P&ID Symbol Diagram Basics - Part 1

Purpose, Owner & Contents

1.P&ID

Welcome to our Engineering and P&ID Diagrams resource page. Here, you'll find key insights into the world of Process and Instrumentation Diagrams (P&ID) a foundational tool in engineering that helps bring complex processes to life through precise and detailed visualization. Whether you're new to P&IDs or looking to deepen your understanding, our resources are designed to guide you through essential concepts, industry standards, and best practices. For a more comprehensive exploration of related topics, we invite you to visit the links provided below. