Process Control

Process control regulates continuous variables — temperature, pressure, flow, level, composition — in chemicals, refining, food, pharma, pulp/paper, power, water/wastewater, and oil & gas.

Overview

The control engineer’s job is to hold variables at setpoint despite disturbances, while respecting equipment limits, safety, economics, and operator workload.

Process Types

• Continuous — steady flow (refining, papermaking).
• Batch — recipe-driven, time- and event-stepped (pharma, specialty chemicals).
• Discrete — count-based parts (handled by PLC/motion).
• Hybrid — semi-continuous, e.g. food/beverage.

Control Strategies

• Feedback (PID) — the workhorse.
• Feedforward — pre-act on measured disturbance.
• Cascade — outer loop sets the SP of an inner loop (faster process).
• Ratio — keep one variable proportional to another.
• Split-range — one controller driving two valves over different ranges.
• Override / select — high/low signal selector for constraint control.
• MPC — model predictive control for multivariable, constrained systems.

Process Dynamics

• First-order plus dead time (FOPDT): K, τ, θ.
• Integrating processes — level, gas pressure.
• Inverse response, runaway, oscillatory dynamics.
• Identify with step / doublet / PRBS tests.
• Tune with Lambda (IMC), Ziegler-Nichols, Cohen-Coon, or auto-tune.

DCS vs PLC

• DCS — distributed I/O on plant network; integrated HMI, engineering, history; tuned for analog/regulatory control.
• PLC + SCADA — discrete-strong, often cheaper; bolt on HMI/historian.
• Modern lines blur: most PLCs handle PID well; many DCSs handle sequencing.
• DCS vendors: Emerson DeltaV, Honeywell Experion, Yokogawa CENTUM, ABB 800xA, Siemens PCS 7 / PCS neo, Rockwell PlantPAx.

Standards

• ISA-5.1 — instrumentation symbols (P&ID).
• ISA-88 — batch control.
• ISA-95 — enterprise-control integration (MES).
• ISA-101 — HMI design.
• IEC 61511 / ISA-84 — safety instrumented systems.