Valve P&ID Symbols: A Practical Legend for Engineers
Quick Answer
Valve symbols on a P&ID are built from a base valve glyph defined by ISA 5.1. The shape inside the glyph shows the valve type (globe, gate, ball, butterfly, check), marks above it show the actuator, and letters such as FC, FO or FL show the failure position. Learning the base plus a few modifiers lets you read most diagrams.
The Base Symbol
The generic valve is drawn as two triangles pointing at each other (a bowtie). Modifiers are added rather than redrawing the whole symbol.
Common Valve Type Marks
| Valve type | Mark inside base symbol |
|---|---|
| Gate | Vertical line through the bowtie |
| Globe | Solid filled bowl shape |
| Ball | Small circle in the bowtie |
| Butterfly | Two triangles forming a disk |
| Check | A free arrow or half-arrow showing flow direction |
| Relief / safety | Spring symbol above the bowtie |
| Control valve | Bowtie with a line and actuator above |
Actuator Marks
Actuators sit above the valve glyph:
- Diaphragm (spring return) for pneumatic control valves.
- Piston for higher thrust.
- Solenoid for on-off electric operation.
- Handwheel or gear for manual operation.
- Motor for electric modulating drive.
Failure Position
Control and on-off valves show behavior on loss of signal:
- FC: fail closed (spring return holds it shut).
- FO: fail open.
- FL: fail last (stays in position, common with motor or double-acting pneumatic).
Reading a Real Symbol
A globe control valve with a diaphragm actuator and FC marking means: throttling globe valve, pneumatically actuated, springs shut on air loss. For the leakage classes behind these valves, see the valve leakage classes guide. For the full standards map, see the valve standards overview.
Frequently Asked Questions
What standard defines valve P&ID symbols?
ISA 5.1 defines the instrumentation and piping diagram symbols, including how valve type, actuator and failure position are marked.
How do I tell a gate valve from a ball valve on a P&ID?
A gate valve has a vertical line through the base symbol; a ball valve shows a small circle inside the base symbol.
What does FC mean on a valve symbol?
FC means fail closed: the valve moves to the closed position if the actuating signal (air or power) is lost.
How is a check valve shown?
A check valve is shown with a flow-direction arrow (or half-arrow) inside the base symbol, indicating it permits flow in one direction only.