Plant Engineering Infobase · Chapter 04 of 14
Process control engineering
Process control loops (formerly I&C loops) are shown in the P&ID according to DIN EN 62424:2017-12. It replaces the withdrawn DIN 19227-1. The US equivalent is ANSI/ISA-5.1-2024.
Open-loop and closed-loop control
- Open-loop control
- Input variables influence a process variable whose actual value is not fed back. Disturbances are therefore neither detected nor compensated: open control loop.
- Closed-loop control
- The controlled variable x is measured continuously and compared with the reference variable w. From the control deviation e = w − x, a manipulated variable is formed that brings x into line with w: closed control loop. Cyclic digital sampling control also counts as continuous.
- Process
- The totality of interacting operations in which matter, energy or information is converted, transported or stored, e.g. power generation in a power plant or pig iron production in a blast furnace.
The terms are now defined in DIN IEC 60050-351 (International Electrotechnical Vocabulary, control technology), which replaces the former DIN 19226. In English, control covers both; only closed-loop control for "Regelung" and open-loop control for "Steuerung" are unambiguous.
Summary based on Wikipedia: Control engineering
Structure of a PCE loop
The process control loop is shown as an oval. The letter codes are at the top, the loop number at the bottom. A horizontal line means operation and indication in the control room; without a line, it means local.
Identification letters according to DIN EN 62424
| Letter | First letter (PCE category) | Succeeding letter (processing function) |
|---|---|---|
| A | Analysis | Alarm, message |
| B | Optical measurement (e.g. flame monitoring) | Limitation |
| C | – | Closed-loop control |
| D | Density | Difference |
| E | Voltage | – |
| F | Flow | Ratio |
| G | Distance, length, position | – |
| H | Manual input, manual intervention | Upper limit, on, open |
| I | Electric current | Analog indication |
| J | Electrical power | – |
| K | Time-based function | Rate of change over time |
| L | Level | Lower limit, off, closed |
| M | Moisture | – |
| N | Electrical actuation (motor, heating) | – |
| O | – | Status indication of binary signals (local or DCS) |
| P | Pressure | Test point connection |
| Q | Quantity, count, material properties, quality | Integral, quantity or total |
| R | Radiation variables | Recording |
| S | Speed, frequency, rotational speed | Binary control or switching function (not safety-related) |
| T | Temperature | – |
| U | – | – |
| V | Vibration, mechanical analysis, torque | – |
| W | Weight, mass, force | – |
| X | free for meanings not listed | free for meanings not listed |
| Y | Hydraulic or pneumatic actuation | Computing function |
| Z | – | Binary control or switching function (safety-related) |
Differences from the old DIN 19227-1: There, V stood for viscosity, W for weight force/mass and E for electrical variables in general; A (analysis) and I (current) were not assigned as first letters, and Z meant "emergency intervention, protective device". Loops were drawn as circles instead of ovals. Therefore, do not adopt as-built documentation according to DIN 19227 without checking it.
Overall concept · I&C / process control engineering
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Standards status for this page 4 standards · researched September 2026
| Standard | Contents | Current edition | Replaced / successor |
|---|---|---|---|
| DIN EN 62424 (VDE 0810-24) | Representation of process control engineering requests in P&I diagrams and data exchange between P&ID tools | DIN EN 62424 (VDE 0810-24):2017-12 | replaces DIN EN 62424 (VDE 0810-24):2010-01 |
| DIN IEC 60050-351 | International Electrotechnical Vocabulary – Part 351: Control technology | DIN IEC 60050-351:2014-09 | replaces DIN IEC 60050-351:2009-06 |
| DIN 19226 | Control technology – Closed-loop and open-loop control engineering; terms | withdrawn | now: DIN IEC 60050-351 (replacement 2009-06; current 2014-09) |
| DIN 19227-1 | Control technology – Graphical symbols and identifying letters for process control engineering | withdrawn | now: DIN 28000-4:2014-07 and DIN EN 62424 (VDE 0810-24), currently 2017-12 |
Researched at DIN Media, ISO, IEC and ASME, as of September 2026. Before binding use, please check the current edition with the standards publisher. Overview of all standards: Plant Engineering Infobase · Standards status.