Compressor Surge Modeling and Control
Axial and centrifugal compressor control is exceptionally challenging due to the extraordinary speed and severity of problems and the extreme consequences in terms of plant safety and performance. The fastest and most dangerous phenomenon is surge. An axial or centrifugal compressor in surge can reverse flow in 0.03 seconds going from a large positive flow to a large negative flow. Often, the negative flow is not measurable by flow meters leaving the actual situation to your imagination.
A high fidelity dynamic real time model has been developed and integrated into a digital twin to enable a greater understanding of the special requirements in terms of automation system speed of response and special algorithms. The model uses a momentum balance thoroughly researched and verified. The normally unseen compressor characteristic curve to the left of the surge point is used enabling the model to show the jumps in flow on the positive slopes of the characteristic curve. The unique shape and speed of surge flow oscillations is used in the digital twin to explore, develop, prototype, test, justify, deploy, commission and maintain the best surge control system. The digital twin also enables not only training of operators but education of process and automation engineers on the extreme dynamics and control system opportunities.
A simple but powerful technique is developed for detecting a potential surge and updating the surge curve. Control strategies are detailed that proactively prevent surge while maximizing compressor efficiency. Unnecessary surge valve openings are avoided and the surge controller setpoint is optimized to lie on the longitudinal axis of efficiency ellipses. Feed-forward is used to deal with sudden disturbances from changes in production rates and shutdown of downstream users. Additionally, a method of optimizing the compressor discharge pressure based on user downstream valve positions is developed. All of these control system opportunities can be demonstrated with the actual control system connected to the model.
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