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Reading P&ID instrument tags
How instrument tag letters describe what is measured and what the device does, with examples like LT, LSHH, PDT and FIC.
The idea
On a piping and instrumentation diagram (P&ID) every instrument has a tag such as LT-101. The letters describe the function and the number identifies the loop. The letter system used widely in industry follows ISA-5.1 style identification; projects state any project-specific letters on their legend sheet.
First letter: what is measured
The first letter is the measured or initiating variable: for example F flow, L level, P pressure, T temperature, A analysis. An optional modifier can follow it, such as D for differential or Q for totalised, so PD is differential pressure and FQ is a flow total.
Following letters: what the device does
The next letters give the function: T transmitter, I indicator, C controller, S switch, A alarm, V valve, E primary element. Alarm and switch levels are often added at the end: H high, L low, HH high-high, LL low-low.
Examples: LT level transmitter; LSHH level switch high-high; PDT pressure differential transmitter; FIC flow indicating controller; TE temperature element; FV flow valve.
Where the instrument is
The symbol shape tells you where the function lives: a plain circle is mounted in the field, a circle with a line is on a local panel, a circle inside a square is shown on a shared operator screen, and a diamond inside a square is a PLC function.
Try it yourself
Open the Drawings tab and use the Tag decoder. Then tap instruments on the live P&ID and add the missing ones to the instrument index.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.