Advances in Industrial Control For further volumes: www.com/series/1412 Se Young Yoon r Zongli Lin r Paul E. Allaire Control of Surge in Centrifugal Compressors by Active Magnetic Bearings Theory and Implementation Se Young Yoon Paul E. Brown Dpt of El. of Mechanical & Aerospace Engin.
University of Virginia University of Virginia Charlottesville, USA Charlottesville, USA Zongli Lin Charles L. Brown Dpt of El. University of Virginia Charlottesville, USA ISSN 1430-9491 ISSN 2193-1577 (electronic) Advances in Industrial Control ISBN 978-1-4471-4239-3 ISBN 978-1-4471-4240-9 (eBook) DOI 10.1007/978-1-4471-4240-9 Springer London Heidelberg New York Dordrecht Library of Congress Control Number: 2012941917 © Springer-Verlag London 2013 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed.
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Printed on acid-free paper Springer is part of Springer Science+Business Media (www.com) To Our Families Series Editors’ Foreword The series Advances in Industrial Control aims to report and encourage technol- ogy transfer in control engineering. The rapid development of control technology has an impact on all areas of the control discipline. New theory, new controllers, actuators, sensors, new industrial processes, computer methods, new applications, new philosophies,. Much of this development work resides in industrial reports, feasibility study papers and the reports of advanced collaborative projects.
The series offers an opportunity for researchers to present an extended ex- position of such new work in all aspects of industrial control for wider and rapid dissemination. Monographs in the Advances in Industrial Control series can be considered to range in type from the “art of the possible”, a “proof of principle” type and then a “state of the art” type, where the latter often reports on control as it exists in today’s industry. For example, some “art of the possible” monographs explore a new theoretical development and demonstrate how it might find application in the control field. A good example of this type of monograph is Process Control by J.
Other monographs examine the present “state of the art” of control and its technology as found in current industrial practice and look at how better control might enhance efficiency and minimise pollution. Recent exemplars of this category are Advanced Control and Supervision of Mineral Processing Plants by D. del Villar (ISBN 978-1-84996-105-9, 2010) or the monograph Hydraulic Servo-systems by M. However, this present, comprehensive Advances in Industrial Control mono- graph Surge Control of Active-Magnet-Bearing-Suspended Centrifugal Compres- sors: Theory and Implementation by Se Young Yoon, Zongli Lin and Paul E.
Allaire is an example of the “proof of concept” monograph. It is an excellent addition to the series since its content has broad but complementary contributions from a new tech- nology, from advanced control and from an advanced controller demonstration and assessment using an industrial-standard experimental rig. The phenomenon of surge and stall in compressor technology is long standing and when the widespread industrial use of compressors is considered, a successful vii viii Series Editors’ Foreword control strategy that optimally maximises performance and eliminates compressor downtime would be of significant economic benefit to industry. This particular con- trol problem has received exposure in the Advances in Industrial Control series pre- viously through the published monograph Compressor Surge and Rotating Stall by J.
Egeland (ISBN 978-1-85233-067-5, 1999), a monograph that is often cited in the literature of the compressor control field. Some related material can be found in another monograph in the series, namely, Dynamic Modelling of Gas Turbines edited by G. However, this monograph by Se Young Yoon, Zongli Lin and Paul E. Allaire is distinctive in that it investigates the particular technology of active-magnet-bearing- suspended centrifugal compressors and assesses the authors’ own original advanced control strategies.
The assessment takes place using “an industrial-size centrifugal compressor test rig. designed, built, and commissioned by the Rotating Machinery and Controls Laboratory (ROMAC) at the University of Virginia” (USA). A descrip- tion of this experimental set-up can be found in Chap. 4 of the monograph.
Access to and use of this industrial-sized test rig is just one of the distinctive features of the research reported in the monograph. Another feature is the compre- hensiveness of the contents since the authors have taken special care to address the requirements of two readerships, one being readers from the control field, and the second being a more general engineering readership. The industrial and academic control community will be interested in the outcome of the linear-quadratic-Gaussian (LQG) and H∞ advanced control trials performed using the experimental rig. This group of readers will also find the technical knowl- edge extracted in terms of models and parameters needed for computer simulation tests before the instrumented control trials of value.
However, to ensure that the industrial and academic control community can fully comprehend the fundamen- tals of compressor technology there are invaluable and detailed presentations on the problem of surge and stall (Chap. 1), rotor dynamics (Chap. 2), magnetic bearings (Chap. 3) and on the experimental rig and its associated instrumentation (Chap.
