(117d) Simple and Improved Level Control Scheme for Tanks In Series

The present work proposes a novel, simple and effective scheme to control the level in a cascade of tanks. Level control often involves two conflicting issues, rejection of disturbance and the minimization of outlet flow variations. Normally level control is detuned intentionally to reduce flow oscillations downstream. However, due to constraints in level variations, one cannot detune beyond a certain point. The proposed scheme translates the system of cascade tanks from multiple level controls into interacting tanks in series, i.e. multiple SISO to a MIMO system. This transformation substantially increases the time constant of the tank system and therefore provides additional flexibility to reject disturbances and at the same time avoid process constraints. Feedback controls based on classical linear PI controllers are used and simple, generalized charts are prepared to decide optimal tuning parameters based on process constrains. The present control scheme is compared to the existing schemes and the results are presented. The control scheme has been implemented successfully in a large chemical manufacturing plant. Simulated results and actual performance of the industrial plant shows that the performance has improved significantly in comparison to previously installed non-interacting control schemes.