Improve Control of Liquid Level Loops

54 www.aiche.org/cep June 2008 CEP Because most processes are self-regulating, it can sometimes be challenging to tune a controller for an integrating process. The principal characteristic of a self-regulating process is that it naturally seeks a steadystate operating level if the controller output and disturbance variables are held constant for a sufficient period of time. For example, a car’s cruise control is self-regulating. By holding the fuel flow to the engine constant (assuming the car is traveling on flat ground on a windless day), the car is maintained at a constant speed. If the fuel flowrate is increased by a fixed amount, the car will accelerate and then settle at a different constant speed. The temperature of a process stream exiting a heat exchanger is also self-regulating. If the shellside cooling fluid flowrate is held constant and there are no significant external disruptions, the tubeside exit stream temperature will settle at a constant value. If the cooling flowrate is increased, allowed to settle, and then returned it to its original value, the tubeside exit stream temperature will move to a new operating level during the increased flowrate and then return to its original steady-state. Tanks that have a regulated exit flow stream do not naturally settle at a steady-state operating level. This is a common example of what process control practitioners refer to as a non-self-regulating (or integrating) process. Integrating processes can be remarkably challenging to control. This article explores their distinctive behaviors. Armed with this knowledge, you may come to realize that some of your facility’s more-difficult-to-control level, temperature, pressure and other loops have such character.