An Empirical Solution To An Anti-Surge Control Problem.

Abstract

Steps taken by a du Pont team to prevent surge in a 23,000 hp process air compressor are reviewed. The process has several reactors which must interlock down quickly for safety reasons. These interlocks can occur on one, several, or all reactors, simultaneously. During large upsets, the original control scheme could not prevent surge, which then caused shutdown of the entire system. Several constraints complicated the solution. To protect the axial compressor casing from excessive blade stresses, the compressor has to be interlocked on surge. The control margin between the surge control and actual surge line has to be small to conserve energy and improve turndown. A second control margin was required to feed any excess air to be fed to an energy recovery expander. The improved control scheme uses a dual-gain controller which shifts to high gain during upsets. High speed recording equipment was used to track process variables, control signals and actual valve position. This information was then used along with computer simulations to tune controllers and control valve boosters.