P&ID Line Symbols
The line types that connect instruments on a P&ID: process lines, pneumatic, electric, hydraulic and capillary signals and data links.
Process line
Main process piping: heavy solid line, with arrows showing flow direction.
Instrument supply or process connection
Thin solid line connecting an instrument to the process or to its supply.
Pneumatic signal
Instrument air signal: line with pairs of short diagonal marks.
Electric signal
Electronic or electrical signal, for example 4 to 20 mA: dashed line.
Hydraulic signal
Hydraulic signal line: line with the letter L marks.
Capillary tube
Filled capillary system such as a remote seal or filled thermal system: line with crosses.
Data link or software link
Communication link or internal system (software) link, for example between DCS functions: line with small circles.
Fieldbus or bus line
Many projects draw fieldbus segments as a line with circles or a distinct line type defined on the legend sheet; always check the project legend.
How to Read These Symbols
The line types that connect instruments on a P&ID: process lines, pneumatic, electric, hydraulic and capillary signals and data links. 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
How is a 4 to 20 mA signal shown on a P&ID?
As a dashed line between the instrument bubbles.
What does a line with two slashes mean?
A pneumatic signal, such as 0.2 to 1 bar instrument air.
What is a line with small circles?
A data or software link, such as communication within a DCS or a fieldbus.
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