P&ID Explained: ISA-5.1 Symbols & How to Read One
A P&ID (Piping and Instrumentation Diagram) is an ISA-5.1 standardised drawing showing every pipe, vessel, valve, instrument, and control loop in a plant. The visual companion to the I/O list — every instrument tag on the P&ID appears in the I/O list and vice versa.
ISA-5.1 instrument symbol legend
How to read a P&ID
- Find the process flow direction. Follow the major pipes from raw-material feed at one edge to product output at the other.
- Identify equipment. Vessels, pumps, heat exchangers, columns. Each has a tag (V-101, P-201, E-301).
- Trace each loop. Find the transmitter (FIT, LIT), the controller (FIC, LIC), and the final element (control valve FCV, motor M). Signal lines connect them.
- Check signal types. Continuous lines = electrical, dashed = pneumatic, dotted with X = hydraulic, double-line = capillary, line with dots = software/internal signal.
- Cross-reference to I/O list. Every instrument tag should match a row in the I/O list with the same range and signal type.
Standards governing P&IDs
- ISA-5.1 — Instrument symbology (balloons, function letters, line types). Most-cited standard.
- ISA-5.2 — Binary logic diagrams (sometimes embedded in the P&ID for simple sequences).
- ISA-5.3 — Symbols for distributed control / shared display / shared control / computer systems.
- ISA-5.5 — Graphic symbols for process display.
- DIN EN 62424 — European P&ID standard (overlaps with ISA but has different valve symbols).
- BS 5070 — UK piping engineering symbols.
Most international projects standardise on ISA-5.1 even when working in Europe; some plants use DIN EN 62424, especially those with a strong German engineering heritage.
Frequently asked questions
What is a P&ID?
A P&ID stands for Piping and Instrumentation Diagram. It is an ISA-5.1 standardised drawing showing every pipe, vessel, valve, instrument and control loop in a plant. Critical in process industries; less central in pure machine-build projects.
Who creates the P&ID?
The process engineering team creates the initial P&ID during the FEED (Front End Engineering Design) phase. The instrumentation and control team adds instrument tags during detailed design. The P&ID evolves through HAZOP review and is locked at the IFC (Issued For Construction) revision.
How does the P&ID relate to the I/O list?
They must match exactly. Every instrument tag on the P&ID appears as a row in the I/O list and vice versa. The P&ID is the visual; the I/O list is the database. When they disagree (which happens), the IFC revision of the P&ID wins for physical equipment; the I/O list wins for PLC/HMI references.
What is the difference between PFD and P&ID?
A PFD (Process Flow Diagram) is a higher-level drawing showing major equipment, process flow rates, temperatures and pressures — used for process design and material balance. A P&ID is the detailed engineering drawing showing every pipe, valve and instrument. PFDs are produced first; P&IDs are derived from them and contain the additional control system detail.