Module 5 - Control Loops

Module from the Instrumentation Engineering Curriculum curriculum.

CONTROL LOOP FUNDAMENTALS · Learning Objectives · 1. Understand PID control action (proportional, integral, derivative) 2. Tune PID controllers using systematic methods 3. Apply cascade control for disturbance rejection 4. Implement feedforward and ratio control strategies 5. Design split-range and override control schemes · Control Strategies — When to Use Each

Strategy	Description	When to Use	Example Application	Advantages	Disadvantages
Simple PID (feedback)	Single loop: measure PV, compare to SP, adjust output	Process with single manipulated variable, moderate dynamics	Level control on a drum, pressure control on a vessel	Simple, robust, handles unmeasured disturbances	Reacts after upset occurs (lag), cannot handle fast disturbances
Cascade	Two nested loops: outer (master) sets SP of inner (slave)	When measurable intermediate variable exists between disturbance and controlled variable	Reactor temperature control: outer=reactor temp, inner=heater outlet temp	Faster disturbance rejection, inner loop handles fast upsets	More complex tuning (tune inner first), requires additional measurement
Feedforward	Measure disturbance directly, adjust output before PV is affected	When disturbance is measurable and model is known	Feed rate change → adjust heat input in proportion	Proactive correction — no error needed before action	Requires accurate process model, does not correct for unmeasured disturbances — combine with feedback
Ratio	Maintain fixed ratio between two flows	Two related streams must maintain proportion	H2/HC ratio control, air/fuel ratio, chemical addition	Simple, effective for proportioning	Cannot handle ratio changes without operator intervention
Split-Range	One controller output drives two (or more) valves over different ranges	When control requires opposing actions or different capacity ranges	Reactor pressure: 0-50% = vent valve, 50-100% = makeup valve	Extends control range, smooth transition between valves	Gap/overlap at split point can cause cycling, careful calibration needed
Override / Selector	Multiple controllers, selector chooses one output (high or low wins)	Safety constraint must override normal control	Column reboiler: normal = temperature control, override = high level cuts steam	Protects against safety limits while allowing normal operation	Windup on non-selected controller, bumpless transfer design needed
Model Predictive (MPC/APC)	Multi-variable optimization using dynamic process model	When multiple interacting variables need simultaneous optimization	Crude unit optimization (multiple draws, constraints, product specs)	Handles interactions, constraints, optimizes economics	Expensive, requires detailed model, maintenance of model accuracy
PID Tuning — Quick Reference
Loop Type	P Band (%)	Integral (min/rep)	Derivative (min)	Response	Typical Process
Flow	100-200	0.1-0.5	0	Fast, tight	Any flow loop
Level (averaging)	50-200	5-20	0	Slow, loose	Surge drum, accumulator
Level (tight)	20-50	1-5	0	Moderate	Distillation column bottoms, separators
Pressure (gas)	20-100	0.5-5	0	Moderate	Vessel pressure, header pressure
Temperature	20-100	2-20	0.5-5	Slow	Reactor, reboiler, heater outlet
Composition / pH	50-200	5-30	1-10	Very slow	Analyzer-based loops
Source: FOS Chief Files — Auto Controllers.PDF, Controller Tuning folder, MRPL Controller Equations, IPE-EP-12-1-1 (Control Systems)

Source: Instrumentation_Engineering_Curriculum_v1.xlsx · Sheet: Module 5 - Control Loops

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