Quick Answer
Valve symbols in P&ID (Piping and Instrumentation Diagram) drawings use two triangles pointing toward each other as the base symbol. Different shapes added to the center indicate valve type: a vertical line for gate valves, a filled circle for globe valves, a filled diamond for ball valves. The standard is ANSI/ISA-5.1-2009, though variations exist across companies and industries.
Basic symbol: ◁▷ (Two triangles) + Center shape = Specific valve type
Complete Valve Symbols Chart
Common Valve Types – Quick Reference
┌─────────────────────────────────────────────────────────────────────┐
│ VALVE SYMBOLS REFERENCE CHART │
├──────────────────────────┬──────────────┬───────────────────────────┤
│ VALVE TYPE │ SYMBOL │ DESCRIPTION │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Generic Valve (2-way) │ ◁▷ │ Base symbol │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Gate Valve │ ◁|▷ │ Vertical line │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Globe Valve │ ◁●▷ │ Filled circle │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Ball Valve │ ◁◆▷ │ Filled diamond │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Needle Valve │ ◁|▷ │ Thin vertical line │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Plug Valve │ ◁○▷ │ Open circle │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Butterfly Valve │ ◁—▷ │ Horizontal line │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Diaphragm Valve │ ◁~▷ │ Wavy line │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Check Valve │ ◁▷| │ Barrier on outlet │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Relief/Safety Valve │ ╱╲ │ Spring symbol above │
│ │ ◁▷ │ │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ 3-Way Valve │ | │ Three-port junction │
│ │ ◁┼▷ │ │
│ │ | │ │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Control Valve │ [ ] │ Actuator on top │
│ (with actuator) │ | │ │
│ │ ◁▷ │ │
└──────────────────────────┴──────────────┴───────────────────────────┘
Actuator Types
┌─────────────────────────────────────────────────────────────────────┐
│ ACTUATOR SYMBOLS │
├──────────────────────────┬──────────────┬───────────────────────────┤
│ ACTUATOR TYPE │ SYMBOL │ DESCRIPTION │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Manual (Handwheel) │ ⊕ │ Hand-operated │
│ │ | │ │
│ │ ◁▷ │ │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Pneumatic Actuator │ ┌───┐ │ Compressed air │
│ │ │ │ │ │
│ │ └─┬─┘ │ │
│ │ | │ │
│ │ ◁▷ │ │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Electric Actuator │ (M) │ Motor-driven │
│ │ | │ │
│ │ ◁▷ │ │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Hydraulic Actuator │ ● │ Hydraulic fluid │
│ │ | │ │
│ │ ◁▷ │ │
└──────────────────────────┴──────────────┴───────────────────────────┘
Fail-Safe Positions
┌─────────────────────────────────────────────────────────────────────┐
│ FAIL-SAFE POSITIONS │
├──────────────────────────┬──────────────┬───────────────────────────┤
│ FAIL-SAFE TYPE │ SYMBOL │ ACTION ON FAILURE │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Fail Closed (FC) │ ┌───┐ │ Valve closes │
│ │ │ FC│ │ │
│ │ └─↓─┘ │ │
│ │ ◁▷ │ │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Fail Open (FO) │ ┌───┐ │ Valve opens │
│ │ │ FO│ │ │
│ │ └─↑─┘ │ │
│ │ ◁▷ │ │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Fail Last Position (FLP) │ ┌───┐ │ Valve stays in position │
│ │ │FLP│ │ │
│ │ └─┬─┘ │ │
│ │ ◁▷ │ │
└──────────────────────────┴──────────────┴───────────────────────────┘
End Connections
┌─────────────────────────────────────────────────────────────────────┐
│ END CONNECTION TYPES │
├──────────────────────────┬──────────────┬───────────────────────────┤
│ CONNECTION TYPE │ SYMBOL │ DESCRIPTION │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Flanged │ ─┤◁▷├─ │ Bolted flange │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Threaded │ ○◁▷○ │ Screwed connection │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Welded (Butt Weld) │ ■◁▷■ │ Permanent welded │
├──────────────────────────┼──────────────┼───────────────────────────┤
│ Socket Weld │ □◁▷□ │ Socket welded │
└──────────────────────────┴──────────────┴───────────────────────────┘
Complete Control Valve Examples
EXAMPLE 1: Pneumatic Control Valve, Fail Closed, Flanged
┌───┐
│ FC│ ← Fail Closed
└─↓─┘
|
─┤◁▷├─ ← Flanged connections
Tag: FCV-101 (Flow Control Valve #101)
EXAMPLE 2: Electric Actuated Ball Valve, Fail Open
(M) ← Electric motor
|
◁◆▷ ← Ball valve
Tag: TCV-205 (Temperature Control Valve #205)
EXAMPLE 3: Manual Gate Valve with Check Valve
⊕ ← Handwheel
|
─┤|▷├─ ← Gate valve, flanged
| ← Check valve downstream
◁▷|
Understanding P&ID Drawings
What Is a P&ID?
