Keeping tabs on all your assets
Published: 14 October, 2016
The wide availability and low price of vibration monitoring equipment gives maintenance engineers an effective way of protecting all their assets – including those of relatively low value explains Chris Hansford, managing director of Hansford Sensors.
Maintenance has undergone a revolution over the last few decades. In the past, machines were serviced at regular intervals – whether or not there was a problem. Under this regime, machines were also liable to fail unexpectedly, as the condition of components such as bearings was not monitored continuously. This all changed with the concept of predictive maintenance – and the widespread acceptance of techniques such as vibration monitoring.
While there are several ways to monitor the ongoing health of a machine, vibration monitoring is one of the most common and reliable. Very simply, it works by analysing the vibrations emanating from elements of the machine – such as the bearings – and making maintenance recommendations. The technique is so powerful because it can detect problems in their early stages, and remedy them before they become serious.
As maintenance regimes increasingly favour prevention over cure, techniques like vibration monitoring are being used more widely – and this is likely to continue. A recent report from Markets & Markets estimated that demand for vibration monitoring systems will reach nearly $1.5 billion by 2020. That’s an increase of around 50% compared to today – with a compound annual growth rate (CAGR) of more than 6.5%.
“Vibration monitoring can detect faults and machine deterioration before the occurrence of other symptoms like heat, sound, greater electrical consumption, and lubricant impurities,” says the report. “Therefore, it is an integral part of the machine condition monitoring programme and remains the most preferred condition monitoring tool.”
Another reason for this growth is that vibration monitoring is now used to monitor relatively small assets: where once it was reserved for high value assets such as a steam turbines, modern maintenance teams collect data from many lower value assets – including blowers and pumps.
As a result of this, the number of channels – and the volume of recorded data – will also increase: monitoring the performance of assets will then become more complex and challenging than ever before.
The relatively low cost of vibration sensors means that they can be fitted to just about every machine, no matter how small. (At the same time, multiple sensors can be fitted to larger, more complex machines.) The challenge is then to manage the mountain of data that is created, in order to run an efficient maintenance programme.
Online Vs offline
Data can be gathered in two distinct ways: offline – usually via a handheld data-gathering device that ‘interrogates’ a fixed sensor; or online, where the data is automatically transferred to a centralised control system, with real time data monitoring. Though more expensive, this second method is safer and more efficient.
For offline monitoring, maintenance teams must develop the most efficient data collection route through a plant – based on the criticality of equipment. The route is normally held in the data collector. Because the frequency of data collection is based on the criticality of the equipment there may also be periodic additions of non-critical pieces of equipment that are not visited regularly.
The two approaches can be mixed, depending on the type of asset. High value assets might be monitored automatically – with a direct link back to a control centre – while lower-value assets are monitored at set intervals using offline techniques.
There is another split, depending on the type of asset being monitored: while AC accelerometers are typically used to monitor high value assets like turbines, 4-20mA accelerometers are for components such as motors, fans and pumps.
Regardless of how data is captured, it is critical to analyse different frequency spans: these are dictated by the fault frequencies of the fastest-turning component in the machinery being monitored. A slow turning ball mill, for example, will have a narrower frequency span than a high speed fan. Once the span is known, the resolution is set within the vibration software for spectrum analysis so that fault frequencies of rotating components are not mistaken for other – correct – machine frequencies.
Carefully collected and analysed data can help to enhance plant efficiency: readings from a cooler fan, for instance, may reveal that the bearings were incorrectly aligned with the shaft during installation; or, that the outboard bearing was never locked down correctly; or, that the grid coupler was found to be dry. In truth, it could be a combination of these and other issues.
The results of an inspection can throw up surprises: a vibrating dust collector fan may suggest that the rotor needs balancing, though it could be caused by an unrelated issue – such as filter bag failure causing dust build-up on the edge of the rotor. Whether the cause was predicted or not, it is clear that vibration monitoring is critical in alerting the engineer before further damage can occur.
And it’s important to look after the nuts and bolts. Condition monitoring depends on stability. A poorly mounted accelerometer may give readings that relate to its own instability, rather than any change in machinery conditions. Installers should mount the accelerometer directly onto the machine on a flat, smooth, unpainted surface that is larger than the accelerometer base. The surface must be grease- and oil-free, close to the source of vibration and perpendicular to the axis of rotation. This improves the accuracy of vibration measurements – and this, in turn, improves the maintenance regime.
Correct mounting of the sensor is vital to ensure true readings. A vibration monitoring system must account for issues including imbalance, misalignment, bad bearings, mechanical looseness, hydraulic forces (cavitation, resonance) and rubbing. To detect these, sensors should be located to ensure that horizontal, vertical and axial movement are measured effectively.
As well as correct installation, sensors need to be sealed effectively to take account of environmental conditions such as temperature, humidity and corrosive chemicals, and even whether the atmosphere is combustible. Modern accelerometers operate over a wide temperature range, measuring both high and low frequencies with low hysteresis characteristics and high accuracy. Stainless steel sensor housings help to prevent ingress of contaminants.
As well as finding its way into lower-value assets, vibration monitoring is being used across the spectrum of industrial applications – in some cases, very tough ones. In cement production, for instance, it can monitor the motor and gearbox assemblies of key components such as kiln drives, crushers and screens, despite the very punishing operating conditions found there. And it doesn’t end there: in the marine sector, it is used for everything from motors and shafts to checking the air conditioning systems on cruise ships; in steel production, it checks the tolerances on CNC machines, and the correct working of hot and cold rollers; and, in printing presses, it ensures there are no catastrophic failures.
While these applications show how robust vibration monitoring is, its real growth will come from the small applications – pumps, fans, motors – that would once have been ignored and allowed to fail.
For further information please visit: www.hansfordsensors.com