Dynamic Simulation as a Tool for Optimizing Pressure Control Valve Performance

Abstract

Control valves installed for pump station pressure control are typically tuned and commissioned at the low end of the flow range and well below the safe operating limits in order to avoid pressure excursions and line shutdowns during commissioning. Tuning parameters selected for best performance at low flowrates often produce poor performance at high flowrates requiring dampened tuning parameters and slower valve actuator speeds. This results in sluggish responses to pressure changes. Enbridge has undertaken a case study to examine three control valves which exhibited poor control characteristics. The goal of this study was to produce an optimal tuning strategy that could be implemented with a high degree of confidence over the entire range of operating conditions. To accomplish this, the IDEAS (AMEC Technologies, Inc.) dynamic simulation software package was utilized. The pipeline was modeled from the pump station upstream of the station of interest to the downstream pump station. The model consists of pipeline sections, pumps, control valves and other process elements that are hydraulically linked. Station discharge and suction pressures are controlled via PI controllers with adjustable set points, ramp rates and tuning constants. Valve full stroke actuator speed can also be varied. Information required to develop the simulation model included station elevations, pipeline lengths, pump curves and control valve Cv curves. The three simulation models developed for this study have been calibrated against process data by adjusting piping resistances. The inherent nonlinearities present in the control valve system were quantified through use of the simulation model. Various strategies to alleviate the adverse effects of these nonlinearities have been studied. Use of a simulation tool also resulted in increasing the awareness of trade-offs present in design and tuning of control valve systems.