Control System Applications
Core concepts
Application loops differ in dynamics. Flow loops are fast and noisy, often using square-root extraction and fast tuning. Level is integrating and forgiving, so averaging level control can absorb surges. Pressure loops are fast. Temperature loops are slow, with dead time and lag, so derivative must be used carefully. Advanced applications include cascade, feedforward, ratio, split-range, and override control. Know boiler three-element drum level, combustion cross-limiting, distillation column control, reactor temperature control, compressor surge control, pH neutralization, and batch sequencing. Safety interlocks and permissives must remain independent of normal regulatory control.
Traps
Tuning a level loop too tightly can transmit disturbances downstream; averaging level control is often intentional. Flow loops should not be tuned like temperature loops. pH is highly nonlinear near neutrality, so a single PID setting may not work across the curve. Compressor surge protection uses recycle or blowoff and must act quickly. In three-element drum level, steam flow is the feedforward signal, drum level is feedback, and feedwater flow is the inner loop.
Worked example
Three-element drum level control sets feedwater demand from steam flow plus a level-controller correction, then uses feedwater flow as a fast inner loop. If steam flow rises, feedwater demand rises immediately before drum level falls; the level controller trims the long-term inventory.