In addition, to facilitate and ensure a full appreciation of the advanced control developments presented in Chaps. 7 and 8 by a more general readership from the mechanical, manufacturing, mechatronics, rotating machinery and other engineer- ing disciplines, the authors have included an introductory chapter on control systems theory (Chap. Even readers from the control community might find this chapter useful as a “refresher course” before reading the chapters covering the advanced LQG and H∞ control strategies. The original contributions made by the authors in describing the various aspects of the technology, in devising and testing the advanced control strategies and the careful and thorough construction of this monograph make it a very welcome ad- dition to the Advances in Industrial Control series and to the wider literature of compressor technology.
Industrial Control Centre M. Grimble Glasgow, Scotland, UK M. Johnson Preface Compressors are essential machines for a large number of modern manufacturing processes. Like the hearts pumping life to the production lines, compressors are vital to the operation of key industrial sectors, such as the petrochemical and the mining industries, which rely on compressors for critical tasks, ranging from tem- perature control to gas transportation and mixing.
As a result, there have been con- tinual efforts by the academic and industrial communities to improve the reliability and performance of such turbomachinery as new technologies become available. Active magnetic bearing (AMB) is one such enhancing technology that has been gaining strong momentum in recent years. Among other benefits, the low main- tenance requirements and small parasitic energy losses have made these bearings highly desirable for high performance compressors, particularly those designed to operate in harsh or inaccessible environments. Additionally, with their ability to ac- tively change the rotor-dynamic characteristics of the compressor by controlling the bearing parameters in real time, the AMBs can provide a smoother and more reliable operation of the compressor over a wider range of operating conditions.
Stability is a critical factor that limits the performance of compressors. The max- imum mass flow output of a compression system is capped by choke, which is gen- erally not a destabilizing phenomenon, and it is caused by the compressed medium reaching sonic conditions. At the opposite end, the minimum mass flow is limited by the compressor instabilities known as stall and surge. Stall is a localized phe- nomenon that can be observed in some compression systems, and it is sometimes accompanied by a sudden drop in the average compressor output flow.
On the other hand, surge is a system-wide instability that is characterized by large amplitude oscillations in the output pressure and mass flow. These oscillations can cause ex- tensive damage to the compressor casing and internal components due to high vi- brational loads. They can even lead to a catastrophic mechanical failure of the com- pressor if they are not addressed properly. A conservative way of dealing with surge is to avoid it, by operating far away from the instability.
A more efficient way is to implement an active method to stabilize surge and stall, so that the stable operating region of the compression system is extended, resulting in both higher productivity and safer operation. ix x Preface Unfortunately, a majority of current compressors operate conservatively to avoid surge. In other words, many compressors trade the peak performance at the maxi- mum pressure rise for the stability at the higher mass flow rates. The focus in surge avoidance is on guaranteeing the mechanical integrity of the machines and the safety of the work place by keeping a precautionary margin between the operating output flows and the known surge points.
Additionally, a reset mechanism is built in the system that quickly releases the built-up pressure in the compressor if surge is de- tected by the different safety triggers. An active surge controller, on the other hand, stabilizes the compressor flow during the initiation of surge, effectively extending the operational range of the compressor with no loss in performance. The implemen- tation of a control mechanism is much rarer in industrial applications than the surge avoidance strategies for several reasons. The main reason is that the modifications to compressors in the field required for the installation of a surge control mechanism are very often complicated and involve very specialized equipments.
More impor- tantly, there has not been an univocal experimental demonstration of the potential benefits that an effective surge controller could offer to an actual industrial-size compressor. Recently, promising results have been presented in the literature on an active surge control scheme that modulates the impeller position to stabilize the flow in an AMB supported single stage centrifugal compressor. With the AMB acting as a high bandwidth actuator to regulate the displacement of the impeller, the compressor flow states can be restored to the equilibrium operating point during the early stages of the surge instability, when the amplitude of the limit cycle is relatively small. The main advantage of this active surge control scheme is that it can be easily implemented in existing AMB suspended compressors, generally with a simple modification in the control software.
The purpose of this book is to present the fundamentals on the integration of the AMBs for the suspension of the rotor in compressors, and how this relatively new bearing technology can be employed to actively control and potentially eliminate the compressor surge. The material presented here is intended to serve as a com- prehensive reference in the areas of compressor surge control and AMB application in turbomachinery.