(37a) Design of Refinery Steam Network Process Control Philosophy by Using Dynamic Simulation

A dynamic simulation of a refinery steam system provided a high degree of confidence that the process control system design will handle all major plant upsets with minimal impact on operations and lesser operator intervention. Refinery steam system is highly integrated network of multi pressure level steam distribution headers, generators and consumers. Instabilities in operating steam systems can lead to major plant upsets resulting in loss of revenue. The steam system is difficult to design for stability when there are multiple steam generators and users with many differing requirements. In order to ensure smooth operation, it is essential to conduct transient analysis of the system during various operational upsets with current control design in place.

Recently, Bechtel engineers completed a comprehensive dynamic simulation of the steam utility system for a major refinery upgrader project. The steam system for this project included two steam generators (utility boiler and hydrogen plant), power and steam distribution network comprising four pressure levels of steam with steam flows to and from process units. During transient situation the steam producers and consumers ramp up or down in a non-linear fashion. This adds complexity to the dynamics and makes it virtually impossible to assess the process conditions without conducting a detail dynamic analysis. Whether the steam system and its controls would respond effectively to major operational upsets, such as power failure, hydrogen plant trip, steam turbo-generator trip and process gas compressor trips can only be determined from evaluating dynamic responses to these incidents.

The study involved developing a high-fidelity dynamic simulation model of the steam system for evaluating steam system responses to various operational upset scenarios. The responses were used to identify limitations of the "as designed" system if any. The model also proved to be an effective tool for verifying adequacy of the proposed controller set point across let down valves.