PI control (proportional + integral) in process control When integral (I) action is added to proportional (P) control in a feedback loop, which key performance outcome is enabled under steady disturbances and setpoint changes?

Introduction / Context:
In classical feedback control for process industries, proportional (P) control provides immediate corrective action, but it typically leaves a steady-state error (known as offset) in the presence of constant disturbances or load changes. Adding integral (I) action creates PI control, the workhorse of refinery, chemical, and utility loops. This question asks what benefit I-control brings when used alongside P-control.

Given Data / Assumptions:

• Loop is linearized around an operating point.
• Disturbance is constant (step-like) or setpoint is a step.
• Controller is ideal PI with finite tuning and a stable plant.

Concept / Approach:
Integral action accumulates error over time and continues to adjust the controller output until the error becomes zero. In steady state under a constant load, the only way integral of error can stop growing is if the error itself is driven to zero. Therefore, PI control eliminates offset for constant disturbances in stable, properly tuned loops. P-only cannot do this unless process static gain is infinite (which it is not in real systems).

Step-by-Step Solution:

Verification / Alternative check:
Final value theorem applied to closed-loop transfer with an integrator in the controller shows steady error term goes to zero for step inputs/disturbances.

Why Other Options Are Wrong:

• Reduction of offset: Under proper tuning, the target is elimination, not mere reduction.
• Reduction of stability time: Integral action often slows response if not tuned; it is not the defining benefit.
• None of these: Incorrect because PI's hallmark is offset elimination.

Common Pitfalls:
Assuming integral always speeds up response; it primarily removes steady error but can lengthen settling if overused.

Final Answer:
Elimination of offset