What are the main advantages of adopting acoustic emission’s AE technology over traditional vibration analysis? (VA)
“With well-defined ISO standards, traditional vibration techniques including vibration monitoring and vibration analysis have provided a trusted approach to condition monitoring for the past thirty years.Yet, it remains a complex science and requires sophisticated knowledge and understanding from a seasoned expert. In contrast, AE technology extends and simplifies the science, placing the power of vibration techniques directly into the hands of every engineer. Signals can be processed at the AE sensor into an easily understandable form.
“Let’s be clear, vibration analysis (VA) as a technique will have a place for many years to come for many end users, however there is no escaping from the fact that there is often a requirement for a costly and unsustainable level of knowledge required to affect a good diagnosis. For VA, the defect repetition frequencies are critically dependent upon the machine component design and geometry, as well as the precise running speed. Vibration can occur independently in X, Y or Z axis and so orientation of the sensor is as important as location. For a detailed interpretation it is also necessary to know the internal machine geometries, shaft speeds, meshing frequencies etc and to analyse the data before making a diagnosis. So, in objective summary, VA is valuable, but too often overly complicated.
“In fact the areas in which vibration and AE both apply can be illustrated as overlapping circles. AE provides an earlier warning detecting wear and small defects, whereas with vibration, damage must have occurred to detect a signal. AE will pick up a lack of lubrication, friction, and cracking, which vibration will not. Although it must be acknowledged that the totality of information obtained from AE will be more limited than that derived from vibration”
Is AE more reliable than VA?
“It’s not more reliable per se, but as it doesn’t require a vibration expert to interpret the results, it could be viewed as more reliable when used by maintenance staff in general.
“AE in no way invalidates traditional vibration techniques, it simply extends the impact way beyond what we’ve been able to achieve to date. The signal processing required by AE is, in itself, not something that can be performed by just anyone; it’s a high frequency signal so the user must have the knowledge to interpret the squiggly line on a stethoscope. But recent developments have enabled this processing at the sensor level. The sensor output can now provide pre-characterised numbers that tell you about the condition of the machine. AE technology has been effectively deskilled, enabling much wider application use.
“Suitable for continuously running machinery as well as machinery operating intermittently, slowly or for short durations, AE allows the user to diagnose problems with machinery at an early stage, carry out maintenance procedures and then monitor the improvement. It provides real time information with early sensitivity to faults and applicability to a wide range of rotational speeds.
” As awareness of the unique capabilities of AE increases, so too does the number of applications that it is suited to – many of which have proven difficult for other forms of condition monitoring to address. For example, the analysis signals, whether from AE sensors or VA accelerometers, requires a sufficiently long period of machine running at constant speed so that a statistically meaningful signal characterisation can be made. But that is where the similarity stops. AE can be effective after around 10 seconds of measurements. 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. Comparing this where Fast Fourier Transform (FFT) based vibration analysis typically needs 60-120 seconds measurement time and tight tolerances on machine speed for an effective signal interpretation.
“In those cases where a hand-held instrument is used for periodic Condition Maintenance (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 require continuous online monitoring with permanently installed sensors inputting CM data or status into SCADA systems or PLC’s. Kittiwake Holroyd’s AE product range includes portable instruments, permanently installed remote sensors for areas of difficult access, as well as stand-alone programmable smart sensors for continuous surveillance.”
From a financial viewpoint are there any benefits associated with AE over VA?
“For vibration techniques to be effective you need equipment that’s far from cheap coupled with clever people to get the best from it. Every result must be analysed to understand what’s good and what’s bad. For those that cannot afford the luxury of in-house vibration experts, there are many vibration specialists who offer a contract monitoring service; again, requiring not insignificant investment. While for some, the criticality of certain applications coupled with the scale of some companies might justify this cost, others could still benefit from the efficiencies realised by similar CM techniques.
“Ultimately, maintenance personnel are responsible for keeping machinery running. If they are empowered to monitor condition themselves, identify where action is needed and then check that the action taken has solved the problem, then deskilled AE technology has significant advantages of cost, speed, flexibility and ease of field application in comparison to traditional vibration analysis techniques. It is the efficient and effective approach to CM.”
Is AE suitable for both land and marine based applications?
“Based on frequencies much higher than are monitored in the repetitive synchronous movement of vibration, AE technique is absolutely suitable for both land and marine based applications. These frequencies are the result of shock, impact, friction and cracking for example. By this means it is possible to detect impending failure before damage occurs, as well as monitoring its progress thereafter.
“AE technology spawned from the aviation industry where vibration analysis simply couldn’t be easily applied, short of a suicidal maintenance technician hanging off the wings. It is specifically designed to allow users with little knowledge of the subject to check bearings and major slideways for condition in a way that would be near impossible using traditional vibration techniques.
“In land based applications throughout industry, AE is favoured by maintenance personnel as a front line technique. Whether food and drinks, manufacturing, utilities or building services, the use of AE allows maintenance staff to quickly assess machine condition, without knowing the bearing’s ISO number, speed, size or history. This allows them to focus maintenance activities when and where they are required with minimum disruption to operations and, most importantly, to check there and then that a repair has been effective.
In a marine setting, vibration analysis is typically undertaken on ships using outside, third party consultants, if at all. With AE technology, you give it to a ship’s engineer right out of the box and by the end of their shift they will have an accurate assessment of all the engine room pumps, turbine and generator bearings, crane slewing rings and any ancillary air leaks in the engine room.”
Trevor Holroyd, managing director, Kittiwake Holroyd
Diesel & Gas Turbine Worldwide, January 2012
