Piping and instrumentation diagrams (P&IDs) serve as crucial blueprints for systems involving fluids and gases in the world of industrial engineering and process design. These detailed diagrams represent the functional relationships of piping, equipment, and instrumentation within a process plant. Valves are one of the key components in a P&ID, as they regulate the flow and pressure of fluids throughout the system. They help engineers, operators, and technicians visualize and interpret the purpose and functioning of a system. Therefore, understanding these symbols is essential for accurate system design, operation, troubleshooting, and maintenance.

In this blog, we will explore the P&ID valve symbols — what they are, their functions, why they are important, and how to read and interpret them, along with best practices for reading these symbols effectively.

Valves play an essential role in:

• Starting and stopping flow
• Regulating pressure or temperature
• Controlling fluid direction
• Protecting equipment from overpressure
• Isolating equipment for maintenance

Since different valves serve distinct purposes and have unique operational characteristics, their accurate representation on P&IDs is necessary to:

• Understand process logic
• Ensure operational safety
• Optimize maintenance procedures
• Supporting automation and controlling logic

Valve symbols represent the type, function, and sometimes actuation method of a valve. Most symbols consist of:

• Body shape indicating valve type.
• Lines and arcs suggesting flow direction or control.
• Additional markers for automation, manual operation, or remote control.

A well-crafted P&ID symbolically communicates all necessary valve attributes without text clutter.

Understanding Valve Tags and Labels

Valve symbols are often accompanied by tags that provide more context. A typical valve tag might look like: “V-101”

Where: V: Indicates a valve & 101: A unique number in the system

Some companies use naming conventions based on plant area, medium, or function. Tags may also indicate the valve type or material, like:

• VGV: Vent Gas Valve
• FCV: Flow Control Valve
• SDV: Shut Down Valve
• MOV: Motor Operated Valve

Valve tags in digital P&IDs are usually linked to a database, enabling faster access to specs, maintenance data, and vendor details.

Let’s break down the most frequently used valve symbols in P&IDs and how they’re represented:

Types of Valves

2-Way Valves:

A 2-way, on/off valve is represented by two equilateral triangles pointing towards each other. Different types of lines are used to represent different valve types, and the direction of the flow is indicated by an arrow at the end of the line.

The most depicted 2-way valves include:

• Ball Valve
• Butterfly Valve
• Plug Valve
• Gate Valve
• Globe Valve
• Pinch Valve
• Needle Valve
• Diaphragm Valve

3-Way & 4-Way Valves:

For multi-port valves, extra triangles are added to the symbol. L-port and T-port valves are shown with lines inside the ball symbol, and the flow path is indicated with small arrows next to the symbol.

S.No.	Name/Type of Valve	Purpose	Variants
1	Gate Valve	On/off service; minimal pressure drop	Rising stem gate valve, Non-rising stem gate valve
2	Globe Valve	Flow regulation	Angle globe, Straight globe
3	Ball Valve	Quick shut-off, low torque operation	Full port, Reduced port
4	Butterfly Valve	Flow regulation; compact design	Lug-type, Wafer-type
5	Check Valve	Prevents backflow	Swing check, Lift check, Ball check
6	Diaphragm Valve	Corrosive fluid control
7	Plug Valve	Quick shut-off
8	Needle Valve	Precise flow control
9	Pressure Relief Valve / Safety Valve	System protection from overpressure	Pressure Safety Valve (PSV), Pressure Relief Valve (PRV), Rupture disc (depicted differently)
10	Control Valve	Automated flow control based on process inputs	Flow control (FCV), Pressure control (PCV), Temperature control (TCV)

Valve States

Valves can be classified based on how they behave when no power or control signal is applied. The three main types are:

• Normally Open (NO): The valve stays open when it’s not powered or actuated.
• Normally Closed (NC): The valve stays closed when it’s not powered and only opens when it’s actuated.
• Bistable: These valves hold their position even if power is lost. They need a separate action to change their state—like a latching solenoid valve that stays in its last position until it gets a new command.

There are also variations of these, such as valves that are usually open or usually closed under typical operating conditions.

Though it is important to note that just because a valve symbol is filled in or blacked out doesn’t automatically mean it’s NO or NC—these visual cues are often used interchangeably and can be confusing. A much clearer approach is to label valves directly with “NO” (Normally Open) or “NC” (Normally Closed), as shown in our symbol examples.

