P&ID line types
Lines on a P&ID tell you what connects what. Thick solid lines are process piping. Thinner lines with marks along them are signals between instruments: pneumatic, electrical, hydraulic, capillary or software.
- Process lineA process line is a pipe carrying the main process fluid between equipment.
- Minor process lineA minor process line carries secondary or utility flows: small bypasses, drains, vents and utility connections.
- Pneumatic signal lineA pneumatic signal line carries an air pressure signal between instruments, for example from a positioner to a control valve.
- Electrical signal lineAn electrical signal line carries an electronic signal, such as 4 to 20 mA, between instruments and the control system.
- Hydraulic signal lineA hydraulic signal line carries a hydraulic pressure signal, for example to a hydraulic valve actuator.
- Capillary tubeA capillary tube is a sealed, fluid-filled tube connecting a sensor to an instrument, used in filled thermal and pressure systems.
- Data linkA data link is a software or digital communication connection between systems, such as a fieldbus or a link between the DCS and a PLC.
- Mechanical linkA mechanical link is a physical connection between devices, such as a float arm or linkage.
- Electromagnetic signalAn electromagnetic or sonic signal travels without a wire or tube: radio, light, radar or sound.
- Heat traced lineA heat traced line has an electric cable or small steam tube along it to keep the fluid from freezing or becoming too viscous.
- Jacketed lineA jacketed line is a pipe inside a larger pipe, with a heating or cooling medium in the space between.
Line types in ISO 10628-2
About ISO 10628-2The marks follow ISA 5.1: two slashes for pneumatic, dashes for electrical, small circles for a software or data link. Process lines also carry a line number with size, service, number and specification.
The cards at the top use the forms of US legend sheets (PIP PIC001 practice). The grids below them show the same symbols as ISO 10628-2 or IEC 62424 draw them. See ISA vs ISO P&ID symbols for the differences.
Frequently asked questions
What are line types on a P&ID?
Lines on a P&ID tell you what connects what. Thick solid lines are process piping. Thinner lines with marks along them are signals between instruments: pneumatic, electrical, hydraulic, capillary or software.
How many line types are there?
This legend covers the 11 most common ones: process line, minor process line, pneumatic signal line, electrical signal line, hydraulic signal line, capillary tube, data link, mechanical link, electromagnetic signal, heat traced line, jacketed line. Project legends may add company-specific variants.
Can I download these symbols?
Yes. Each symbol page has a free SVG and PNG, and the whole legend is available as a printable PDF chart on the P&ID symbols page.