Process automation: unit operations, control and instrumentation

Learning outcome no. 12 requires that the apprentice, on the basis of basic knowledge of process industrial unit operations, control technology and instrumentation, can build, commission, maintain, repair and optimize automated industrial equipment. It is a subject area that differs somewhat from more piecemeal machine automation: it is about keeping a coherent process — such as heating, mixing, distillation or cleaning — stable over time.

§What is a unit operation?

A unit operation is a fundamental discrete step in a production process — e.g. heating cooling mixing separation or transport of liquid. An entire process plant regardless of whether it produces food medicine or energy is typically built up of a series of these unit operations linked together. The automation technician's task is to understand what each unit operation requires in terms of control and how they work together so the entire process runs smoothly without bottlenecks or instability.

§Regulation: from feedback to PID

An earlier article on the history of automation introduced feedback as the field's core idea: measure, compare, regulate. On a process system the most common version of this principle is PID control, where the control combines three contributions — the proportional (P) which responds to the deviation itself; the integrating (I) which over time removes a persistent small deviation; and the differentiating (D) which responds to how quickly the deviation changes. A well-tuned PID controller keeps for example a temperature stable even when the load on the process changes along the way.

• 01P (proportional) — responds proportionally to the deviation itself
• 02I (integral) — removes a persistent small deviation over time
• 03D (differential) — responds to the rate at which the deviation changes
• 04Together: PID control that keeps a process stable under changing loads

§Instrumentation – the process's measurement points

To be able to regulate something you must first be able to measure it reliably. Instrumentation covers the transmitters and meters that convert a physical quantity — pressure, level, flow, temperature — into a signal the control can use. The quality of the instrumentation sets a limit for how well the process can be controlled at all: an inaccurate or drifting sensor gives inaccurate control no matter how good the PID tuning is.

§From food to district heating

Process automation is found widely in Danish industry: in food production, where hygiene and precise temperature control are critical, in the pharmaceutical industry, where documentation and traceability requirements are high, and in the energy and supply sector, where stable operation of, for example, district heating plants is societally critical. Common to them all is that the automation technician must be able to both build, troubleshoot and optimize the same basic control technique — just adapted to the specific process.