What a P&ID is
A P&ID (piping and instrumentation diagram) shows the equipment of a process, the piping that connects it, the instruments that measure it and the loops that control it. It is used for design, construction and commissioning, then throughout operation. It is often the reference document behind a supervision screen.
The instrument tag: PIC-101
A tag reads in three parts: the measured variable, the functions, the loop number.
| Part | Example | Meaning |
|---|---|---|
| First letter | PIC | measured variable: pressure |
| Following letters | PIC | functions: indicator, controller |
| Number | 101 | loop 101, often tied to an area or a piece of equipment |
A modifier letter can follow the first letter: PD for differential pressure, FQ for totalised flow.
The most common first letters
| Letter | Variable |
|---|---|
| A | analysis (pH, conductivity, oxygen and so on) |
| F | flow |
| L | level |
| P | pressure |
| T | temperature |
| S | speed |
| Z | position |
| H | hand (manual) |
The most common function letters
| Letter | Function |
|---|---|
| I | indicate |
| R | record |
| C | control |
| T | transmit (transmitter) |
| S | switch |
| A | alarm |
| V | valve, final control element |
| E | primary element, sensor |
| H, L | high, low (used with S or A) |
A few readings: TT temperature transmitter; LIC level indicating controller; LSH level switch high; PSHH pressure switch high-high; FV flow control valve; ZS position switch.
The letters are read by position, not by meaning alone. In TT, the first T is temperature and the second is transmit. In PSH, the S is a switch because it comes after the first letter; as a first letter, S would mean speed.
The bubble shape: where the function lives
The bubble (the circle around the tag) tells you where the instrument or function sits, and whether the operator can reach it.
| Shape | Usual meaning |
|---|---|
| plain circle | field-mounted instrument |
| circle with a solid horizontal line | in the control room, accessible to the operator |
| circle with a dashed horizontal line | in the control room, behind the panel, not accessible |
| circle inside a square | shared display function of a control system (DCS), shown on screen |
| hexagon | computer function |
| diamond inside a square | function performed by a PLC |
The same shapes with a double line usually mean a local panel outside the control room. The project legend confirms it.
The lines: how the signal travels
| Line | Usual meaning |
|---|---|
| thick solid line | process piping |
| thin line crossed by pairs of short diagonal strokes at intervals | pneumatic signal |
| dashed line | electrical signal |
| thin line with crosses | capillary tube |
| thin line with small circles | data link, software link |
| thin line with small L shapes | hydraulic signal |
An arrow on a pipe shows the flow direction. An off-page connector (a pentagon with a reference) tells you where the line continues.
Equipment
Equipment tags are less strictly standardised than instrument tags. You will often see P-101 for a pump, V- or T- for a vessel or tank, E- for a heat exchanger and K- for a compressor. The symbols follow recognisable shapes:
| Family | Typical shapes |
|---|---|
| Valves | two triangles point to point (the bow-tie), with a circle (ball), a line (butterfly), or an actuator on top (motor-operated, pneumatic, solenoid) |
| Pumps | circle with a tangential outlet (centrifugal), variants for positive displacement, metering and screw pumps |
| Vessels | vertical or horizontal cylinder, sphere, floating roof, jacketed, with coil |
| Heat exchangers | shell and tube, plate, air cooler |
| Separation | separator, cyclone, filter, Y strainer |
A control loop, read end to end
Take level control on a tank:
- LT-201, plain circle on the tank: level transmitter in the field.
- A dashed line, or a line with small circles, runs to LIC-201 in a circle inside a square: indication and control are in the DCS, on the operator's screen.
- Another line runs to LV-201 on the outlet pipe: the control valve, with its actuator.
- LSH-201 and LAH-201, if shown, add a high level switch and a high level alarm.
The number 201 ties together every element of the same loop. It is the number you will find again in the I/O list, in the PLC program and on the supervision screen.
Three mistakes beginners make
- Reading the bubble as the instrument. A circle inside a square is not a device you can touch: it is a function on a screen. The physical transmitter is the plain circle.
- Ignoring the line style. A pneumatic signal and an electrical signal look alike at first glance. On a brownfield site the difference decides what you wire, and what fails safe on loss of air or power.
- Assuming every project uses the same letters. Some sites assign their own meaning to letters the standard leaves to the user. The legend sheet settles it.
From the P&ID to the supervision screen
An HMI process graphic does not copy the P&ID. It keeps what the operator needs to run the plant. Symbols are simpler, the state (running, stopped, fault, open, closed) shows through colour or shape, and values are displayed next to the equipment. High-performance HMI principles ask for a neutral background and little colour, kept for abnormal situations. The guide High-performance HMI: 8 rules for a web screen develops them.
Ready-made symbols
pid-symbols provides 117 symbols drawn on the same grid, in 11 families (valves, pumps, rotating machines, vessels, heat exchangers, separation, water treatment, HVAC, electrical, instruments with the bubbles and the PIC, TIC, FIC, LIC loops, piping and lines), as SVG, PNG and a draw.io library with English names. In a web HMI, states animate through CSS classes. The shapes follow the usual conventions (ISA-5.1, ISO 10628), with no certification or affiliation, and there is no Visio stencil or Figma library in this version (the SVG and PNG files drag and drop into both). To assemble symbols into an animated screen, see the SVG process graphic.
Sources
- ANSI/ISA-5.1, Instrumentation Symbols and Identification (not reproduced; common conventions summarised)
- ISO 10628, Diagrams for the chemical and petrochemical industry (not reproduced)
- ISO 14617, Graphical symbols for diagrams (not reproduced)