A piping and instrumentation diagram (P&ID) is a graphic representation of a process system that includes the piping, vessels, control valves, instrumentation, and other process components and equipment in the system.
Key purposes:
- Shows functional relationships between components
- Guides installation and construction
- Basis for control system programming
- Used for troubleshooting and maintenance
- Required for safety analysis
Important note: These symbols are NOT to scale and are NOT dimensionally accurate. They are merely used to represent a certain type of component.
Standards for Valve Symbols
The International Society of Automation (ISA) has defined a standard for P&IDs. The standard is ANSI/ISA-5.1-2009 and is available on the ISA website.
Major standards:
- ANSI/ISA-5.1-2009 – Most common in North America
- ISO 10628 – International standard
- ISO 14617 – Graphical symbols for diagrams
- DIN standards – Common in Europe
- JIS standards – Common in Japan
Reality check: Despite strict standards, you will find various ways of representing certain valves across different industries and companies. This isn’t problematic since all components are also described by text, tag numbers, and legends.
The Basic Valve Symbol
Generic 2-Way Valve
The generic symbol for a 2-way valve is two triangles pointing to each other with the tips of the inner points touching.
Generic valve symbol:
◁▷ or ◀▶
Structure:
- Left triangle: Inlet side
- Right triangle: Outlet side
- Tips touching: Valve body
- Lines connect to pipe: Process lines
Flow direction:
- Arrows show flow direction
- Arrow at inlet
- Arrow at outlet
- Arrow at every 90° turn
Common Valve Type Symbols
Gate Valve
Symbol: Two triangles with vertical line in center
◁|▷
P&ID representation:
- Vertical line between triangles
- Modification of generic valve
- Manual operation (unless marked otherwise)
Usage:
- On/off service
- Full flow when open
- Common in process lines
Globe Valve
Symbol: Two triangles with filled circle in center
◁●▷
P&ID representation:
- Filled (dark) circle between triangles
- Distinguishes from plug valve (open circle)
Usage:
- Throttling and flow control
- Regulation applications
- Manual or automated
Ball Valve
Symbol: Two triangles with filled diamond in center
◁◆▷
Alternative symbols: Some companies use a filled circle or a small filled square instead of a diamond.
P&ID representation:
- Filled diamond most common
- Quarter-turn operation
- Fast shutoff capability
Usage:
- Quick on/off operation
- Full bore flow
- Low pressure drop
Needle Valve
Symbol: Two triangles with thin vertical line or needle shape
◁|▷ (thinner line than gate valve)
Alternative: Some use elongated triangle pointing into valve
P&ID representation:
- Very thin line between triangles
- Sometimes labeled “NV”
Usage:
- Precise flow control
- Small flow rates
- Instrumentation connections
Plug Valve
Symbol: Two triangles with open (unfilled) circle in center
◁○▷
Key difference from globe valve:
- Circle is NOT filled (globe valve has filled circle)
- Can be confusing—always check drawing legend
Usage:
- Rotary motion valve
- On/off or diverting service
- Oil and gas applications
Butterfly Valve
Symbol: Two triangles with partial/incomplete triangular center or line through center
◁◁▷ (overlapping or partial)
or
◁—▷ (line through center)
Unique feature: The butterfly valve symbol is the only symbol where a full triangle is not used.
