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The cost of machinery damage and process downtime due to compressor surge and overspeed can be from thousands to millions of dollars for large continuous chemical or petrochemical plants. This text demonstrates how to select the proper control schemes and instrumentation for centrifugal and axial compressor throughput and surge control. 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 fast-acting instrumentation are needed to prevent surge due to the unusual nature of this phenomenon. In order to appreciate the special instrument requirements, the distinctive characteristics of centrifugal and axial compressors and the surge phenomenon are described. This text focuses on the recent advancements in the description of surge by E.M. Greitzer (Ref. 15). Simple electrical analogies are used to reinforce the explanation.
Simulation program plots using the Greitzer model of surge are used to graphically illustrate the oscillations of pressure and flow that accompany different degrees of severity of surge. Extensive mathematical analysis is avoided. A few simple algebraic equations are presented to help quantify results, but the understanding of such equations is not essential to the selection of the proper control schemes and instrumentation. The surge feedback control scheme is built around the type of controller set point used.
In order to appreciate the advantages of various set points, the relationship of the location of the set point relative to the surge curve and the effect of operating conditions on the shape and location of the surge curve are described. The need for and the design of an open-loop backup scheme in addition to the feedback control scheme are emphasized. The use and integration of process and manual override control schemes without jeopardizing surge protection are also illustrated.
Many of the transmitters, digital controllers, and control valves in use at this writing are not fast enough to prevent surge. This text graphically illustrates the effect of transmitter speed of response on surge detection. The effects of transmitter speed of response, digital controller sample time, and control valve stroking time on the ability of the control scheme to prevent surge are qualitatively and quantitatively described.
The modifications of control valve accessories necessary for fast throttling and the maintenance requirements are detailed. The interaction between throughput and surge control and multiple compressors in parallel or series can be severe enough to render the surge control scheme ineffective or even to drive a compressor into surge. This text describes the detuning and decoupling methods used to reduce interaction. The computational flexibility and power of modern computers facilitates on-line monitoring of changes in compressor performance and its surge curve.
This text describes how computers can be used to predict impending compressor damage and the extent of existing damage by vibration frequency analysis. It also describes how computers can be used to gather pressure and flow measurement data to update the surge curve on a CRT screen. In order for the reader to understand the physical nature of surge, it is desirable that he or she be familiar with some of the elementary principles of gas flow.
The reader should know that a pressure difference is the driving force for gas flow, that gas flow increases with the square root of the pressure drop until critical flow is reached, and that gas pressure in a volume will increase if the mass flow into the volume exceeds the mass flow out of the volume and vice versa. If the reader is also comfortable working with algebraic equations and understands such terms as molecular weight, specific heat, efficiency, and the speed of sound, he or she can use the equations presented to describe the surge oscillations and the surge curve.