(14e) Analysis of a Propylene Refrigeration Compressor Trip with Dynamic Simulation

Process upsets initiated by trip of the Charge Gas Compressor (CGC) are typically causing the Propylene Refrigeration Compressor (PRC) to trip on high drum liquid level. Review of previous incidents reveals that the PRC trip 5 to 10 minutes after the CGC. Liquid level controller tuning philosophy was suspected as a potential cause. The impact of tuning was tested on a dynamic model of the propylene refrigeration system (C3R). The dynamic loss of heat duty caused by the trip of the CGC was used as input to the model. The results suggested that drum level controller tuning alone was not responsible for the PRC trip. The trip was rather caused by the upset on the C3R system heat and material balance resulting in a dead ended flow of propylene, hence leading to accumulation of liquids in the 3rd stage drum. Faster controller tuning would delay but not prevent the trip. This paper describes how Aspen Dynamics has been used to model the transient conditions typically experience on the C3R system during a CGC trip. Model simplifications, dynamic inputs and expected model deviations from actual behaviors are also discussed.