Getting the system control loop right. Feedback loops alone might not suffice to control the steam system through a wide range of upsets. Scenarios where steam supply exceeds demand can be handled by disposing of excess steam for a short while until feedback loops bring the system under control. However, when there's a sudden shortage of steam, the feedback control actions might be too late. Appropriate feedforward control actions must be initiated before the shortage affects operation of the facility. Understanding the extremely non-linear characteristics of a steam system by simulation is critical for successful design of these feedforward controls.

Setting correct priorities in steam shedding. Situations where there's likely to be a severe shortage of steam require an emergency steam-shedding plan to avoid a full-scale shutdown. But which units should be shed and in which order? Such situations require a carefully designed strategy that prioritizes shedding of steam users based on the impact on the overall operation. As with designing feedforward controls, a simulation model can be used to evaluate different steam-shedding strategies in a cost effective and safe manner, thereby ensuring the best possible emergency shedding strategy is determined and deployed in the master controller.

---

IAN WILLETTS is a director of Invensys Operations Management, Carlsbad, Calif. ABHILASH NAIR is a principal consultant for Invensys Operations Management in Carlsbad. CHARLES REWOLDT is an application engineer for Invensys Operations Management in Carlsbad. E-mail them at Ian.Willetts@invensys.com, Abhilash.Nair@invensys.com and Charles.Rewoldt@invensys.com.

Reference
1. Gandhi, S.L., Graham, J., Duffield, M.A., and Cortes, R.M., "Dynamic Simulation Analyzes Expanded Refinery Steam System," p. 3, Hydrocarbon Processing (Nov. 1995).