P&ID Symbols : How to read P&ID Drawing

What are P&ID Symbols ?

Piping and Instrumentation Diagram (P&ID) symbols are graphical representations used in the design and documentation of process plants. These symbols represent various equipment, piping, and instrumentation details in a complex process flow. P&IDs are vital in the maintenance and modification of the process that they describe.

Here are some common categories and examples of P&ID symbols:

1. Piping P&ID Symbols

  • Lines: Solid lines for process pipes, dashed lines for instrument signals, etc.

Lines: Solid lines for process pipes, dashed lines for instrument signals, etc.

  • Valves: Gate valve, globe valve, check valve, ball valve, butterfly valve, etc.

2. Equipment P&ID Symbols

Equipment P&ID Symbols

  • Pumps: Centrifugal pump, gear pump, diaphragm pump, etc.
  • Tanks and Vessels: Storage tanks, reactors, columns, drums, etc.
  • Heat Exchangers: Shell and tube, plate type, air-cooled, etc.

3. Instrumentation P&ID  Symbols

  • Sensors and Detectors: Temperature sensor, pressure gauge, flow meter, level sensor, etc.
  • Controllers and Transmitters: PID controller, I/P converter, transmitters, etc.

4. Valve P&ID Symbols

  • Different types of valves are represented by specific symbols, often indicating the valve type and actuation method.

5. Flow Elements P&ID Symbols

  • Flow Meters: Orifice plate, venturi meter, rotameter, etc.

6. Process Control P&ID Symbols

Process Control P&ID Symbols

  • Symbols representing control loops, interlocks, and other control systems components.

Each symbol in a P&ID is standardized and follows specific industry standards, such as those from the International Society of Automation (ISA), the American National Standards Institute (ANSI), and the Instrumentation, Systems, and Automation Society (ISA). These standards help ensure that P&IDs are universally understandable by engineers and technicians across different companies and countries.

Piping P&ID Symbols Lines

Piping and Instrumentation Diagrams (P&ID sysbols) are crucial in the engineering and operation of process systems, providing a detailed graphical representation of the piping and instrumentation. Here’s an overview of some common piping symbols used in P&IDs:

1. Lines

  • Solid Lines: Represent process pipes that carry fluids. These lines are the main arteries of the process system, showing the direction of flow and connections between equipment.
  • Dashed Lines: Often used to represent instrument signals and connections that are not physical pipes. These can indicate electrical, pneumatic, or hydraulic connections between instruments and control systems.

2. Pipe Types and Connections

  • Straight Pipe: Represented by a simple straight line, indicating a section of pipe.
  • Reduced or Expanded Pipe: Shown with two parallel lines that converge or diverge, indicating a change in pipe diameter.
  • Fixed Pipe (Rigid): A straight line, similar to a standard pipe, but may have annotations to indicate rigidity.
  • Flexible Hose: Depicted with a series of curved or wavy lines, indicating a flexible connection that can accommodate movement.

3. Fittings

  • Elbow: Represented by a curved line or an angled line (usually 45° or 90°), indicating a change in the pipe direction.
  • Tee: Shown as a “T” shape, representing a junction where a pipe splits into two directions or two pipes join into one.
  • Cross: Illustrated by a “+” shape, indicating a four-way junction.
  • Reducer/Expander: Depicted by two lines of different widths coming together, representing a change in pipe size.

4. Valves

Valves are critical components used to control the flow of fluids. They come in various types, each designed for specific functions:

  • Gate Valve: Symbolized by a general gate valve symbol, it’s used where a straight flow of fluid and minimum restriction is desired.
  • Globe Valve: Characterized by a globular-shaped body, this valve is used for regulating flow in a pipeline. It’s represented by a symbol that includes a spherical shape within the valve body.
  • Check Valve: Allows flow in one direction and prevents backflow. It’s symbolized by a single swing check valve symbol, often with an arrow indicating the direction of flow.
  • Ball Valve: Known for quick shut off, represented by a ball inside a valve body. It’s ideal for on/off control without pressure drop.
  • Butterfly Valve: Used for isolating or regulating flow, this valve is represented by a symbol resembling a butterfly because of the disc that moves around a central axis.

Each valve symbol can be further detailed with annotations or additional symbols to indicate specific characteristics like actuation method (manual, pneumatic, electric, etc.), normally open or normally closed position, and fail-safe position in case of loss of actuating power.

