Centrifugal compressors are critical for moving large volumes of gas in the natural gas pipeline, hydrocarbon processing, and general energy industries. The operation of centrifugal compressors is limited by a number of factors including the occurrence of surge at low flows. The exact low flow conditions at which surge occurs cannot be determined from external measurements. The precise flow and head at which surge occurs is effected by factors including the installed piping configuration, the dynamic impedance of the piping system, the pulsating pressures in the piping, and the gas properties such as compressibility among other variables. The results of the many factors affected when surge occurs and the sudden nature of surge onset are such that there are no previously identified reliable ways to detect an approaching surge. Current surge control methods rely on external measurements of head or speed and flow to estimate the conditions at which surge will occur and then recycle flow at some margin above the expected surge conditions in order to avoid surge. However, this type of surge control is inefficient and frequently leads to recycling more flow than necessary with the result that fuel and energy are wasted. If a means were available to measure a fundamental pre-cursor to surge then a reliable indication of the operating margin above surge could be developed and the amount of recycle flow and the loss of efficiency could be minimized. Such a pre-cursor has been found and a sensor technique base on drag probe technology has been developed. This paper presents some of the background on pre-surge detection in centrifugal compressors and then describes the known behavior of the impeller inlet outer wall re-circulating flow that develops as surge approaches. In addition, this paper reports on the development of a drag probe sensor to measure these internal flow components. Evidence from direct surge control testing is presented to support the finding that these internal flows are a basic surge pre-cursor and a useful control for centrifugal compressors. The measured flow changes prior to surge are identified and data is shown. The result of this research will lead to an improved surge control system for a significant class of signal stage modern centrifugal compressors and will increase the operating range and overall efficiency of such machines.

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