The Olefins Unit that is the subject of this case study has been operating for 25 years. Recently, the unit initiated a control system retrofit program to increase availability, save energy, and improve access to operating information. New control systems were installed on the ethylene, propylene and furnace gas compressor trains. The gas compression equipment consists of single and multi-case compressor trains driven by steam and gas turbines and electric motors.
Significant control system improvements were made to the turbine/compressor speed/load regulation and temperature limiting. The systems also execute “safety system” logic for process and machinery protection. In addition to integrated turbine compressor control, the strategy that incorporates all control functions for the entire compresor train, a new anti-surge algorithm is included. To meet more stringent availability requirements imposed by integration, a triple modular redundant (TMR) system structure is employed. The surge algorithm described in this paper accurately predicts where surge occurs, regardless of changes in gas composition and conditions (e.g., molecular weight, temperature, etc.), and allows the integrated controls to compensate accordingly. A PC workstation-based operator display is provided for enhanced operator awareness of machinery conditions. This paper describes the requirements and implementation of this new approach to control.