NONLINEAR CONTROL OF LIQUID LEVEL

In a process in which liquid flows to a vessel and is then withdrawn from the vessel, method and apparatus is provided for implementing nonlinear liquid level control by providing a variable set point to a conventional level controller. The variable set point is a nonlinear function of the actual liquid level in the vessel. The variable set point enables the vessel to absorb disturbances in the flow rate of the liquid flowing to the vessel while the actual liquid level in the vessel remains between a desired high and low limit thus providing a smooth flow of liquid from the vessel.