ISO 10628 is the international standard that governs the classification, content and layout of flow diagrams, block diagrams, process flow diagrams and piping and instrumentation diagrams, for the chemical and petrochemical industry. It has two parts: ISO 10628-1 specifies how a diagram is classified and drafted, and ISO 10628-2 supplies the graphical symbols for the process equipment shown on it. A companion standard, ISO 14617, supplies a wider symbol library used across many kinds of technical diagrams; two of its parts, covering measurement and control devices and measurement and control functions, are the ones a P&ID actually draws on for instrument symbols. Neither ISO standard replaces ISA 5.1: they govern diagram content and symbol shape, not the letters used to identify a specific instrument tag.
Key takeaways
- ISO 10628 governs diagram classification, content and layout (Part 1) and process equipment symbols (Part 2) for chemical and petrochemical flow diagrams, including P&IDs.
- ISO 14617 is a separate, broader symbol library; Parts 5 and 6 cover measurement-and-control devices and functions, the instrument symbols an ISO-drawn P&ID uses.
- Neither ISO standard defines a tag identification scheme. The letters and numbers inside the bubble come from a separate convention, chosen per project.
- ISO 10628 replaced the older German DIN 28004 flow-diagram standard once it was adopted as a European norm, DIN EN ISO 10628.
- DIN 19227 and IEC 62424 sit alongside ISO 10628/14617 on many European projects, handling function-letter identification and PCE data representation and exchange respectively, not diagram layout or symbol shape.
- On an ISO-drawn set, check the legend sheet for the symbol convention in use and the coding standard named in the title block for tag identification. Never assume ISA habits carry over unchecked.
What ISO 10628 actually governs
ISO 10628-1 classifies flow diagrams into the families a chemical or petrochemical project actually issues, block diagrams, process flow diagrams and piping and instrumentation diagrams, and specifies the drafting rules each family follows: how a diagram is laid out, how flow direction and connections are drawn, and what content each diagram type is expected to carry. It is explicitly scoped to the chemical and petrochemical industry and does not apply to electrical engineering diagrams, which sit under a different family of IEC standards.
ISO 10628-2 supplies the graphical symbols for the process equipment those diagrams show, vessels, columns, pumps, heat exchangers and the like. Together, the two parts answer "what does a compliant flow diagram or P&ID look like and what goes on it", for the equipment side of the drawing.
What ISO 14617 actually governs
ISO 14617 is a separate, multi-part library of graphical symbols for technical diagrams generally, developed in cooperation with the International Electrotechnical Commission and sharing some heritage with the IEC's own diagram-symbol standards. It is not specific to process diagrams; its parts span symbols for mechanical, fluid-power, and other technical drawing disciplines besides the process industry.
Two parts matter for a P&ID reader. Part 5 covers measurement and control devices, the physical instrument symbols. Part 6 covers measurement and control functions, symbols that represent a control function in the abstract, and which in simple applications can also stand in for the device that performs it. A P&ID drawn under ISO 10628's layout rules typically draws its instrument bubbles from ISO 14617 Parts 5 and 6, the same way an ISA 5.1 drawing draws its instrument circles from ISA 5.1's own symbol table.
Where the ISO family fits with ISA 5.1
| Standard | What it governs | Scope | Region / where it is cited | What it does not cover |
|---|---|---|---|---|
| ISO 10628 (Parts 1 and 2) | Classification, content and layout of flow diagrams, including P&IDs, plus process equipment symbols | Diagram structure and equipment symbols | International; the base diagram standard cited on many European and licensor-driven P&ID sets | Electrical engineering diagrams; the instrument symbol set; tag identification letters |
| ISO 14617 (multi-part) | A general graphical symbol library for technical diagrams; Parts 5 and 6 supply measurement-and-control device and function symbols | Symbol shapes and their registration | International; used alongside ISO 10628 wherever an instrument symbol set is needed | Diagram layout rules (ISO 10628's territory); tag identification |
| ISA 5.1 | Instrument identification (the letter-and-loop-number tag) and instrument symbols, combined in one standard | Symbol and identification together | North America, and most global oil and gas or EPC work outside Europe | Sheet layout and drafting rules for the wider flow-diagram family |
Where DIN 28004, DIN 19227 and IEC 62424 fit in
Older German-engineered plants may still reference DIN 28004, the national predecessor standard for chemical-industry flow diagrams. It was withdrawn once the international standard was adopted as a European norm, DIN EN ISO 10628, so current European work cites ISO 10628 even where the as-built documentation on a legacy plant still carries a DIN 28004 number.
DIN 19227 is a separate German standard covering control-technology graphical symbols and identifying letters for process control engineering. It defines its own function-letter grammar for naming instruments and control functions, in the same spirit as ISA 5.1's letter system but not identical to it, and remains common on German-engineered plants and their control systems. Because ISO 10628 and ISO 14617 do not define a tag grammar of their own, DIN 19227 is one of the conventions a project may cite for that job.
IEC 62424 governs how process control engineering requests are represented in a P&ID and how that data is exchanged between a P&ID tool and a separate process-control-engineering CAE tool, using a neutral interchange format. It is a representation and data-exchange standard, distinct from ISO 10628's diagram-layout rules and from IEC 81346's reference-designation structure. A European project can cite all four, ISO 10628, ISO 14617, DIN 19227 or IEC 81346, and IEC 62424, at once, each governing a different layer of the same drawing.
What an ISA-trained reader should check on an ISO-drawn set
An ISA-trained reader opening a P&ID drawn to ISO conventions will recognise the drawing's purpose immediately, a process flow with instrument bubbles and line connections, but several habits do not carry over automatically.
| Element | ISO convention | ISA 5.1 habit | Where the answer lives on the sheet |
|---|---|---|---|
| Instrument symbol shapes | Drawn from ISO 14617 Parts 5 and 6, which register their own symbols rather than reproducing the ISA 5.1 table | ISA 5.1's symbol table, read by habit | The drawing's own symbol legend |
| Measured-variable / function representation | A Part 6 function symbol, which can represent a control function independent of the device implementing it | ISA letters read as a single instrument identity | Cross-checked against the legend and the instrument index, not assumed |
| Tag identification lettering | May follow IEC 81346 reference designations, DIN 19227-style function letters, KKS, or an operator's own convention; ISO 10628/14617 mandate none of these | ISA 5.1's letter-and-loop-number tag | The legend sheet, or the coding standard referenced in the title block |
| Line and connection conventions | ISO 10628-1's own line-type and flow-direction rules | Paired with ISA/ANSI drafting practice, which differs in places | The general notes or the drafting-standard reference in the title block |
| Equipment symbols | ISO 10628-2 symbol set for vessels, pumps and columns | Often company standard, drawn close to but not identical with ISO or ISA conventions in North America | The symbol legend and the equipment list |
Building an instrument index from an ISO-conventions P&ID
The practical consequence of all of the above: the drawing tells you what a bubble measures and what it does, but it does not tell you, by ISO 10628 or ISO 14617 alone, what identification scheme its tag letters follow. That decision was made once, at the front of the project, and it is recorded on the legend sheet or in the project coding standard referenced from the title block, never assumed from habit.
A tag built to IEC 81346 reference designations reads differently from one built to DIN 19227 function letters, KKS, or an operator's own convention, even though all four can sit on a drawing laid out to identical ISO 10628 rules and drawn with identical ISO 14617 symbols. The safe habit on any ISO-drawn set is the same one that works everywhere else: read the symbol legend first, confirm the identification scheme in the title block or general notes, and only then start building the tag list.