors such as these vary in different locations and have varied effects on the vibration monitoring equipment in the environment. There is also the presence of corrosive chemicals and explosive gases to consider. These factors are present in underground mines and must be taken into account during the specification process because they demand sensors with intrinsic safety.
Because of the impact incorrectly specifying vibration sensors could have on productivity and safety, to ensure correct specification it is advised to work with a provider that has many years of experience. As well as ensuring the appropriate vibration sensors are specified, an experienced provider can specify the appropriate supporting equipment to make sure vibration monitoring is at its most effective, such as diagnostic kits, cables and connection points.
Installation and operation
Although modern vibration sensors are generally easy to install, there are a number of important factors that maintenance engineers must take into account to maximise the reliability of data. Effective condition monitoring depends on vibration sensors being stable, otherwise inaccurate readings are likely. For this reason sensors should be located as close as possible to the source of vibration, in the load zone, and be mounted on a smooth, flat, unpainted and oil-free surface, which must be larger than the base of the accelerometer itself. Installers must also decide between drilling, tapping or gluing, but first they must consider how each of these methods may affect the warranties on the equipment. In the event that the sensor is poorly mounted and unstable, it will record the instability of the sensor itself and not the vibrations of rotating components – therefore limiting the effectiveness of the maintenance regime.
Following the correct installation of the vibration sensors, the next consideration for maintenance teams is the regularity with which vibration data is collected and analysed. If the mining machinery is new, reconditioned or recently installed, data should be collected and reviewed once a month for six months to confirm the equipment is operating efficiently. However, if equipment is beyond this stage there are a number of factors that decide when checks should be conducted. If a piece of equipment is of high criticality it is advised that it is checked on a four week cycle, while less critical equipment can be checked less frequently, with least critical equipment checked every 12 weeks. The availability of spare parts, the time to change out the unit and the machine history also influence when checks are scheduled, which should become more frequent if a problem is identified.
There is sometimes a misconception that data collection is time consuming because of the high number of vibration sensors that may be used on a site. This is not the case and, in fact, sensors can either be hard wired or wirelessly connected directly to higher level plant control systems for automatic data gathering and analysis. Another option is for data to be captured with handheld instruments. In this method of data collection multiple sensors are wired back to local connection boxes, allowing data from multiple sensors to be gathered simultaneously and reducing the risk of exposure to machinery. To further improve the efficiency of this process, maintenance teams can develop routes based on equipment criticality, which can be stored in the data collector. This enables engineers to check and analyse mining machinery more time efficiently, so more equipment can be checked in less time.
Vibration is an excellent indicator of wear or imbalance within a system. Used correctly, vibration monitoring is a valuable tool for mining companies in Africa and around the world for maximising machine uptime, extending the operating life of components and systems, and enhancing system performance and energy efficiency – helping mining companies operating in Africa to better control profit margins through optimising productivity and reducing unnecessary costs.