5. Flanges and Joints

  • Flanged Joint: Illustrated by a pair of short lines perpendicular to the pipe at the connection point, indicating a flanged connection.
  • Welded Joint: Often represented by a small filled circle or by the absence of a symbol at the connection point, indicating a welded joint.
  • Screwed Joint: Shown with small circles or dots along the pipe connection, indicating a threaded connection.

6. Special Items

  • Sight Glass: Represented by two short, parallel lines perpendicular to the pipe, indicating a window for observing the flow.
  • Strainer or Filter: Shown with a symbol resembling a small diamond or an annotated symbol, indicating a device for removing particulates from the flow.
  • Trap: Illustrated by a specific symbol, often resembling a “U” shape or an annotated symbol, indicating a device for trapping substances while allowing others to pass.

These piping symbols are standardized to ensure consistency and clarity across engineering drawings and P&IDs. Familiarity with these symbols allows engineers, technicians, and operators to understand the layout, components, and functionality of a piping system at a glance.


Understanding these symbols is essential for anyone involved in the design, operation, or maintenance of process systems, as they provide a standardized way of representing the complex network of piping and instrumentation.

Equipment P&ID Symbols

Centrifugal pump, gear pump, diaphragm pump, etc. Tanks and Vessels: Storage tanks, reactors, columns, drums, etc. Heat Exchangers: Shell and tube, plate type, air-cooled, etc

In Piping and Instrumentation Diagrams (P&IDs), equipment symbols are crucial for depicting various types of machinery and storage units used in the process industry. Here’s an overview of common equipment symbols:

1. Pumps

Pumps are devices used to move fluids and increase fluid pressure. Common pump symbols include:

  • Centrifugal Pump: Represented by a symbol with a circle and one or two triangles pointing outwards, indicating the direction of fluid movement. It’s used for liquids and is known for its variable flow rate and head.
  • Gear Pump: Shown with a symbol that includes two meshing gears, representing its internal mechanism. It’s used for pumping viscous liquids at a constant flow rate.
  • Diaphragm Pump: Symbolized by a circle with a diagonal line through it, often with an additional symbol to indicate the diaphragm mechanism. It’s used for pumping fluids that may contain solids or need to be isolated from the pump components.

2. Tanks and Vessels

Tanks and vessels store fluids, gases, or solids and are integral to many processes. Symbols include:

  • Storage Tanks: Typically represented by a simple cylinder or rectangular shape, sometimes with a fixed roof or floating roof to indicate the type of storage.
  • Reactors: Symbolized by a tank with additional features like agitators (shown as a small circle with a line) to indicate mixing capabilities, used for chemical reactions.
  • Columns: Shown as a vertical cylinder, often with internal lines to indicate trays or packing, used for distillation or gas absorption processes.
  • Drums: Similar to tanks but often smaller, used for temporary storage or separation of liquids and solids, with symbols that might include additional details like outlets or weirs.

3. Heat Exchangers

Heat exchangers transfer heat between two or more fluids. Common types include:

  • Shell and Tube Heat Exchanger: Symbolized by a large rectangle with a smaller rectangle inside, often with lines indicating the tubes. Used in a wide range of industries for heating or cooling processes.
  • Plate Type Heat Exchanger: Represented by a series of closely spaced lines, indicating the plates where heat exchange occurs. Known for their efficiency in heat transfer and compact size.
  • Air-Cooled Heat Exchanger: Shown with a symbol that includes fan blades to indicate air cooling, used where water is scarce or expensive.

These symbols provide a standardized way to represent the equipment in process diagrams, allowing engineers, operators, and maintenance personnel to understand and communicate about the process system efficiently.

Instrumentation P&ID Symbols

Temperature sensor, pressure gauge, flow meter, level sensor, etc. Controllers and Transmitters: PID controller, I/P converter, transmitters, etc.


Sensors and Detectors:

  • Temperature Sensor: Often symbolized by a circle with “T” inside, indicating a device that measures temperature.
  • Pressure Gauge: Depicted by a circle with “P” inside, used to measure the pressure within pipes or vessels.
  • Flow Meter: Shown as a circle with an “F” inside, representing a device that measures the flow rate of the fluid.
  • Level Sensor: Illustrated by a circle with an “L” inside, used to measure the level of material within tanks or vessels.

