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Reading Instrument Tags and P&ID Symbols

Documentation · 8 min read

Piping and instrumentation diagrams (P&IDs) show the instruments on a process, how they are connected and how they control it. Most of them use the symbols and letter codes of the ISA-5.1 standard. Once you can read a tag like FIC-101, a P&ID becomes a map of the control system. This note covers the basics.

Anatomy of an instrument tag

Figure 1 — The first letter is what is measured. The letters after it are what the device does. The number identifies the loop.

Common identification letters

Letter	As first letter (measured variable)	As succeeding letter (function)
A	Analysis (pH, O₂, conductivity…)	Alarm
C	—	Control
E	Voltage	Primary element (sensor)
F	Flow	—
G	—	Glass / gauge (local viewing device)
I	Current	Indicate
L	Level	Light (as a function); "low" as a modifier
P	Pressure	Point (test connection)
S	Speed / frequency	Switch; "safety" as a modifier (e.g. PSV)
T	Temperature	Transmit
V	Vibration	Valve, damper
Y	Event / state	Relay, compute, convert (e.g. I/P converter)

Some examples: PT pressure transmitter, TE temperature element (a thermocouple or RTD), LAH level alarm high, PDT differential pressure transmitter, FV flow control valve, PSV pressure safety valve.

Instrument bubbles: where is the device?

The shape around the tag and the line through it tell you where the device is located or where its function is available.

Figure 2 — Common ISA-5.1 bubble styles. The line through the middle shows the location; the outer shape shows the type of device or system.

Signal lines

Figure 3 — The line style shows what kind of signal travels between instruments.

Putting it together: a flow control loop

Figure 4 — Loop 101. The orifice plate (FE) creates a pressure drop. The transmitter (FT) converts it to 4–20 mA. The controller in the DCS (FIC) compares it with the setpoint. The I/P converter (FY) drives the control valve (FV) with air.

Every device in the loop carries the same number, 101. That is how you find all the parts of one control loop on a drawing, in the loop folder and in the field. The letters change with each device's role.

Note: many companies add their own conventions on top of ISA-5.1, such as area prefixes (20-FIC-101) or special symbols. Always check the legend sheet at the front of the P&ID set first.