Actuator Types and Symbols

Valves in P&IDs are often associated with actuators which automate valve operation. Common actuator types and their P&ID symbols include:

Actuator Type	Symbol Representation	Application
Manual (Handwheel)	Simple circle or line over the valve symbol	Manual operations
Electric Actuator	Box symbol with ‘E’ or lightning bolt	Remote electrical control
Pneumatic Actuator	Box with arrow and “P” or airflow lines	Air-operated control
Hydraulic Actuator	Box with ‘H’ or fluid arrows	High-pressure control
Solenoid Actuator	Box with solenoid coil symbol (zigzag or loop)	Fast operation for on/off flow

Fail-Safe Position

Certain actuators are made to automatically shift the valve to a safe open or closed position if power or control is lost. This is known as the fail-safe position. Usually, diagrams with an arrow connecting the valve and actuator display this. “FO” (Fail Open) or “FC” (Fail Closed) are additional letters often used to denote this.

End Connections

Valve end connections can be shown in a few different ways in diagrams:

• Generic Connection: Often, pipes are shown simply running into the valve, like in many standard examples.
• Flanged Connection: Represented by short perpendicular lines at the ends of the pipe near the valve, with a small gap. This shows the valve can be removed without cutting the pipe.
• Threaded Connection: These are semi-permanent and shown with small open (hollow) circles where the pipe connects to the valve.
• Welded Connection: Permanent connections are represented with small solid squares. If it’s a socket weld, the square is hollow instead.

Valves regulate, isolate, or direct flow within a process. As such, they are crucial control elements in any process facility. The correct interpretation of valve symbols in a P&ID:

• Ensures accurate system understanding
• Supports design verification and safety checks
• Assists in maintenance planning
• Prevents operational mishaps
• Facilitates communication between multidisciplinary teams

Different valve types and actuation methods are used depending on the fluid, pressure, temperature, and control needs of the system. Hence, precise representation in P&IDs is essential.

Let’s look at some common valve symbols and functions along with their symbolic representations:

To maintain uniformity and consistency across industries, standard valve symbols are used in P&IDs. The following standards include:

• ISA S5.1 – Instrumentation Symbols and Identification (by International Society of Automation)
• ANSI/ISA-5.1-2009 – Instrumentation Symbols and Identification
• ISO 10628 – Diagrams for the chemical and petrochemical industry
• PIP (Process Industry Practices)

These standards define graphical representations and tag identifiers for various valve types.

To correctly interpret valve symbols on a P&ID:

• Use the P&ID Legend: Always refer to the legend or symbol key provided in the P&ID drawing set. Different companies or projects may use slightly varied standards.
• Understand Flow Direction: Look for arrows or pipeline orientation to understand how flow moves through the valve.
• Read the Control Loop: Check associated instruments like transmitters and controllers to understand how the valve operates in the loop.
• Check for Redundancies: Look for fail-safe setups like DBB, bypass valves, and interlocks. These are critical for process safety and maintenance.
• Cross-Reference with Line Lists: Use line designation tables and valve lists to confirm sizes, pressure classes, and materials.

• Always refer to the legend or key that accompanies P&IDs.
• Use standard-compliant symbols (ISA, ISO, PIP) across all drawings.
• Ensure consistency between P&ID and other deliverables like piping isometrics, layout drawings, and instrumentation data sheets.
• Update valve symbols and tags when system changes occur (MoC compliance).
• Use software tools like AutoCAD P&ID, SmartPlant P&ID, or AVEVA Diagrams for symbol libraries and automatic tagging.

• Misidentifying similar shapes – Ball and plug valves look alike but function differently.
• Ignoring actuator type – Leads to wrong assumptions in automation capabilities.
• Forgetting flow direction – Especially important in check and control valves.
• Overlooking tag codes – These provide context; don’t ignore them.
• Assuming graphical scale – Symbols are not to scale; placement is logical, not physical.

When designing a new facility or upgrading an existing one, it’s crucial to correctly understand valve symbols in P&IDs. A solid grasp of these symbols ensures that the valve specifications match the process needs and operational goals. Valve symbols in P&IDs form the backbone of process and equipment design documentation, playing a critical role across engineering disciplines.

With a skilled multidisciplinary design team offering specialized equipment engineering services, you can trust that every valve is accurately represented and properly integrated. Their deep understanding of different valve types, symbols, and how they fit into the bigger picture helps ensure smooth coordination between design, commissioning, operations, and maintenance teams. By using industry-standard symbols, clear tagging, and advanced design tools, accurate valve representation remains central to efficient and error-free plant engineering.

Our engineering team delivers detailed, code-compliant documentation tailored to your project’s needs.