P&ID representation:
- Incomplete triangle shape
- Or horizontal line through center
- Alternative symbols clearer
Usage:
- Large diameter pipes
- Low pressure drop
- Compact design
Diaphragm Valve
Symbol: Two triangles with wavy or curved line
◁~▷ or ◁⌢▷
P&ID representation:
- Wavy line represents flexible diaphragm
- Curved line between triangles
Usage:
- Corrosive fluids
- Slurries
- Sterile applications
Check Valve (Non-Return Valve)
Symbol: Two triangles with solid barrier on outlet side
◁▷| or ◁▷▌
Key feature:
- Arrow shows permitted flow direction
- Barrier on one side indicates closure
- Prevents backflow
Variations:
- Swing check: May show hinged element
- Lift check: Vertical element shown
- Spring check: Spring symbol added
Usage:
- Prevents reverse flow
- Automatic operation
- Pump discharge lines
Relief Valve / Safety Valve
Symbol: Two triangles with spring symbol above or special marking
╱╲
╱ ╲
◁▷
P&ID representation:
- Spring symbol above valve
- Or arrow pointing to discharge
- Often labeled “PRV” or “PSV”
Usage:
- Pressure protection
- Safety critical
- Set pressure marked
Control Valve
Symbol: Two triangles with actuator symbol on top
[ ] ← Actuator
|
◁▷
Actuator types shown:
- Square/rectangle: Pneumatic actuator
- Circle: Hydraulic actuator
- Motor symbol: Electric actuator
Tag example: FCV-101 (Flow Control Valve #101)
Usage:
- Automated flow control
- Process regulation
- Remote operation
Actuator and Operator Symbols
Manual Operation
Symbol: Handwheel or lever drawn above valve
⊕ ← Handwheel
|
◁▷
Indicates:
- Manual operation required
- No automation
- Local control only
Pneumatic Actuator
Symbol: Rectangle or diaphragm shape above valve
┌───┐
│ │ ← Actuator
└─┬─┘
|
◁▷
Indicates:
- Compressed air operation
- Remote control capable
- Fastest actuation
Electric Actuator
Symbol: Motor symbol (circle with M) above valve
(M) ← Motor
|
◁▷
Indicates:
- Electric motor driven
- Precise positioning
- No air supply needed
Hydraulic Actuator
Symbol: Solid circle or cylinder symbol
● ← Hydraulic
|
◁▷
Indicates:
- Hydraulic fluid powered
- High force capability
- Slower than pneumatic
Fail-Safe Position Symbols
Fail Closed (FC)
Symbol: Line from actuator with “FC” or arrow pointing down
┌───┐
│ FC│
└─↓─┘
◁▷
Meaning:
- Valve closes on power/air loss
- Safety position is closed
- Common for isolation valves
Fail Open (FO)
Symbol: Line from actuator with “FO” or arrow pointing up
┌───┐
│ FO│
└─↑─┘
◁▷
Meaning:
- Valve opens on power/air loss
- Safety position is open
- Common for cooling water
Fail Last Position (FLP)
Symbol: “FLP” or “FL” notation
┌───┐
│FLP│
└─┬─┘
◁▷
Meaning:
- Valve stays in current position
- Locked on failure
- Requires manual intervention
End Connection Symbols
Flanged Connection
Symbol: Short perpendicular lines at each end
─┤◁▷├─
Indicates:
- Bolted flange connection
- Removable without cutting pipe
- Most common in process piping
Threaded Connection
Symbol: Open circles at ends
○◁▷○
Indicates:
- Screwed connection
- Temporary or removable
- Smaller sizes (≤2″)
Welded Connection
Symbol: Filled squares at ends
■◁▷■
Indicates:
- Permanent welded connection
- Butt weld
- Cannot remove without cutting
Socket Weld Connection
Symbol: Open squares at ends
□◁▷□
Indicates:
- Socket welded
- Smaller sizes
- Stronger than threaded
Process Line Symbols
Different Pipe Types
Main process line:
━━━━━ (Solid single line)
Pneumatic signal:
─ ─ ─ (Dashed line)
Electric signal:
─ ─ ─ ─ (Dashed line with dots)
Hydraulic line:
━━━━━ (Thick solid line)
Pipe Crossing (Not Connected)
Method 1: Break one line
|
────┘ └────
|
Method 2: Add hump
|
────⌢────
|
Important: This is NOT physical representation—pipes may not actually cross in the system.
