Valve and Instrument Symbols Chart

How to read ISA 5.1 instrument bubbles, tag letters, signal lines and common valve and actuator symbols on a P&ID.

Ref PV-0057ISA 5.1

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A P&ID (piping and instrumentation diagram) shows the process piping, equipment, valves, instruments and control loops. ISA 5.1 sets the symbols and the letter codes used to identify instruments. Each instrument is a bubble with a tag: letters that say what is measured and what the device does, and a loop number.

How to read it

Read the first letter of the tag. It is the measured variable.
Read the following letters. They are the functions.
Read the line through the bubble. It shows where the device is mounted.
Follow the signal lines to see what the instrument connects to.
Check the project legend sheet for any company-specific symbols.

Tag letters (common)

Letter	As first letter (measured variable)	As later letter (function)
A	Analysis	Alarm
F	Flow	—
L	Level	Light
P	Pressure	Point (test connection)
T	Temperature	Transmitter
C	User's choice	Controller
I	Current	Indicator
E	Voltage	Sensor (primary element)
S	Speed or frequency	Switch
V	Vibration	Valve, damper or louver
R	Radiation	Recorder
Y	Event or state	Relay, compute or convert

Example: PIT-101 is a pressure indicating transmitter in loop 101.

Bubble and line symbols

Symbol	Meaning
Circle, no line	Field mounted
Circle with a solid line	Primary location, normally accessible to the operator, such as a control room panel
Circle with a dashed line	Location not normally accessible, such as behind a panel
Circle inside a square	Shared display or control, such as a DCS
Hexagon	Computer function
Diamond inside a square	PLC function
Solid process line	Process piping
Line with short double slashes	Pneumatic signal
Dashed line	Electric signal
Bow tie (two triangles)	Valve body, general
Dome on a stem	Diaphragm actuator
Box with "S"	Solenoid actuator
Circle with "M"	Motor actuator

Key points

The same bubble can carry different line styles. The line, not the shape, sets the location.
Fail positions are shown as FO (fail open), FC (fail closed) or FL (fail locked, last position).
Valve body types often add a mark inside the bow tie, such as a circle for a ball valve.

Common mistakes

Reading the second letter as the measured variable.
Confusing pneumatic and electric signal lines.
Assuming symbols from an older edition mean the same thing today.
Ignoring the project legend sheet.

Governing standards: ANSI/ISA 5.1 (instrumentation symbols and identification); the project P&ID legend.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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