P&ID vs PFD: the difference between the two diagrams
Deskely Completions team Singapore
A process flow diagram (PFD) shows how a process works: the main equipment, the main streams and their conditions. A piping and instrumentation diagram (P&ID) shows how it is built and controlled: every pipe, valve, instrument and control loop.
Both are drawn by the process engineer, but at different stages and for different readers.
- PFD: the main flow, equipment and conditions. One sheet per unit.
- P&ID: every line, valve, instrument and loop. Many sheets per unit.
- The PFD comes first, in concept and FEED. The P&ID is developed from it.
- The P&ID is the drawing the plant is built, commissioned and operated from.
Side by side
| PFD | P&ID | |
|---|---|---|
| Purpose | How the process works | How the plant is built and controlled |
| Equipment | Major equipment only | All equipment, including spares and packages |
| Piping | Main process streams | Every line, with size, number and specification |
| Valves | Only key control valves | Every valve, with type and normal position |
| Instruments | Main control loops, if any | Every instrument, loop, alarm and trip |
| Data | Flow, pressure, temperature, heat and mass balance | Line numbers, tags, set points, design conditions |
| Stage | Concept and FEED | FEED, detail design, construction, operation |
| Sheets | A few per unit | Many per unit |
What a PFD contains
- Major equipment: columns, vessels, exchangers, pumps and compressors.
- Main process streams, numbered, with flow direction.
- A stream table with flow rate, composition, temperature and pressure for each stream.
- The main control strategy, sometimes.
What a P&ID adds
- Every line, with a line number giving size, service and piping specification.
- Every valve, including isolation, drains, vents, bypasses and check valves.
- Every instrument, with ISA 5.1 tags and the control loops between them.
- Safety devices with set pressures, and shutdown valves linked to the safety system.
- Utility connections, insulation and tracing, and system boundaries.
See how to read a P&ID and the full P&ID symbols legend.
Same pump, two diagrams
On a PFD a pump is one symbol on a stream, with its flow and discharge pressure in the stream table. On the P&ID the same pump is a duty and standby pair, each with suction and discharge isolation valves, a strainer, a check valve, a pressure gauge, a minimum-flow line and a relief valve, all tagged.
Where the P&ID goes after design
Once construction starts, the P&ID becomes the source of the tag register: every tag on it gets inspection and test records, belongs to a system and a subsystem, and has to be completed and certified before handover. Each new revision has to be compared against the last. That is the work Deskely Extract and Delta do.
The Deskely products for this
Frequently asked questions
Is a P&ID more detailed than a PFD?
Yes. A PFD shows the main process; a P&ID shows every line, valve and instrument needed to build and operate it.
Which comes first, the PFD or the P&ID?
The PFD. It is produced in concept and front-end design, and the P&IDs are developed from it.
What is a UFD?
A utility flow diagram: a PFD for utility systems such as steam, cooling water, instrument air and nitrogen.
Is a P&ID the same as a process and instrumentation diagram?
Yes. Both names are used for the same drawing.
- In production since 2014
- Thousands of daily users
- On MODEC FPSO projects since 2016