Condition Monitoring: why acoustic emission represents the next generation of vibration

History, experience and familiarity count for a lot where conditioning monitoring is concerned. But that doesn’t negate the need for change, innovation and the advancement of tried, tested and trusted techniques. The late Steve Jobs commented: “Innovation is the ability to see change as an opportunity – not a threat”. Condition monitoring (CM) is transforming rapidly and so too must the mindset of CM practitioners and users. It’s not good enough to simply disregard a disruptive technology in an effort to protect the ‘old guard’. When combating downtime, there’s no place for historical sentiment.

Steadily disrupting traditional vibration techniques is acoustic emission (AE). As the mechanical condition of machinery deteriorates, energy loss processes such as impacts, friction and crushing generate sound wave activity that spans a broad range of frequencies. AE technique is based on frequencies much higher than are monitored in the repetitive synchronous movement of vibration. By detecting only the high frequency part of this signal, it is possible to detect miniscule amounts of activity, for example a slight rub, a brief impact or the crushing of a single particle in the lubricant. By this means it is possible to detect impending failure before damage occurs, as well as monitoring its progress thereafter.

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 in to an easily understandable form.

Let’s be clear, vibration analysis (VA) as a technique will have a place for many for 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. There is not doubt that VA is valuable, but it is too often overly complicated.

In fact the areas in which vibration and AE both apply can be illustrated as overlapping circles However, 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.

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 lines on an oscilloscope. But recent developments pioneered by Kittiwake Holroyd 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.

While for some, the criticality of certain applications coupled with the scale of some companies might justify the cost of vibration techniques, others could still benefit from the efficiencies realised by similar CM techniques. AE 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. Vibration analysis is typically undertaken on ships using outside, third party consultants, if at all. With AE technology, you give it to a ships 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.

As awareness of the unique capabilities of acoustic emission increases, so too does the number of applications that it is suited to and the formats in which it is available. Kittiwake Holroyd, for example, provides a range of portable instruments, permanently installed remote sensors for areas of difficult access, as well as stand-alone programmable smart sensors for continuous surveillance.

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 AE 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; an easy way to implement a ‘no surprises’ maintenance policy.

Martin Lucas, managing director, Kittiwake 

Marine Propulsion, February 2012