How to Read Valve Symbols in P&ID Diagrams

Quick Answer: How to Read Valve Symbols in P&ID Diagrams

Valve symbols on a P&ID combine three elements: the basic valve shape (ball, butterfly, gate, globe), the actuator symbol attached to it (hand wheel, piston, motor, solenoid), and letter codes in a tag like “FV-101” that describe function and instrument type. Read them left to right: identify the valve type from the symbol geometry, the actuator from the attachment, and the tag code for the service and control function.

The Valve Body Symbols

Symbol	Valve Type	Typical Use
Circle with ball inside	Ball valve	On/off isolation
Circle with butterfly shape	Butterfly valve	Isolation and regulation
Two triangles touching	Gate valve	Isolation
Bowtie / lens shape	Globe valve	Throttling and control
Diagonal line in circle	Check valve	One-way flow
Circle with plug	Plug valve	On/off, multi-port
Circle with center line	Diaphragm valve	Corrosive and hygienic service

The two triangles of a gate valve point toward each other at the center; the bowtie of a globe valve has a distinct central disc. Ball and butterfly symbols both use a circle, distinguished by the internal element.

Actuator Symbols

• Manual: a small handle or wheel symbol above the valve — no letter code for actuation.
• Pneumatic / hydraulic: a semicircle (bowl) or rectangle over the valve, often with a letter: “A” (air), “H” (hydraulic).
• Electric motor: a circle with an “M” or a small motor symbol.
• Solenoid: a square with diagonal lines or the letter “S”.
• Spring return: a zigzag spring line added to the actuator symbol, indicating fail-safe behavior.
• Fail-open / fail-closed: arrows or letters (FO, FC) near the actuator indicate the fail-safe position.

Instrument Tag Codes

Tags follow ISA 5.1 conventions: the first letter indicates the measured variable, the second the function. Examples:

Tag	Meaning
FV-101	Flow control valve, loop 101
PV-204	Pressure control valve, loop 204
XV-150	Shutdown / isolation valve (on/off), loop 150
TV-301	Temperature control valve, loop 301
HV-410	Hand-operated valve
MOV-505	Motor-operated valve

The prefix (FV, XV, MOV) tells you the control function and how the valve is operated before you even look at the symbol.

Reading a Complete Symbol

Take “FV-101” with a bowtie symbol, a semicircle actuator with a spring, and “A” marked on it: this is a pneumatic, spring-return globe control valve regulating flow in loop 101. The spring position and fail arrows tell you whether it fails open or closed. Reading all three layers — body, actuator, tag — gives you the complete story of the valve’s function in the process.

Common Pitfalls

• Confusing ball and butterfly symbols (look at the internal element).
• Missing the fail-safe indication on spring-return actuators.
• Ignoring “FS” (fail to last position) or “FL” (fail locked) notes on critical shutdown valves.
• Assuming a valve is manually operated when a small solenoid symbol is present.

Conclusion

P&ID valve symbols are a compact language that combines valve type, actuation, and control function. Learn the core body shapes, the actuator attachments, and the ISA tag codes, and you can interpret any valve on a process diagram. The tag code is your shortcut — it states the function before you decode the geometry.

Tags: Valve Standards