Valve Tag Numbering
Understanding Tag Numbers
Based on STANDARD ANSI/ISA S5.1, the P&ID uses identification tags to label instruments and valves.
Tag structure:
FCV-028
│││ │
│││ └─ Loop/Sequence Number
││└──── Function (Valve)
│└───── Modifier (Control)
└────── Measured Variable (Flow)
Common first letters:
- F = Flow
- P = Pressure
- T = Temperature
- L = Level
- A = Analysis
Common second/third letters:
- C = Control
- I = Indicator
- T = Transmitter
- V = Valve
- S = Switch
Examples:
- FCV-101 = Flow Control Valve #101
- PV-205 = Pressure Valve #205
- TCV-308 = Temperature Control Valve #308
- LV-412 = Level Valve #412
Special Valve Symbols
3-Way Valve
Symbol: Three triangles meeting at center or T-shape
|
◁┼▷
|
Usage:
- Flow diversion
- Mixing service
- One inlet, two outlets (or reverse)
4-Way Valve
Symbol: Four triangles in cross pattern
|
───┼───
|
Usage:
- Flow switching
- Reversing service
- Hydraulic systems
Block and Bleed Valve
Symbol: Two valves with bleed between (often marked DBB)
◁▷─◁▷
↓
Usage:
- Double isolation
- Bleed port for testing
- Safety critical applications
How to Read P&ID Valve Symbols
Step-by-Step Process
1. Identify the basic valve symbol
- Look for two triangles pointing toward each other
- This is the base valve symbol
2. Check the center marking
- Vertical line = Gate valve
- Filled circle = Globe valve
- Filled diamond = Ball valve
- Empty circle = Plug valve
3. Look for actuator symbols above valve
- Rectangle = Pneumatic
- Circle with M = Electric
- Circle = Hydraulic
- Handwheel = Manual
4. Check for fail-safe notation
- FC = Fail closed
- FO = Fail open
- FLP = Fail last position
5. Read the tag number
- First letter = Process variable (F, P, T, L)
- Middle letters = Function (C, I, V)
- Number = Loop identification
6. Check end connections
- Lines = Flanged
- Circles = Threaded
- Squares = Welded
7. Note flow direction
- Follow arrows on process lines
- Check valve orientation (especially check valves)
8. Refer to legend
- Always check drawing legend
- Standards may vary by company
- Special symbols explained there
Common Mistakes to Avoid
Installation Errors
Mistake 1: Installing check valve backwards
- Arrow shows permitted flow direction
- Installing wrong way causes failure
- Always verify before welding
Mistake 2: Ignoring actuator type
- Wrong assumptions about automation
- Air supply may not be available
- Electric vs. pneumatic crucial
Mistake 3: Misreading similar symbols
- Globe (filled circle) vs. plug (empty circle)
- Gate (line) vs. needle (thin line)
- Always cross-check with legend
Mistake 4: Forgetting fail-safe position
- Critical for safety analysis
- Affects process during shutdown
- Must match process requirements
Mistake 5: Assuming symbol scale
- Symbols NOT to scale
- Cannot determine size from drawing
- Check line list for actual sizes
Best Practices
For Creating P&IDs
1. Use standard-compliant symbols
- Follow ISA, ISO, or company standards
- Be consistent throughout project
- Document any deviations
2. Include comprehensive legend
- Define all symbols used
- Note any non-standard representations
- Update legend as project evolves
3. Use clear tag numbering
- Follow logical sequence
- Group related components
- Make numbering system obvious
4. Show all necessary information
- Actuator types
- Fail-safe positions
- Connection types
- Flow directions
5. Cross-reference with other documents
- Line lists
- Valve data sheets
- Equipment lists
- Instrument index
For Reading P&IDs
1. Always check the legend first
- Different companies use different symbols
- Standards vary by industry
- Custom symbols explained
2. Understand flow direction
- Follow arrows carefully
- Trace flow paths
- Identify dead-ends
3. Read control loops
- Start with measured variable
- Follow to transmitter
- Trace to controller
- End at control valve
4. Check for redundancies
- Backup systems
- Double block and bleed
- Safety interlocks
- Emergency shutdowns
5. Cross-reference sizes and materials
- Symbols show type, not size
- Check line designation tables
- Verify pressure ratings
- Confirm materials of construction
Software and Tools
P&ID Drawing Software
Common tools:
- AutoCAD P&ID
- SmartPlant P&ID (Intergraph)
- AVEVA Diagrams
- Visio (Microsoft)
- Lucidchart
Features to look for:
- Standard symbol libraries
- Automatic tag numbering
- Data integration
- Error checking
- Export capabilities
Frequently Asked Questions
Why do valve symbols vary between companies?
