P&ID Symbols 1 - Symbol Diagram Basics

How to read a P&ID?

1. P&ID vs PFD

Interpreting technical diagrams is often a challenge for young engineers. Identifying and understanding the different symbols that appear on a diagram as well as the connection between the different elements may seem trivial at first but sometimes requires additional knowledge.

This article is a brief introduction to the question of how to read a P&ID and what information is contained on a P&ID. This article will review general concepts and basic rules to offer in the last section a final list of the different identifications and symbologies necessary to make a good interpretation of a diagram.

How to Read a P&ID?

The P&ID starts from the PFD and includes the control loops. If we compare both kinds of documents, PFD and P&ID, the P&ID Diagram has more details about the plant topology and displays equipment (including spare units), pipelines (with details about scheduling and isolation), instrumentation and valves, and signal-related.

So, the P&ID Diagram lets you visualize in just one time the process flow and all the related control functions and monitoring functions.

Other related articles:

If you are interested to know more about subjects like Purpose, Owner and Contents & Conventions, read our free introduction to these concepts in our article P&ID Diagram Basics Part 1 Purpose, Owner and Contents.
If you are interested to know more about subjects like Standards and Guidelines, read our free introduction to these concepts in our article P&ID Diagram Basics Part 2 International Standards.
If you are interested to know more about subjects like Functional Identification and Instrument Numbering, read our free introduction to these concepts in our article P&ID Diagram Basics - Part 3 - Functional Identification and Naming Conventions.

2. What information is contained on a P&ID?

The information usually displayed in P&ID diagrams is:

Plant equipment, normally we will find all types of process pumps, compressors, and any other type of rotating element, such as centrifuges or air coolers. It is possible that in the tanks or vessels we can find the different design levels (maximum, normal, and minimum).
It is a good practice to represent the relative size, shape, and location of the actual equipment in the plant. Pitting, nozzles, and access routes should normally be defined in this type of diagram.

Pitting, nozzles and access routes

Regarding the instrumentation, we will find in this type of diagram all the information related to the connection of the instrument with the control system, either through a current loop connection or through a data bus.
In the P&ID diagrams we will find information on all the process pipes, control valves, and ON / OFF valves, any type of derivatives of the process flow, relief valves, safety valves, drains, and any other type of element or device in line such as, for example, check valves, filters, or reductions/extensions.

Process pipes, control valves and ON / OFF valves

Typically you will also find the design and set pressure of all the relief and safety valves, as well as the same information for the rupture discs, pressure, and temperature regulators. In the case of lines with a layout, we will normally find the type of layout if it is steam or electric, and if it also has an automatic control system, it will appear in the diagram.
When it is necessary to indicate the existence of sloping lines, the slope will normally be displayed. A common case can be the arrangement of the pulse lines of an orifice plate with the corresponding pressure instrument.
In the case of motors, its voltage and power can be displayed or displayed. Usually, the motors have a typical and standard connection diagram for the entire plant.
The logic associated with starting and stopping the motor is usually included in P&ID diagrams.
Elements related to control and instrumentation:
- All instruments are represented by standardized symbols that indicate the location of the equipment, whether it is in the plant or in the technical room, and the type of connection of the instrument with the associated control system.
- Depending on the type of connection and the equipment where it is connected, a different symbol will be used. The P&ID will use symbols and circles to represent each instrument and how they are interconnected in the process.
- For example, the same symbol is not used for an instrument that is connected to a distributed control system (DCS) or for an instrument connected to a Safety Instrumented System. In addition, we will also find represented the signal transmission method and the actions of the control valve in case of air/electrical failure, that is, failure to open (FO), failure to close (FC), or last lock failure position (FL).

More information can be found in our article P&ID Symbol Diagram Basics - Part 1 - Purpose, Owner & Contents.

3. P&ID Diagram Design

General Rules

The design of a P&ID diagram generally follows from left to right in the drawing.

Notes are usually added to the right side of a P&ID above the title block and are used to:

Describe elements of the P&ID.
Provide guidance in understanding the P&ID information.
Make reference to other documents.

