In order to frame the work performed up to this point, this chapter begins with a short discussion of each section alongside key findings. In this way, the development ‘story’ can be outlined.
8.1.1 Introduction
At the beginning of this work, the general question of localising imbalance in rotating machinery was raised. Information from Rolls Royce, one of the world leaders in the field, suggested that the push for improved maintenance procedures and corresponding reduction in downtime will have a significant importance in future IVHM systems for rotating machinery. It was further suggested that common rotordynamic faults are still of significant concern to such manufactures, and whilst improvements in this field continue to be researched relatively little of this research finds its way to industry. Thus the general question of ‘how can common faults in rotating machinery be diagnosed and prognosed for efficient maintenance solutions in future IVHM systems?’
8.1.2 Key Concepts & Review of Literature
With the formation of a general research question, the topics for the review of literature were clear. Important rotordynamic principles were outlined alongside eight common faults in rotating machinery. These faults were discussed from the perspective of diagnosis, prognosis, localisation, modelling and common sensing methods. It was found through this review that imbalance faults are often considered to be the most common, with potentially serious consequences. Whilst imbalance faults can be potentially complex in nature,
with a variety of underlying causes, a large amount of research has been performed over the years into diagnosing these faults. The existing literature however exposed a lack of research into localising imbalance faults in complex systems. Through this finding, it became clear that improved maintenance procedures could be implemented into complex machinery if technicians have access to accurate information on not just the type, but location of the fault. Some existing systems for fault localisation were described; however a clear gap was identified for an accurate system which could operate in both rigid and flexible systems using a reduced sensor suite. The study of published research yielded some further information that guided the direction of the study. This included the fact that a synergy between physics-based and data-driven approaches would be required, alongside indications that machine nonlinearities could play a role in imbalance detection and localisation. The importance of relating the system to next generation IVHM systems was also highlighted.
8.1.3 Methodology
The information gathered from the review of literature resulted in a clear methodology being created. This set the framework for the remainder of the project, and outlined the work flow of the following chapters.
8.1.4 Imbalance Localisation using Four Disc MFS
In this chapter the experimental approach to localising imbalance faults was established, consisting of the largest contribution to knowledge within this project. The recommendations from the literature were considered in order to aid the creation and development of the localisation system. The results demonstrate the potential for the effects of imbalance upon nonlinearities to be used for fault localisation. Comparison of the ‘nonlinear’ approach has been made against a ‘standard’ ANN. It is acknowledged that the ‘standard’ ANN used for comparison does not represent a state of the art approach in fault localisation. Despite this, it provides a useful indication that nonlinear features possess advantages over a linear approach, especially in the rigid regime.
The vast array of fault combinations and operating conditions available on the MFS prevented every potential combination from being tested within a practical timespan. Nevertheless, the extensive combinations of speed, sensor position and machine configuration combined with imbalance size, position and type provide a solid indication that the system can be considered validated for the MFS. Throughout this testing, the only limitations occur when a very small imbalance weight is applied. Whilst this is undesirable, the indications from the MFS are that as soon as the imbalance becomes large enough to cause undesirable vibration and resulting operations, then localisation can be performed. The difficulties in localising couple imbalance also stem from this limitation, which was in any case negated through the ‘duel ANN’ work in the following chapter.
8.1.5 Extended Testing & Validation
The novel system developed and tested on the MFS was extended and further researched. The addition of two other rigs for testing allowed the system to be adapted with success in these applications. Through the course of this chapter, the system was developed to incorporate other faults – including rub and misalignment, operate during a run-up, run-down cycle and tested to ensure it worked with high accuracy in both flexible and rigid regimes.
The most extensive testing and validation of the system has occurred on the MFS. Whilst the other two rigs both allow for imbalance faults, as they are not designed as ‘fault simulators’, the combinations which can be studied are limited. However, this limitation also acted as an advantage in this case, as the two ‘validation’ test rigs display much smoother operating conditions. Whilst the relative size of the 1/3X feature in the MFS could almost be described as a fault in its own right (under certain operating conditions), this is certainly not the case in the two other rigs, both of which possess small nonlinearities. The two additional rigs only allow for imbalance to be located between two positions (as opposed to the four of the MFS). Whilst given the opportunity, more combinations would have been studied, even the ability to localise between two sections of a complex machine has already stated benefits.
