P&ID Symbols & Instrument Tags

This page follows the widely used ISA-5.1 conventions in general terms. Projects often add their own rules in a symbols legend sheet: always check the legend of the drawing set you are using.

Reading a tag number

A tag such as FIC-101 has two parts:

Letters (functional identification): the first letter is the measured or initiating variable; the following letters say what the instrument does.
Number (loop number): every instrument in the same loop shares it, e.g. FT-101, FIC-101 and FV-101 are one flow loop. Many projects prefix the area or unit, e.g. 21-FIC-101.

So FIC-101 = Flow · Indicating · Controller in loop 101.

Common letter codes

First letter	Measured variable
A	Analysis (pH, O₂, gas…)
F	Flow
L	Level
P	Pressure
PD / dP	Pressure differential
T	Temperature
S	Speed / frequency
V	Vibration
Z	Position
H	Hand (manual)
X	Unclassified (see legend)
W	Weight / force

Following letter	Function
T	Transmitter
I	Indicator
C	Controller
R	Recorder
E	Primary element (sensor)
G	Gauge / glass
V	Valve, damper
Y	Relay / compute / converter (e.g. I/P)
S	Switch
A	Alarm (with H/L: high/low)
Q	Totalise (2nd letter)
Z	Safety / emergency action (e.g. SDV, XV)

Examples: LT level transmitter · TE temperature element · PSH pressure switch high · FQI flow totaliser · PDT differential pressure transmitter · TY temperature converter · LAHH level alarm high-high.

Tag decoder

Type a tag to see what it means:

Bubble types: where is the instrument?

The outline tells you where the function lives and whether the operator can access it.

Signal lines

Common line types. Projects may use slight variations: check the legend sheet.

Valve symbols

Control valve: two triangles tip-to-tip (a "bow tie") with an actuator on top: a dome for a diaphragm actuator, a box for a piston.
Fail action is written beside it: FC fail closed, FO fail open, FL fail last (locked).
Shutdown / on-off valves are usually tagged SDV, ESDV, XV or BDV (blowdown), with a solenoid "S" box on the actuator.
Relief valves (PSV) show an angle body with a spring.

A worked loop: flow control

FE-101 (orifice) → FT-101 (DP transmitter, field) → FIC-101 (controller in the DCS) → FY-101 (I/P converter) → FV-101 (control valve, fail closed). Same loop number throughout.

Reading it out loud: "Flow element 101 is measured by field transmitter FT-101, sent electrically to the DCS controller FIC-101, whose output goes to the I/P converter FY-101 and pneumatically to control valve FV-101, which fails closed."