Any piece of equipment or instrument should be shown only once in a single P&ID diagram. If it is necessary to show it in more than 1 P&ID diagram, it is necessary to indicate or represent the equipment or instrument through dotted lines or clouds.

3.1 What is included in a P&ID Drawing?

Shows the relative location of process equipment, sensors, and actuators in a process.
Conceptual outline of a chemical plant.
Provide a common language for discussing a plant.
Show control connections between sensors and actuators.

3.2 What is not included in a P&ID Drawing?

Not an architectural diagram of a process. Positions in a P&ID do not correspond to a 3D position, but more connectivity.
Not to scale.
Not a diagram of the reaction kinetics.
Not a control diagram (block diagram), influence graph, incidence graph, Bayesian network, or correlation network.

Reactors, tanks, vessels, and equipment are not drawn to scale.

What do we mean? The elements present in the diagram are not to scale, which implies that it is possible to find that the location of some elements, for example, drains, is not in their exact position.

3.3 Direction of Flow

In general, the main direction of flow proceeds from left to right and from top to bottom. Inlet and outlet arrows are used for indicating the inlet and outlet of flows into or out of the diagram. Arrows are incorporated in the line for indicating the direction of the flows within the flow diagram. If necessary for proper understanding, arrows may be used at the inlets to equipment and machinery (except for pumps) and upstream of pipe branches.

Process lines entering and leaving the diagram from/to other drawings in the Unit shall be terminated at the left-hand or right-hand side of the drawing.

Process lines entering and leaving the diagram

3.4 Crossing Lines

To obtain a clear representation, different line widths shall be used. Main flow lines or main piping shall be highlighted.

To indicate that two lines are connected we will use the diagram on the left, to indicate that there is no connection between the lines we will use the diagram on the right.

Connected Lines

Non-Connected Lines

As shown in the figure below, the pipe and signal lines, also known as the major and minor lines, should be broken according to the hierarchy in the order of major-minor and primary-secondary lines, when they cross.

Crossing lines - Walker, 2009

3.5 Instrument Locations

Instrumentation symbols reflect process instruments, for example, transmitters, gauges, valves, analyzers, or controllers.

Instrument functions and locations - Example of a typical implementation

These symbols and their corresponding labels identify the instrument's functions and their locations. To learn more about the different instrument naming systems you can read our article:

P&ID Symbol Diagram Basics - Part 3 - Functional Identification and Naming Conventions.

The following figure from the ISA standard document lists common instrumentation symbols.

Instrument functions and locations - Common instrumentation symbols

As shown in the previous figure, we can find 4 types of graphical shapes, such as circles, squares, hexagons, and diamonds, possess specific denotation.

A circle indicates a device is located in the field area of a plant.

P&ID Diagram - Instrument Representation

A circle with an external square represents devices or functions which belong to a shared display and control system, for example a DCS system.

P&ID Diagram - DCS Representation

A hexagon represents a computer function.
A diamond with an external square defines functions within a programmable logic controller, for example an alarm configured in a SIS (Safety Instrumented System).

Regarding the lines inside the symbols:

No horizontal line - located somewhere in the field, probably close to the general area shown on the P&ID.
Single horizontal line - One line inside means the device is located in the central control room or some computer screen in the main control room.
Two lines inside indicate the device is located in a local panel.
Dotted line inside means the device is installed behind or inside of a panel and inaccessible to the operator (normally not in view).

4. Legend Sheets

Also called "Lead Sheets" or “Master Sheets”, this kind of documents are required to explain line identifications and describe all the symbols used on P&IDs. The detail shown on such a legend P&ID will vary with corporate culture, but typically includes these main sections:

Line Identification
Equipment Identification
Instrument Identification
Fluid Identification
Other Legends
Valve Positions
Tracing & Insulation Codes
Transducer Functions
Line symbols
Instrumentation symbols
Valve symbols
Vents & Drains symbols
Equipment
Piping & Connecting Shapes
Tags & Labels

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Information and Definitions

Actuator Symbols
Actuator symbols in P&ID diagrams represent the mechanical devices that control the position of valves or other components in response to a control signal. These symbols indicate the type of actuator used, such as pneumatic, electric, or hydraulic, and help identify how the system will execute control actions. Understanding actuator symbols is essential for interpreting how automation and manual controls interact within the process, ensuring proper design, installation, and operation of control elements in the system.

