P&ID Instrument Symbols (ISA 5.1)
The ISA 5.1 instrument symbols used on every P&ID: field and panel instruments, DCS shared display, PLC and computer functions, and the meaning of the lines through the bubble.
Field mounted instrument
Discrete instrument located in the field, near the process. No line through the circle.
Instrument in main control room
Discrete instrument on the main control panel, normally accessible to the operator: one solid line.
Instrument on local (auxiliary) panel
Discrete instrument on a local or auxiliary panel, normally accessible: double line.
Instrument behind panel
Discrete instrument not normally accessible (behind the panel or in a cabinet): dashed line.
DCS shared display (field)
Shared display or shared control function, such as a DCS faceplate, shown as a circle inside a square.
DCS function in control room
Shared display or control function accessible to the operator on the DCS: square and circle with a solid line.
DCS function not accessible
Shared function not normally accessible to the operator (for example a DCS internal calculation): dashed line.
Computer function
Function performed by a host computer: hexagon.
PLC function
Programmable logic controller function: diamond inside a square.
Interlock or logic
Binary or sequence logic, often with an identifier such as I 1 inside: diamond.
Instrument with tag number
Letters at the top show the function (ISA 5.1), the number below is the loop number.
How to Read These Symbols
The ISA 5.1 instrument symbols used on every P&ID: field and panel instruments, DCS shared display, PLC and computer functions, and the meaning of the lines through the bubble. Instrument symbols and identification letters follow ISA 5.1. Valve, equipment and some line symbols vary between companies and with ISO 10628, so every project issues a legend sheet: when it differs from this page, the legend sheet wins.
Frequently Asked Questions
What does a line through an instrument bubble mean?
It shows the location: no line is field mounted, a solid line is the main control room, a double line is a local panel and a dashed line is behind the panel or not accessible.
What does a circle in a square mean on a P&ID?
A shared display or shared control function, typically a DCS faceplate.
How is a PLC shown on a P&ID?
As a diamond inside a square.
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