Monitoring to cut downtime
September 1st 2011

The condition monitoring (CM) arena has been influenced by a number of innovations which allow today's engineers to enjoy the benefits of onsite and online testing, working in unison with offsite laboratory analysis.Martin Lucas,MD of Kittiwake Developments, explains

Where critical plant machinery and equipment is concerned, there is a clear 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 routine, scheduled test and analysis.

For both acoustic emission and oil analysis, 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, alerting engineers to problems at the earliest possible stage.

Acoustic emissions 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 condition monitoring directly into the hands of every engineer.

Providing real time information with early sensitivity to faults and applicability to a wide 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 with machinery at an early stage, carry out maintenance and then monitor the improvement.

As awareness of the capabilities of acoustic emission increases, so too does the number of applications to which it is suited – many of which have proven difficult for some other forms of condition monitoring to address. For example analysis of signals, whether from acoustic emission sensors or accelerometers, requires a sufficiently long period of machine running at constant speed so that a statistically meaningful signal characterisation can be made. This is easily achieved on machinery that is continuously running but is on the impossible side of difficult when it comes to machinery that operates only intermittently and for short durations. For example the algorithm used to derive the widely used acoustic emission parameters of Distress & 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 (FFT) based vibration analysis to require comparable or even longer measurement periods and tighter tolerances on speed variation.

In those cases where a handheld 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 towards CM automation, which requires continuous online monitoring with permanently installed sensors inputting CM data or status into SCADA systems or PLCs.

Kittiwake Holroyd's MHC-Sigma is a smart sensor that addresses the two key challenges of coping with short signal durations and the rejection of run-up and slow-down periods. It forms just part of a full product range that includes portable instruments, permanently installed remote sensors for areas of difficult access, as well as stand-alone programmable smart sensors for continuous surveillance.

Oil analysis Another weapon in the CM armoury, oil analysis is usually conceded to be the most revealing form of non-destructive testing.

On-site test kits and wear debris monitors can provide accurate information in minutes.

For example the ANALEX fdMplus accurately measures total ferrous wear in samples of any oil or grease, from gearbox lubricants through to hydraulics. It is widely accepted that in systems containing ferrousbased, moving equipment, the ferrous levels are the first to increase as 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, occasional laboratory results can be unrepresentative and sometimes cause false alarms.While temperature, pressure and vibration sensors all have a part to play in a CM package, early detection of changes in oil and lubricant condition can provide a greater insight.

Downtime costs money and impacts profitability. Successful troubleshooting using a combination of the CM technology available provides the first means of diagnosing problems with essential machinery and equipment. By 'deskilling' technology, all maintenance professionals are empowered to make informed decisions quickly and with confidence, ultimately enabling them to positively and significantly impact a company's bottom line.