Control Valve Symbols
Control valve symbols are standardized graphical representations used in P&ID diagrams to indicate valves that regulate fluid flow based on control inputs. These symbols show not only the presence of a control valve but also the specific type, such as globe, ball, or butterfly, and how the valve is actuated. By reading these symbols, engineers can understand the valve's role in the process, the control strategy applied, and how it integrates into the overall system design to maintain flow, pressure, or temperature control.

Instrument Symbols
Instrument symbols in a P&ID are graphical codes that represent measuring and control devices such as sensors, transmitters, controllers, and indicators. These symbols are based on standardized conventions and provide key information about the function of the instrument, its location (field or panel), and the process variable it measures or controls. Accurate use of instrument symbols helps ensure that process operations are clearly documented, making it easier to design, operate, and troubleshoot complex systems.

Line Symbols
Line symbols in P&IDs define the different types of connections between equipment and instruments, such as process piping, electrical signals, pneumatic tubing, or data communication lines. Each line type is represented by a specific style—solid, dashed, dotted, or combinations—that conveys the nature and direction of flow or signal. Recognizing line symbols is critical for understanding how the system components are interconnected and how signals or materials move throughout the process.

Measurement Symbols
Measurement symbols are graphical indicators in P&ID diagrams that show devices used to monitor process variables like pressure, temperature, flow, or level. These symbols identify what is being measured and whether the device includes additional functions such as indication, recording, or control. Proper interpretation of measurement symbols ensures accurate understanding of the monitoring system, supports system design and diagnostics, and allows for efficient placement and calibration of instrumentation.

P&ID Symbol Standards
P&ID symbol standards are formalized guidelines, such as those from ISA or ISO, that define the shapes, codes, and conventions used to represent equipment, instruments, and control elements in diagrams. These standards ensure consistency, clarity, and universal understanding across different disciplines and organizations. Following these standards is essential for effective communication between engineers, operators, and technicians and helps maintain accuracy and safety in system design and documentation.

Piping Symbols
Piping symbols in P&IDs are graphical representations that show the layout and type of piping used in a process system, including details like diameter, material, and insulation. These symbols may also indicate piping components such as reducers, elbows, tees, and flanges. Understanding piping symbols helps engineers visualize how fluids flow through the plant, identify critical connections, and design systems that meet operational and safety requirements.

Signal Line Symbols
Signal line symbols in a P&ID identify the types of signals exchanged between instruments and control devices, such as electrical, pneumatic, hydraulic, or digital signals. Different line styles—such as dashed for electrical or dotted for pneumatic—are used to differentiate signal types and show the path and direction of information flow. These symbols are essential for mapping out the control strategy and ensuring that signals are correctly routed and interpreted during design, installation, and troubleshooting.

Tagging in Symbols
Tagging in symbols refers to the use of alphanumeric codes placed near P&ID symbols to uniquely identify and describe each instrument, valve, or piece of equipment. These tags follow naming conventions that reflect function, loop number, and device type, providing quick reference and traceability across documents and systems. Accurate tagging ensures that each component can be clearly identified during design, maintenance, and operation, enhancing system integrity and simplifying communication across departments.

Valve Symbols
Valve symbols are graphical representations used in P&ID diagrams to indicate the presence and type of valves in a piping system. These symbols convey whether the valve is manual or automated, its function (such as on/off, check, or relief), and specific design types like gate, ball, or diaphragm. Proper interpretation of valve symbols allows engineers to understand flow control strategies, ensures correct installation, and supports maintenance planning throughout the lifecycle of the process system.