Two additional faults were studied in this section, misalignment and rub. This was deemed sufficient to validate the localisation of imbalance in the presence of other faults. The extreme ‘rub-misalignment-imbalance’ combination resulted in highly undesirable (and noisy) operation of the MFS, and yet through the described method imbalance could still be localised. In future studies, the other eight common rotordynamic faults may be introduced for full validation of this statement, however the results conducted here can be considered ‘proof of concept’. Whilst it is always possible to test the system under ever more complex situations, the main benefit of further work would come from application to a full scale turbine, or other aspects (see ‘Future Work) which fall firmly outside of the possibilities of this initial study, and therefore at the end of this chapter the localisation system can be seen as sufficiently validated for the purposes of this thesis.
8.1.6 Nonlinearity Modelling and Design for IVHM
Having seen sufficient development of the system performed on rotordynamic rigs, in order to provide enablers for the system to be adapted to more complex systems, two further aspects were studied in this chapter. Firstly, through nonlinearity modelling the theory behind the experimentally developed system was demonstrated. This in turn enables ‘design for IVHM’ to be considered, whereby synergy with other existing research was considered.
The nonlinear studies have been aimed at qualitatively demonstrating the phenomena occurring in the MFS, validating the theories proposed in the previous two chapters. In this case the system appears to be successful, with differences in the 1/3X demonstrated to provide larger differences than the 1X under given conditions. In this study, quite a large nonlinear effect has been implemented in order to replicate the MFS and demonstrate the point in question, however it should be noted that this applied to smaller bearing nonlinearities as well. As a theoretical validation and demonstration of the phenomena, it was not deemed necessary to provide quantitative validation in this case for the modelling, especially as the approach described is mainly data-
driven in nature. If a ‘design for IVHM’ approach was to be followed however, a quantitatively validated model would likely be required.
The full scale gas turbine model provides an initial investigation into the possibilities for scaling up the system. This study is limited in nature, as full benefit is only likely to be obtained alongside practical experiments on a large turbine (or similar). The questions of “if inherent nonlinearities are present and detectable” require solving in the first instance, before extensive further modelling can be beneficial. The results nevertheless indicate positive potential for the application in such a system, if given bearing nonlinearities are present (or may be implemented). Whilst within the model, no significant adverse effects were detected by introducing a bearing nonlinearity, this would need extensive further verification if one were to consider deliberately introducing such a nonlinearity into an actual turbine. Nevertheless, the potential for this to occur is already evident, as detailed in numerous publications highlighted in the latter part of the chapter.
8.1.7 Localising Imbalance in Future IVHM Systems
In the final chapter of this research, the research was placed into the broader picture of IVHM. During the review of literature, it was noted that much published research is left at the initial design stages (equivalent to Chapter 4 in this study). As the novel approach discussed in this thesis is intended to make inroads towards practical application of an imbalance localisation system, considering future constraints and requirements was deemed necessary.
The main conclusion of this chapter can be seen in the ‘graphical methodologies’. Imbalance localisation is rarely discussed by authors as an issue in its own right, more as an addition to other performed work. These methodologies therefore provide a guide which represents the workflow found to be most relevant throughout this work for creating an imbalance localisation system. By framing the technical aspects (determining critical speeds, linear/nonlinear feature identification etc) alongside other considerations (business case, maintenance decision support), it is possible to calculate precisely what a proposed system must achieve. Determining such facts before
design of such a system is fully developed is crucial in order to avoid impracticalities – e.g. the system works, however requires too many sensors.
8.1.8 Summary of Work Performed
It can thus be seen that the desire for improved maintenance procedures in next generation industrial applications results in the necessity for accurately localising imbalance faults in complex rotating machines. A novel system for localising these faults through application of a single sensor has thus been developed. This system has been thoroughly described and tested, performing localisation based upon machine nonlinearities. Simulation has been used alongside current research to demonstrate the potential impact of such systems in future IVHM applications.