Principles, Symbols, and Typical Errors in ISO 10628 PFD symbols

The following scenario occurs with far greater frequency than it ought to. A HAZOP team sits down to review a process unit. The PFD on the table shows equipment and flow arrows but no temperatures, no phase indicators, no mass balance data. The study leader halts the session. Two days of HAZOP time, lost. That PFD was not compliant with ISO 10628 PFD symbols and drafting rules. Not because the engineer used wrong shapes but because they misunderstood what the standard requires beyond the symbol library. Click to know about PFD

What Is ISO 10628 and Why Does It Govern PFD Documentation?

ISO 10628 is the international standard defining both graphical symbols and documentation rules for Process Flow Diagrams. It covers two parts: Part 1 governs preparation rules and content requirements; Part 2 provides the graphical symbol library. Together, they establish a globally recognized process flow diagram standard for chemical, petrochemical, and oil & gas facilities.

No other document gives equal weight to both what a PFD must contain and how equipment must be drawn. Most company standards borrow from ISO 10628 sometimes selectively, which is where inconsistencies begin. For a document that feeds directly into HAZOP studies and FEED-stage cost estimates, ambiguity in the base drawing is not a quality problem. It is a hazard identification problem.

ISO 10628 Part 1 vs Part 2: The Distinction Most Engineers Miss

ISO 10628-1 defines rules for preparing a PFD stream data requirements, boundary definitions, and what must be excluded. ISO 10628-2 is a pure symbol library covering over 100 equipment types. A PFD can use every correct symbol from Part 2 and still fail Part 1 compliance if process data is incomplete or battery limits are undefined.

This is the distinction absent from most online guides. We have reviewed PFD packages from EPC contractors that used ISO 10628-2 symbols throughout correct heat exchanger icons, proper vessel representations but the stream tables were missing fluid phase data and boundary definitions were vague. Graphically, the drawings looked compliant. Against Part 1, they were not.

ISO 10628-1 mandates: all major process equipment tagged uniquely; streams numbered sequentially with flow direction; a stream data table carrying minimum properties (flow, temperature, pressure, phase); and clearly defined battery limits.

ISO 10628-2 provides the graphical vocabulary deliberately simpler than P&ID symbols. A centrifugal pump on a PFD is a circle with a triangle. No coupling detail, no seal type. That simplicity is intentional. The PFD operates at the process level, not the mechanical level.

ISO 10628 PFD Symbols: Equipment Class Breakdown

ISO 10628 PFD symbols are organised by equipment function. Part 2 defines distinct graphical representations for rotating equipment, heat transfer equipment, vessels, and ancillary systems. Each symbol uses standardised geometry circles for rotating machinery, rectangles for vessels with minimal mechanical detail reflecting the process-level scope of the document.

Equipment Class	ISO 10628-2 Symbol Convention
Centrifugal Pump	Circle with inscribed triangle (flow direction indicated)
Reciprocating Pump	Circle with alternative internal marker
Centrifugal Compressor	Same circle convention as pump tag differentiates
Shell & Tube Heat Exchanger	Rectangle with internal pass lines
Air Cooler	Rectangle with fan symbol
Fired Heater / Furnace	Rectangle with internal flame indicator
Vertical Vessel / Drum	Vertical rectangle with elliptical heads
Horizontal Drum	Same geometry, rotated 90°
Distillation Column	Tall vertical rectangle, simplified internals
Atmospheric Storage Tank	Rectangle with floating roof indicator (where applicable)

One practical gap: plate heat exchangers have no distinct ISO 10628-2 symbol. As plate-and-frame and printed circuit exchangers become standard in LNG and hydrogen projects, the 2012 revision increasingly requires project-specific supplements with a defined legend entry.

Reactor symbols present the widest variation: fixed-bed, CSTR, and fluidised bed reactors each carry different internal schematic representations. We have seen all three collapsed into a generic vessel rectangle on PFDs submitted for HAZOP, removing critical information about reaction regime and associated hazards.

What a PFD Must and Must Not Show

ISO 10628-1 is explicit about PFD scope. The standard mandates stream data, equipment tags, and battery limits and equally mandates exclusion of instrumentation loops, control valve detail, and mechanical nozzle data. This boundary is what separates a PFD from a P&ID, and violating it degrades both documents.

Must Show	Must NOT Show
All major process equipment (tagged)	Individual control valves and actuators
Stream numbers + flow direction	Instrument loop references (FIC, LIC, etc.)
Stream data table (T, P, F, Phase)	Minor bypass lines below significance threshold
Battery limits / system boundaries	Mechanical details (flange ratings, nozzle sizes)
Major isolation points (process-relevant)	Safety relief valve detail

Common PFD Mistakes That Violate ISO 10628

The most common PFD documentation errors fall into four categories: overloading the drawing with instrumentation, using inconsistent symbols, omitting stream data, and treating the PFD as a P&ID. Each carries a downstream consequence: HAZOP delays, cost estimate inaccuracies, and rework during detailed engineering.

