Level Control by Regulating Control Valve at the Bottom of a Gravity- Drained Tank

Larry K. Jang


In this work, the author explores the feedback control of liquid level in an open, gravitydrained tank with a linear, direct-acting control valve located at the bottom of the tank. The open-loop dynamic model of liquid level (h), subject to changes in incoming feed rate (qin , disturbance or load) and control valve stem position (x, manipulated variable), is derived. One crucial step in deriving the model is the linearization of the discharge flow rate from the control valve, q = k x (h)1/2 , with k as a constant. Transfer functions for a closed-loop, feedback PIcontrol system are derived for the setpoint-tracking case and also for the disturbance-rejection case. Simulation for the disturbance-rejection case is performed for this hypothetical unit using the internal model control (IMC) tuning rule.

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