P&ID Diagram International Standards References

1 ASME - B31 Series "For Pressure Piping”

2 ASME - B16.1 "Cast Iron Pipe Flange and Flanged Fittings, Class 25, 1st.Ed.,1989”

3 ISA - S5.1 "Instrumentation Symbols and Identification 1st. Ed., 1984”

4 ISA - S5.2 "Binary Logic Diagrams for Process Operations 2nd. Ed., 1981 (Reaffirmed 1992)”

5 ISA - S5.3 "Graphic Symbols for Distributed Control / Shared Display Instrumentation, Logic and Computer Systems ìEd.,1983”

6 ISA - S5.4 "Instrument Loop Diagrams Ed., 1991”

7 ISA - S5.5 "Graphic Symbols for Process Displays 1st. Ed., 1985”

8 ISA - S18.1 "Annunciator Sequences and Specifications 1st. Ed., 1979 (Reaffirmed 1992)”

9 ISA - S50.1 "Compatibility of Analogue Signals for Electronic Industrial Process Instruments 1st. Ed., 1975 ( Reaffirmed 1995)”

10 ISA - S51.1 "Process Instrumentation Terminology 1st. Ed., 1979 (Reaffirmed 1993)”

11 ISO - 3098 "Drawings-Lettering, Part 1: Currently Used Characters 1st. Ed. 1974”

12 ISO - 3511 "Process Measurement Control Functions and Instrumentation Symbolic Representation

13 ISO - 6708 "Pipe Work components-Definition and Selection of DN(Nominal size) “

14 API - Standard 602 "Compact steel gate valves flanged, threaded, welding and extended body ends 7th Ed., 1998”

15 Graphical Symbols for Process Flow Diagrams, ASA Y32.11 (New York: American Society of Mechanical Engineers, 1961).

16 Austin, D. G. Chemical Engineering Drawing Symbols.

17 Instrument Symbols and Identification Research Triangle Park, NC: Instrument Society of America, Standard ISA-S5-1, 1975.

18 Bausbacher, E. and R. Hunt, Process Plant Layout and Piping Design (Upper Saddle River, NJ: Prentice Hall PTR, 1998).

19 DIN - 19226 - Control Technology

20 DIN - 19227 - Graphical symbols and identifying letters for process control engineering.

21 Hervé Baron (2013) The Oil & Gas Engineering Guide

22 P&IDs Notation, Construction, & Interpretation by Peter Woolf University of Michigan

23 Walker, V., 2009. Designing a Process Flowsheet. Chemical Engineering Progress.

Another calculators or articles that may interest you ...

1 P&ID Diagram Basics - Introduction to reading and understanding P&ID symbols and conventions.

2 P&ID Diagram Basics 2 - International Standards - Learn about standard symbols used in process and instrumentation diagrams worldwide.

3 P&ID Diagram Basics 3 - Functional Identification - Understand functional identification and naming conventions in P&IDs.

4 Pressure Measurement - Explore different pressure measurement techniques and instruments used in process control.

5 Temperature Measurement - Learn about temperature sensors and measurement methods in industrial applications.

6 Instrument Selection Principles - Key factors for choosing the right instrumentation for your process.

7 Introduction to Hazop Analysis - Understand HAZOP methodology for risk assessment and process safety.

8 Introduction to Instrumentation - Fundamentals of instrumentation and control systems.

Frequently Asked Questions

Q1 What are actuator symbols in a P&ID?

A1 Actuator symbols in a P&ID represent devices that move or operate control elements like valves based on input signals, enabling automated process control. These symbols indicate whether the actuator is electric, pneumatic, or hydraulic and help identify how the component will respond during system operation. Understanding actuator symbols is essential for interpreting how control actions are executed and how energy is delivered to mechanical components. Accurate symbol use allows operators and engineers to plan maintenance, troubleshoot issues, and ensure the correct actuator type is installed for each process requirement.

Q2 What are control valve symbols used for in P&ID diagrams?

A2 Control valve symbols are used in P&ID diagrams to represent valves that regulate process variables like flow, pressure, or temperature by adjusting fluid passageways. These symbols indicate the type of valve, such as globe or ball, and show how it is actuated—manually or automatically. Proper use of control valve symbols allows engineers to understand the function, control strategy, and interaction between process and control systems. Accurate interpretation supports efficient system design, ensures the right components are selected, and enables effective communication between instrumentation, operations, and maintenance teams.

Q3 What do instrument symbols represent in a P&ID?