While ANSI/ISA-5.1-2009 provides standards, companies often develop their own variations based on industry practices, legacy systems, or specific needs. As long as symbols are consistent within a project and clearly defined in the legend, variations are acceptable.
What’s the difference between P&ID and isometric drawings?
P&IDs show functional relationships and control logic using standardized symbols. Isometric drawings show physical pipe routing in 3D perspective with actual dimensions and elevations. P&IDs are for understanding the system; isometrics are for installation.
How do I know which standard to use?
Follow the project specification or company standard. North American projects typically use ANSI/ISA standards, international projects use ISO standards, European projects may use DIN standards. Always check project requirements.
Can I use P&ID symbols without following standards exactly?
Yes, as long as you’re consistent and provide a clear legend. Many companies modify standard symbols for clarity or to match their specific needs. The key is consistency and clear documentation within your project.
What do the letters in valve tags mean?
The first letter indicates the measured/controlled variable (F=Flow, P=Pressure, T=Temperature, L=Level). Following letters indicate function (C=Control, I=Indicator, V=Valve). Numbers identify the specific loop or sequence. Example: FCV-101 is Flow Control Valve number 101.
How are normally open vs normally closed valves shown?
This is typically shown through the fail-safe notation (FO = Fail Open, FC = Fail Closed) rather than the valve symbol itself. Some drawings also use text annotation like “NO” (Normally Open) or “NC” (Normally Closed) near the valve.
Do I need special training to read P&IDs?
Basic P&ID reading can be learned quickly, but full proficiency requires understanding process engineering concepts, control systems, and industry practices. Many companies provide training, and various online courses and certifications are available.
Are valve symbols the same in electrical drawings?
No. Electrical drawings use completely different symbols. P&ID symbols are specific to process and instrumentation diagrams. Don’t confuse P&ID valve symbols with hydraulic schematic symbols, which also differ.
Conclusion
Understanding valve symbols in P&ID drawings is essential for anyone working in process industries. The basic principle is simple: two triangles pointing toward each other form the generic valve, with different center markings indicating specific valve types.
Key takeaways:
Basic structure:
- ✅ Two triangles = Generic valve base
- ✅ Center marking = Valve type
- ✅ Top symbol = Actuation method
- ✅ End markings = Connection type
Remember:
- Always check the drawing legend first
- Symbols are NOT to scale
- Standards vary between companies
- Tag numbers provide critical information
- Flow direction is shown by arrows
For success:
- Learn standard symbols (ANSI/ISA-5.1-2009)
- Understand tag numbering system
- Read legends carefully
- Cross-reference with other documents
- Ask questions when uncertain
Whether you’re an engineer designing systems, a technician installing equipment, or a student learning the basics, mastering P&ID valve symbols opens the door to understanding complex process systems. These symbols are the universal language of process engineering, enabling clear communication across disciplines and borders.
