Where critical plant machinery is concerned, there is a benefit in knowing what is going on at an exact point in time, not just when the engineer can get to a machine for a scheduled test and analysis. The condition-monitoring (CM) arena has been influenced by a number of innovations that now allow engineers to enjoy the benefits of onsite and online testing working in unison with offsite lab analysis.
For both oil analysis and acoustic emission, onsite instruments enable rapid testing and action, and online sensors reduce the risk of human error. Online, of course, refers to sensor technology, which is advancing at a furious pace. Dependable sensors designed to monitor remotely and in real time provide an early warning system.
Oil analysis is usually the most revealing form of non-destructive testing. On-site test kits and wear-debris monitors can provide accurate information in minutes. It is widely accepted that in systems containing ferrous-based moving equipment, the ferrous levels are the first to increase as the equipment wears.
However, the real value comes from continuous monitoring of critical plant systems. Trending of vital lubricant test parameters — including viscosity, water in oil, total base number, insolubles, wear debris and particle content — is important, and the more regular the information the better. Even with the best sampling practices, sometimes results can be unrepresentative and cause false alarms. While temperature, pressure and vibration sensors all have their part to play, early detection of changes in oil and lubricant condition provides far greater insight.
Traditional vibration analysis has provided a trusted approach to condition monitoring for the past 30 years, but it is a complex science and requires sophisticated knowledge and understanding. Acoustic emission technology, however, places the power of CM into the hands of every engineer.
Providing real-time information with early sensitivity to faults and applicability to a range of rotational speeds, the acoustic-emission technique is based on the detection of the high-frequency component of naturally occurring stress waves. Suitable for continuously running machinery as well as machinery operating intermittently or for short durations, acoustic emission allows the user to diagnose problems at an early stage, carry out maintenance procedures and then monitor the improvement.
As awareness of its capabilities increases, so too does the number of applications to which it’s suited, many of which have proven difficult for other forms of condition monitoring to address. For example, the analysis of signals, whether from acoustic emission sensors or accelerometers, requires a sufficiently long period of machine running at constant speed so a statistically meaningful signal characterization can be made. This, easily achieved on machinery continuously running, is close to impossible on machinery that operates intermittently. For example, the algorithm used to derive the widely used acoustic emission parameters of “Distress” and “dB Level” in the MHC range of products from Kittiwake Holroyd requires a 10-second period of running at an approximately constant speed. Similarly, it would not be unusual for Fast-Fourier-Transform-based vibration analysis to require comparable or even longer measurement periods and tighter tolerances on speed variation.
In cases where a hand-held instrument is used to carry out periodic CM, it may be possible to interrupt normal machine operation and put it into a special continuously running mode for the duration of CM measurements. However, such disruption is not always possible and never convenient. Furthermore, it is not compatible with the current trend toward CM automation, which requires continuous online monitoring with permanently installed sensors inputting data into SCADA systems or PLCs.
Downtime costs money and impacts profitability, which must be steadfastly avoided, especially in today’s financial climate. Successful troubleshooting using a combination of the state-of-the-art CM technology provides the first means of diagnosing problems. By deskilling technology, all maintenance professionals are empowered to make informed decisions quickly and with confidence.
Martin Lucas is the managing director with Kittiwake Developments. In Canada, contact Kittiwake Americas president and CEO Peter Pilon at email@example.com.