Method and apparatus for antisurge control of turbocompressors having complex and changing surge limit lines

This invention relates generally to a method and apparatus for antisurge control of turbocompressors having complex and changing surge limit lines. More specifically, it relates to a method for using a function of multiple variables to describe (with high accuracy) a surge limit line under the influence of varying process conditions. Compensating for the complex changing shape and location of a turbocompressor's surge limit line can be difficult and imprecise when using antisurge controllers that do not incorporate sufficient capability. Based upon various operating conditions, surge limit line changes can be attributed to a number of process variables. These effects are particularly relevant to multistage centrifgal and axial turbocompressors operating with variable rotational speed and equipped with adjustable inlet or diffuser guide vanes, or both. This invention relates to an innovative method of antisurge control in which the function of those process variables (comprising the turbocompressor operating condition parameters) is used to calculate distances to the surge reference line. The function is formed as a superposition of the functions of lesser numbers of variables. For instance, these functions are formed as polynomials where the power of each polynomial function is determined by real characteristics of correlation between the shape and location of the surge limit line and the variables to which this polynomial function is being formed.