Vibration monitoring has become a key part of predictive maintenance – and its use is booming thanks to affordable sensors that can be applied to a diversity of applications. Chris Hansford, Managing Director of Hansford Sensors, explains. The nature of maintenance has changed radically in recent years. Where once machines were repaired when they failed – or, at best, serviced according to a fixed schedule – they are now more likely to be covered by a predictive maintenance regime. Here, the condition of machines is monitored using sensors in order to detect problems in their early stages and remedy them before they become serious. The advantage is clear: it reduces the risk of unexpected failure, and thus maximizes machine uptime. Vibration monitoring is a reliable way to monitor ongoing machine health. It analyses vibrations from components such as the bearings, and makes maintenance recommendations. It can, for instance, detect if a bearing is beginning to wear – and so will recommend that the part is changed. The technique is used in a wide variety of applications – from heavy duty machinery in the cement industry, through air conditioning systems on cruise ships to checking the tolerances on CNC machines in steel production. As maintenance regimes switch from preventive to predictive, the use of vibration monitoring will continue to increase. A recent report from Markets & Markets estimates that global demand for vibration monitoring systems will rise by around 50%, to reach nearly $1.5 billion by 2020. This equates to 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 program and remains the most preferred condition monitoring tool.” The relatively low cost of vibration sensors means they can be fitted to most machines, no matter how small. This is contributing to the predicted boom in these systems: vibration sensors were previously reserved for high value assets such as steam turbines, but maintenance teams now collect data from many lower value assets – from blowers and pumps to tunnel vents and elevators. As a result, the number of channels, and the volume of recorded data, will also rise. The challenge is now to manage all the data that is created, in order to run an efficient maintenance program. Gathering data Data is gathered either offline – using a handheld device that ‘interrogates’ a fixed sensor – or online, where data is automatically transferred to a centralized control system, with real time monitoring. Offline monitoring requires maintenance teams to develop the most efficient data collection route through a plant, based on the criticality of equipment. The route is normally stored in the data collector. Because the frequency of data collection is based on the criticality of the equipment there may be periodic additions of non-critical items that are not visited regularly. These two approaches can be combined, depending on the type of asset. High value assets might be monitored automatically – with a direct link back to a control center – while lower-value assets are monitored at set intervals using offline techniques. High value assets such as turbines are usually monitored using AC accelerometers, while more affordable 4-20mA accelerometers are used to monitor components such as motors, fans and pumps. Fault frequencies Regardless of how data is captured, it is critical to analyze 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, has 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 some perfectly correct machine frequencies. The results of an inspection can throw up surprises. If a dust collector fan is vibrating, this may indicate that the rotor needs balancing. However, it could be caused by an unrelated issue such as filter bag failure causing dust to 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 takes place. Precise data analysis can enhance plant efficiency. Readings from a cooler fan, for example, may reveal that the bearings were incorrectly aligned with the shaft during installation. Alternatively, the readings may indicate that the outboard bearing was never locked down correctly – or that the grid coupler was found to be dry. In reality, the underlying problem may be a combination of these and other issues. Stable mounting This may sound basic, but condition monitoring depends on stability. A poorly mounted accelerometer may produce readings that relate to its own instability, rather than indicating 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 should 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 – which, in turn, improves the maintenance regime. In addition, sensors must 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 high and low frequencies with low hysteresis characteristics and high accuracy. Stainless steel sensor housings help to prevent contaminant ingress. The ready availability and affordability of vibration sensors means they will continue to play a key role in the maintenance revolution – bringing condition monitoring to small assets that would once have been allowed to fail in service.