The cost of machine damage and process downtime due to compressor surges and over speed can range from thousands to millions of dollars for large continuous chemical and petrochemical plants. This text shows how to select the proper control scheme and instrumentation for through put and surge control of centrifugal and axial flow compressors. More material is devoted to surge control because surge control is more difficult and the consequences of poor control are more severe.
Special feedback and open-loop backup control schemes and immediate instrumentation are needed to prevent surges due to the unusual nature of this phenomenon. To understand the special instrumentation requirements, the unique characteristics of centrifugal and axial compressors and the surge phenomenon will be discussed. The text focuses on recent advances in the description of surges by E.M. Greitzer.
Simple electrical analogies are used to augment the description, and plots of simulation programs using Greitzer’s surgemodel graphically illustrate pressure and flow oscillations associated with the degree of surge. Extensive mathematical analysis is avoided. Some simple algebraic equations are presented to help quantify the results, but understanding such equations is not essential to selecting the appropriate control scheme and instrumentation.
Surge feedback control schemes are built around the type of controller set point used. To understand the benefits of different set points, the relationship of the set point location to the surge curve and the effect of operating conditions on the shape and location of the surge curve will be discussed. In addition to feedback control schemes, the need for and design of open-loop back up schemes is emphasized. The use and integration of process and manual override control schemes without jeopardizing surge protection is also discussed.
About the Book
This text is intended primarily for instrumentation and process control engineers who design or maintain compressor control systems. Process engineers, mechanical engineers, start-up engineers, and sales engineers will also benefit from reading this book to gain a better understanding of the unusual problem of compressor surges and the special instrumentation needs associated with them.
Since instrumentation maintenance groups are sincerely concerned about the proliferation of different types and models of instrumentation, it is critical that project team members be familiar enough with surge control to justify the use of special instrumentation.