AE07.pdf

As with other condition monitoring methods the

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As with other condition monitoring methods, the evaluation of machine condition from acoustic emission measurements is not an exact science. With acoustic emission testing, however, overreacting when the fault is at an early stage is more likely than reacting too late. The presence of unexpected acoustic emission signals indicates a problem. Analysis of repetition frequencies may yield diagnostic information for remedial maintenance. However, it is an increasing trend in the activity that usually provides the best indicator of the proximity to failure and hence the trigger for more urgent action. For example, in the case of a measure of overall acoustic emission signal level on a rotating machine, an increase of 10 or 15 dB under otherwise identical operating conditions would usually be noteworthy. Effectiveness of Acoustic Emission Testing for Machinery Monitoring A feature common to most mechanical failures of machines is the appearance of additional energy loss processes. These provide the source of the activity that acoustic emission transducers are able to detect. The high sensitivity of acoustic emission techniques to stress waves from these processes provides the basis for an effective and reliable approach to machinery monitoring. The diversity of machine designs, installations and fault types means that it 249 Acoustic Emission Testing for Process and Condition Monitoring
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is not possible for any condition monitoring method to guarantee detection of all machine faults in a timely manner. The additional complication of the long operational life of most machinery and the large numbers of machines to be monitored means that it is seldom feasible for preliminary research to demonstrate fully a condition monitoring method’s detection capability. For these reasons, the acceptance of a machinery condition monitoring method must be supported by a clear understanding of its technical basis and preferably by an adequate demonstration of its applicability in similar applications. There is no serious doubt about the applicability of acoustic emission testing to machinery condition monitoring. Certain simple implementations are widely used on the industrial shop floor by maintenance personnel. Here, acoustic emission testing is often the method of choice as part of a condition based maintenance program because of its speed, simplicity and versatility. The economic justification for applying the method in this way is easily demonstrated. In other cases, acoustic emission testing is applied as a specialist method. Field testing of more involved, large or unique machines is carried out by an experienced acoustic emission inspector with highly customized acoustic emission test equipment. In these cases, preliminary background noise measurements and propagation studies are likely to be carried out as well as an appraisal of the appropriateness of acoustic emission testing. The effectiveness of acoustic emission testing under these circumstances depends largely
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