P&ID vs PFD.
A process flow diagram, PFD sketches process intent. Major equipment, principal streams, mass and energy balance. A piping and instrumentation diagram, P&ID is the working document. Every instrument, every pipe spec break, every relief device, every interlock. PFDs are read. P&IDs are operated against.
Three drawings included on a new account.
What P&ID vs PFD means.
Both documents describe the same physical plant, but at different levels of resolution and for different audiences. The PFD is a communication tool. It shows what the process does and why. It carries stream flow rates, temperatures, pressures, and compositions, often in tabular form on the same sheet. Equipment is shown schematically without physical detail. Instruments appear only as generic blocks representing major control schemes. The P&ID is a specification tool. It commits to every tag number, every pipe specification, every relief valve, every block valve and bypass, every instrument air supply, and every interlock signal path. The audience is the team that will build, commission, and maintain the plant. Errors in the PFD indicate process design problems. Errors in the P&ID cause physical construction mistakes, incorrect PLC configurations, and failed FATs.
Where each lives in the engineering lifecycle
PFDs appear during conceptual design and FEED. They survive into operations as a high-level reference for tours, troubleshooting briefings, and incident reviews. P&IDs appear during detailed engineering, get marked up during construction and commissioning, and become the controlled document under MOC for the life of the plant. A revision of the PFD is a process intent change. A revision of the P&ID can be a single instrument tag change. On most projects the PFD is frozen after FEED while P&IDs continue to accrue revisions through startup and into ongoing operations.
What each document shows
PFD contents. Major equipment items with reference numbers, primary process streams with flow direction, heat and material balance data, often as a table on the drawing, major control loops represented as a single block, utility connections shown at a summary level. P&ID contents. Every piping run with line number and pipe specification, every instrument with ISA 5.1, or project-standard tag, all manual valves and blinds, equipment nozzle designations, instrument air and electrical supply connections, interlock logic references, relief valve and safety device settings, off-page connectors for multi-page sets.
How line specifications work on each document
PFDs carry stream numbers and design conditions, temperature, pressure, phase. They do not encode corrosion allowances, insulation class, or pipe schedule. P&IDs carry the full line designation. Nominal bore, fluid service code, material class, insulation type, and heat trace flag, all encoded in the line number or in a piping class note attached to the line. A commissioning engineer can read a pipe class directly off a P&ID. The PFD tells them nothing about what grade of stainless was used.
Common confusions
Some companies issue simplified P&IDs for operator training that look closer to a PFD. Others draw mechanical flow diagrams that sit between the two. The terminology is not perfectly consistent across operating companies. The functional distinction holds. If a document carries every tag, it is a P&ID. If it omits instrument tags in favor of process clarity, it is a PFD or a derivative. ATEX and SIS reviews are scoped off P&IDs, not PFDs, because the PFD lacks the instrument and interlock detail those reviews require.
Downstream documents that trace back to each drawing
PFD downstream. Process design basis, equipment data sheets, HAZOP node list. P&ID downstream. Instrument index, I/O list, cable schedule, marshalling schedule, commissioning loop sheets, SIS cause-and-effect matrix, MOC records. The P&ID is the root of most controls-engineering documents. Any error in the P&ID propagates through all of them.
P&ID against PFD, what each drawing shows
Both are process drawings of the same plant. The PFD answers what the process does; the P&ID answers what has to be built, connected, controlled and maintained.
| Aspect | Process flow diagram, PFD | Piping and instrumentation diagram, P&ID |
|---|---|---|
| Purpose | Shows process intent, and the material and energy balance | Shows everything that has to be built, connected and controlled |
| Lifecycle stage | Conceptual design and front end engineering | Detailed design, construction, as-built, and the whole operating life |
| Equipment | Major equipment only | Every item, including spares, packages and vendor scope boundaries |
| Piping | Main process streams only | Every line, carrying its full line number |
| Line identification | A stream number keyed to a stream table | A line number carrying size, service, sequence and piping class |
| Instruments | Only the ones that define the process, if any are shown | Every instrument, with its tag and its function |
| Valves | Rarely shown | Every valve, with type and failure position |
| Utilities | Shown as a connection to a header | Full utility distribution, with line numbers |
| Quantitative data | A stream table with flow, composition, temperature, pressure and enthalpy | Design and operating conditions, insulation, slope, spec breaks and elevations where they matter |
| Documents built from it | Heat and material balance, equipment list, hydraulics, plot plan | Line list, instrument index, I/O list, cause and effect matrix, isometrics, hook-up details |
| Revision behaviour | Largely frozen at the end of front end engineering | Revised under management of change for the life of the plant |
Which drawing answers which question
A quick test for which drawing is actually being asked for.
| The question | The drawing that answers it |
|---|---|
| How much material flows through this train at design rate | PFD, through its stream table |
| What is the composition of the stream leaving the column overhead | PFD |
| What is the piping class of this line | P&ID, from the line number |
| Which valve closes on a high high level in this vessel | P&ID, confirmed against the cause and effect matrix |
| Where does the instrument air header branch to this actuator | P&ID |
| How many analogue inputs does this unit need | P&ID, counted into the instrument index and then the I/O list |
| What is the heat duty of this exchanger | PFD |
| Where is the spec break between two piping classes | P&ID |
Common questions
Can you build an I/O list from a PFD.
Should the PFD and the P&ID match.
At what phase of a project does a P&ID supersede the PFD as the working document.
Which document is used as the basis for a HAZOP.
Do both documents require revision control.
Get the P&ID symbol cheat sheet.
The bubble, valve, line and connector symbols on one printable page, with what each one denotes. Plain PDF.