Controllers and Transmitters:

  • PID Controller: Symbolized by a circle with “PID” inside, representing a controller that uses Proportional, Integral, and Derivative control actions.
  • I/P Converter: Shown as a circle with “I/P” inside, indicating a device that converts current signals to pneumatic signals.
  • Transmitters: Depicted by a circle with the measured variable letter and a line extending from the circle, indicating the device transmits the measurement signal to a control system.

These symbols are standardized and used internationally to ensure consistency and clarity in P&ID documentation, aiding in the design, operation, and maintenance of process systems.

Valve P&ID Symbols Different types of valves

Valve symbols on Piping and Instrumentation Diagrams (P&IDs) are essential for indicating not only the type of valve but also details about its actuation method. Here’s a deeper look into how different types of valves and their actuation methods are represented:

Valve Types

  • Gate Valve: Typically symbolized by a simple gate shape, indicating a valve that can be opened by lifting a round or rectangular gate/wedge out of the path of the fluid.
  • Globe Valve: Represented by a spherical shape, showing the internal mechanism that moves perpendicular to the flow, regulating the flow rate.
  • Check Valve: Symbolized by a half-circle that indicates the flow direction, allowing fluid to flow in one direction and preventing backflow.
  • Ball Valve: Illustrated with a ball inside the valve body, indicating a valve that uses a spherical disc to control flow. The position of the ball (parallel or perpendicular to the flow) shows whether the valve is open or closed.
  • Butterfly Valve: Depicted by a disc or “butterfly” inside a circle, representing a valve that uses a rotating disc to regulate flow.

Actuation Methods

  • Manual: Often indicated by a simple lever or handwheel symbol added to the valve symbol, showing that the valve is operated manually.
  • Pneumatic: Represented by a small air symbol or a diaphragm dome above the valve symbol, indicating that the valve is actuated by air pressure.
  • Hydraulic: Illustrated by a small fluid symbol or a cylinder above the valve symbol, indicating actuation by hydraulic fluid pressure.
  • Electric: Shown with a small motor symbol or an “E” near the valve symbol, indicating an electric motor is used for valve actuation.
  • Solenoid: A small coil symbol or an “S” near the valve symbol represents a solenoid-operated valve, typically used for on/off control.

Valve symbols can also include additional details to represent specific characteristics, such as:

  • Normally Open (NO) or Normally Closed (NC): Indications on the valve symbol can show its default state when not actuated.
  • Fail-Safe Position: Additional symbols or annotations might indicate the valve’s position in case of a failure, such as “FO” (Fail Open) or “FC” (Fail Closed).

Understanding these symbols is crucial for anyone involved in the engineering, maintenance, or operation of piping systems, as they provide clear and concise information about the control and flow direction within the system.

Flow Elements Flow Meters P&ID Symbols

Flow elements in Piping and Instrumentation Diagrams (P&IDs) are crucial for measuring the rate of flow within the piping system. These elements come in various types, each suited to specific applications and fluid characteristics. Here’s an overview of the symbols for common flow meters:

Flow Meters P&ID Symbols

  • Orifice Plate: Symbolized by a circle with a concentric ring inside, representing the orifice plate itself, which creates a pressure drop that is proportional to the flow rate. This setup usually includes a differential pressure sensor symbol connected to both sides of the orifice plate to measure the pressure drop.
  • Venturi Meter: Illustrated with a long, narrow “V” shape within a pipeline symbol, indicating the venturi tube’s characteristic narrowing and subsequent expansion. This narrowing causes a pressure drop, which is used to measure flow rate. Like the orifice plate, it’s often shown with differential pressure sensors connected across the constriction.
  • Rotameter: Depicted by a vertical, tapered tube with a float inside, representing the float’s position within the tube, which varies with the flow rate. The symbol may also include a line representing the scale against which the float position is read.

These symbols provide clear and standardized representations of flow meters in a process system, allowing engineers and technicians to understand the system’s flow measurement points at a glance. Each flow meter type has its advantages and is selected based on factors such as the fluid type, required accuracy, flow range, and process conditions.

