Instrumentation Symbols

This download contains 12 instrumentation symbols for P&IDs as named blocks in DWG and DXF, with a PDF legend: five ISA instrument bubbles (field, panel, behind the panel, shared display and PLC), the three actuated valves that close a control loop (diaphragm control valve, motor-operated valve and solenoid valve) and four signal lines (pneumatic, electrical, capillary and data link). The bubbles carry FUNCTION and LOOP attributes, and all block names match the full library, such as IND_PID_SYMBOLS_INSTRUMENT_FIELD.

ISA instrument bubble symbols and their location

The bubble is a 10 mm circle, or a 10 mm square for system functions, placed by its centre. The line inside says where the operator finds the instrument. No line means a discrete instrument in the field. A solid line means a primary location, such as the control room panel, that the operator can normally reach, and a dashed line means the same location but out of normal reach, inside a cabinet or behind the panel. A circle inside a square is a function on a shared display, typically a DCS screen, and a diamond inside a square is a PLC function.

Instrument tag letters and loop numbers

The tag inside the bubble follows ISA-5.1. The first letter is the measured variable: F flow, L level, P pressure, T temperature. The next letters give the function: I indicate, T transmit, C control, A alarm, V valve. So PI is a pressure indicator, TT a temperature transmitter, LIC a level indicating controller and FV the control valve of a flow loop. A modifier can follow the first letter, as D in PDT for a differential pressure transmitter. The loop number goes on the second line and joins every device of the same loop: FT, FIC and FV of one flow loop share it.

Instrument symbols with attributes in AutoCAD

Each bubble has two attributes, FUNCTION 2 mm above the centre and LOOP 2 mm below, both 2.5 mm high, centred and empty by default, on layer G-ANNO-TEXT. AutoCAD asks for both values on insert. Change them later by double-clicking the block, and when the drawing is done, DATAEXTRACTION in full AutoCAD (not LT) can list every tag to start an instrument index. The dashed line of the behind-the-panel bubble is drawn as separate segments, so it does not depend on LTSCALE.

Signal lines in instrumentation diagrams

Each line sample is 20 mm long, with its base point on the left end. The pneumatic signal is a thin line with pairs of short slashes, the capillary tube has small crosses and the data link is broken by small circles. The electrical signal is a dashed line on layer D-PID-SIGN using the DASHED_S linetype, a 2 mm dash and 1 mm gap, so check LTSCALE if the dashes look wrong at your plot scale. ISA-5.1 has changed the communication lines between editions, so check the data link against the edition your client names.

A loop needs a valve at the end, described on the valve symbols page, and process lines to connect it, on the piping symbols page. The bubbles together with all equipment are in the P&ID symbols library.

Bubbles, tags and signal lines are drawn after ISA-5.1, a standard that the International Society of Automation sells. We describe its conventions in our own words and do not reproduce it.