ABSTRACT
Two types of constraints can be distinguished: hard constraints and soft constraints. Hard constraints are constraints that should not be violated because of operational reasons or cannot be violated because of physical reasons. Soft constraints are approachable from both sides of the operating region in contrast with the compressor surge constraint, which is always approached from one side of the constraint. Process operation is optimal near a constraint. In that case there is the desire to maintain the process near the constraint despite process upsets. There may be situations where multiple constraints are defined and multiple variables can be manipulated to drive the plant toward the constraints. If the controller would continue to output once the valve has reached a limit, the controller would saturate or wind up. The integral valve position controller moves the pressure set point to its optimal long-term value.