Process Control P&ID Symbols

Process control symbols in Piping and Instrumentation Diagrams (P&IDs) are vital for illustrating the control strategies and interlocks implemented within a process system. These symbols represent the various components of control loops and systems, providing insights into how process variables like temperature, pressure, flow, and level are monitored and adjusted to maintain desired operational conditions. Here’s a brief overview of some common process control symbols:

Control Loops and Systems P&ID Symbols

  • Control Loops: Typically represented by a series of interconnected symbols that include sensors (for measuring process variables), transmitters (for sending the measured signal to a controller), controllers (which compare the measured value to a setpoint and generate an output signal), and control elements (like valves or variable speed drives) that adjust the process based on the controller’s output.
  • Controllers: Symbolized by a circle containing letters that define the type of control action (e.g., “PID” for Proportional-Integral-Derivative control). The circle may have multiple lines connecting to it, representing input signals from sensors and transmitters, as well as output signals to control elements.
  • Interlocks: Represented by dashed lines or specific symbols connecting the components they influence, indicating the safety or operational conditions that must be met for certain actions to occur. Interlocks ensure that certain parts of the process can only operate under safe conditions.
  • Control Valves: Often depicted with additional symbols to indicate the type of actuation (manual, pneumatic, electric) and the fail-safe position (fail-open, fail-closed). The control valve symbol is usually connected to a controller symbol with a solid or dashed line, representing the control signal.
  • Signal Lines: Solid lines represent physical connections like piping or wiring, while dashed lines indicate non-physical connections such as wireless signals or logical connections in a control system. These lines show how signals flow between different components of the control system.
  • Alarms and Indicators: Represented by specific symbols or annotations near the measurement or control points, indicating where alarms (for alerting operators to abnormal conditions) and indicators (for displaying process conditions) are located within the system.

Understanding these symbols and their arrangements on P&IDs is crucial for anyone involved in the design, operation, or maintenance of a process system, as they provide a clear and concise way to understand the control strategies implemented in the plant.

How to read P&ID drawing

Reading Piping and Instrumentation Diagrams (P&IDs) is a fundamental skill in engineering, process operations, and maintenance. P&IDs provide a graphical representation of the mechanical and control interconnections within a process system.

Here’s a step-by-step guide to understanding P&IDs:

1. Familiarize with P&ID Symbols

  • Start by learning the basic symbols used in P&IDs, such as those for pipes, valves, pumps, heat exchangers, tanks, and instrumentation. Each symbol provides information about the component it represents, like its type and function.

2. Understand the Lines

  • Identify the different types of lines used, including solid lines for process piping, dashed lines for instrument signals, and double lines for electrical connections. The thickness and style of the lines can also indicate the type of fluid or the state of the material (e.g., liquid, gas).

3. Read the Tags and Labels

  • Each component on a P&ID is typically tagged with an alphanumeric code that provides specific information about the component. This tag can indicate the type of equipment, its sequence in the process, and sometimes its location. For example, P-101 might be the first pump in a process.
  • Process lines are often labeled with the fluid they carry and may include flow direction arrows.

4. Review the Legend

  • P&IDs usually come with a legend or key that explains the symbols, line types, and abbreviations used in the diagram. Refer to this legend to decode any unfamiliar symbols or notations.

5. Identify Major Equipment

  • Locate and identify the major equipment items first, such as reactors, distillation columns, tanks, and major pumps. Understanding the primary processing units will help you grasp the overall function of the system.

6. Follow the Process Flow

  • Trace the flow of materials through the process. Start from raw material input points and follow through to product output, waste streams, and recycling loops. This will help you understand how different components are interconnected.

7. Examine Control Loops and Interlocks

  • Identify control loops, which include sensors, transmitters, controllers, and final control elements like control valves. Understanding these loops will give you insight into how the process is regulated.
  • Look for safety and interlock systems designed to prevent hazardous conditions.

8. Review Ancillary Systems

  • Don’t overlook auxiliary systems like steam traps, vents, drains, and sampling connections, as these are important for the operation and maintenance of the process.

9. Consult Supporting Documentation

  • P&IDs are often part of a larger set of engineering documents. Review related documents like process flow diagrams (PFDs), equipment specifications, and instrument datasheets for a more comprehensive understanding.

10. Ask Questions

  • If you’re learning or unsure about something in the P&ID, don’t hesitate to ask experienced colleagues or refer to engineering standards and textbooks. P&IDs can be complex, and collaboration is key to understanding and troubleshooting process systems.

Practicing these steps will enhance your ability to read and interpret P&IDs, which is crucial for designing, operating, and maintaining process systems effectively.

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