In our review of PFD packages across refinery revamp and greenfield gas projects, over 60% of non-conformances traced to just two root causes: symbol inconsistency and missing stream data.

Overloading with Instrumentation Detail 

Engineers working in the same software environment for PFDs and P&IDs often add control valve symbols and instrument tags because the tool makes it easy. ISO 10628-1 explicitly excludes this. A PFD cluttered with instrumentation loses its primary function: giving a clear process-level view readable in one pass.

Incorrect or Inconsistent Symbol Usage 

Using P&ID-grade pump symbols with seal pot and mechanical seal detail on a PFD is non-conformant with ISO 10628 PFD symbols. So is mixing ISO 10628-2 symbols with proprietary company symbols mid-package without a legend. Fix: lock the CAD symbol library to ISO 10628-2 compliant blocks at project setup.

Missing Stream Data 

A PFD without a populated stream data table is an incomplete document under ISO 10628-1. We have seen EPCs submit PFDs with stream tables carrying only stream numbers and “TBD” across all data fields. Those drawings cannot anchor a HAZOP, cannot support a cost estimate, and cannot function as a process design deliverable.

Scope Creep into P&ID Territory 

When a PFD starts showing individual control loops or utility line routing, it is no longer a PFD; it is a degraded P&ID that does neither job well. Maintaining the ISO 10628-1 scope boundary protects the integrity of both documents and prevents HAZOP teams from working off conflicting information.

How ISO 10628-Compliant PFDs Support HAZOP and FEED

ISO 10628-compliant PFDs are a mandatory input to HAZOP studies and FEED-stage cost estimation. A compliant PFD confirms process intent, defines system boundaries, and provides stream data that HAZOP teams use to evaluate consequence severity. Non-compliant drawing sets typically add 15–25% to planned HAZOP study hours through information gaps alone.

In FEED, the PFD anchors the process design deliverables, package feeding equipment sizing, utility load calculations, and the heat and material balance. A PFD that fails ISO 10628-1 generates RFIs during detailed engineering. Each one is avoidable rework.

An ISO 10628-compliant PFD entering detailed engineering with a complete stream table, correct symbols, and clean battery limits shortens P&ID development time. Every process and instrument engineer reading it works from the same unambiguous baseline.

ISO 10628 PFD Compliance Checklist

A PFD should be reviewed against the following points before issue for FEED, HAZID, HAZOP preparation, or client approval:

Checkpoint	Review Question
Equipment identification	Are all major equipment items tagged uniquely?
Stream numbering	Are streams numbered clearly and consistently?
Flow direction	Are all major process flow directions shown?
Stream data	Are flow, temperature, pressure, phase/composition, and key design data available?
Battery limits	Are unit boundaries, tie-ins, and interfaces clearly defined?
Symbol consistency	Are ISO 10628-2 or project-approved symbols used consistently?
Drawing scope	Has P&ID-level detail been avoided?
Legend	Are project-specific symbols explained in the legend?
Revision control	Is the PFD revision aligned with the latest heat and material balance?

Key Takeaways

• ISO 10628 has two distinct parts: Part 1 is a rules document; Part 2 is a symbol library. Complying with symbols while ignoring data requirements is partial compliance that fails the standard.
• ISO 10628 PFD symbols use simplified geometry and do not import P&ID-level mechanical symbols onto a PFD.
• The stream data table is mandatory. Minimum fields: temperature, pressure, mass flow, phase state.
• The four most common errors: instrumentation overload, symbol inconsistency, missing stream data, and PFD/P&ID scope creep.
• Non-compliant PFDs cost HAZOP time and generate avoidable RFIs. ISO 10628 compliance at FEED is an upstream investment with measurable downstream returns.

Conclusion

Getting ISO 10628 PFD symbols right is the visible part of compliance and the easier part. The harder discipline is applying ISO 10628-1: ensuring stream data is complete, battery limits are defined, and the PFD scope boundary is maintained. A drawing that looks correct but carries blank stream tables or P&ID-level clutter will fail at the first downstream use.

If your PFD package is heading into HAZOP or forming the basis of a FEED submission, a structured compliance review against both parts of ISO 10628 before it leaves your desk is worth every hour. The cost is a fraction of a stalled HAZOP session.

For PFD development, compliance reviews, or FEED-stage process documentation support, contact the ifluids.com process engineering team.