A3 Instrument symbols in a P&ID represent devices that measure, indicate, or control process variables such as pressure, flow, temperature, and level. These symbols are standardized and communicate the function, location, and mounting method of the instrument. For example, a circle symbol may include letters like PT for pressure transmitter or TI for temperature indicator. Proper interpretation of instrument symbols ensures engineers can accurately trace control loops, design reliable systems, and conduct maintenance effectively. They are crucial for integrating instrumentation into the process and ensuring safety, performance, and operational clarity.

Q4 What do line symbols in P&ID diagrams indicate?

A4 Line symbols in P&ID diagrams show the types of connections between equipment, instruments, and control devices, representing the nature of flow or signal transmission. These lines can be solid, dashed, or dotted, each indicating different mediums such as process fluids, electrical signals, or pneumatic pressure. Understanding line symbols is critical for reading a P&ID because they reveal how systems interact and where information or materials are transferred. This clarity helps during system design, ensures compatibility between components, and supports troubleshooting and maintenance activities.

Q5 What do measurement symbols show in a P&ID?

A5 Measurement symbols in a P&ID represent devices that monitor key process parameters like pressure, temperature, flow, and level. These symbols communicate the function of the instrument, whether it also includes indication, control, or recording features, and where the device is mounted. They are essential for visualizing how process data is collected and interpreted by control systems. Understanding measurement symbols ensures engineers can evaluate the accuracy and completeness of monitoring systems, design effective feedback loops, and maintain optimal process performance throughout plant operation.

Q6 What is the importance of P&ID symbol standards?

A6 P&ID symbol standards, such as those developed by ISA or ISO, define consistent graphical representations for equipment, piping, and instrumentation, allowing for clear and uniform communication across engineering disciplines. These standards ensure that anyone reading the diagram—regardless of background or location—can interpret it correctly. Using standardized symbols reduces the risk of misinterpretation, supports efficient training, and ensures compliance with industry norms. Adhering to symbol standards is vital for maintaining documentation integrity, ensuring safety, and facilitating design reviews, construction, and long-term system maintenance.

Q7 What role do piping symbols play in a P&ID?

A7 Piping symbols in a P&ID illustrate the type, size, and configuration of pipes used to transport fluids between components in a process system. These symbols may include representations of fittings, reducers, elbows, flanges, and insulation, offering a detailed view of the piping layout. Accurate use of piping symbols is important for planning material specifications, identifying flow paths, and ensuring the system complies with design and safety standards. They help engineers visualize the overall process architecture and guide construction and maintenance teams during system installation and upgrades.

Q8 What types of information are conveyed by signal line symbols?

A8 Signal line symbols in a P&ID show how information or control commands are transmitted between components like sensors, controllers, and actuators. These lines use different styles—such as dashed for electric signals or dotted for pneumatic—to distinguish the communication method and indicate signal direction. Understanding signal line symbols is crucial for analyzing how the system receives, processes, and responds to input. They also assist in verifying correct wiring, diagnosing faults, and ensuring that the chosen control methods are compatible with the system's requirements and operational environment.

Q9 Why is tagging in symbols important in P&IDs?

A9 Tagging in symbols involves assigning unique alphanumeric codes to each instrument, valve, or device in a P&ID to clearly identify its function and loop association. Tags typically follow industry conventions, such as PT-101 for a pressure transmitter, and provide a direct link to documentation and system databases. Effective tagging helps engineers and operators trace components quickly, coordinate maintenance activities, and cross-reference parts in related diagrams or control systems. It improves clarity, enhances safety, and supports efficient plant management throughout the lifecycle of the process.

Q10 Why is understanding valve symbols essential in P&ID interpretation?

A10 Valve symbols in a P&ID indicate the type, function, and actuation method of valves used to control flow within a process system. They help differentiate between manual valves, such as gate or ball valves, and automated control valves driven by actuators. Understanding these symbols allows engineers to visualize flow paths, analyze control strategies, and verify that the correct valves are specified for design and operation. Accurate interpretation ensures proper installation, supports effective troubleshooting, and contributes to maintaining process safety and efficiency under different operating conditions.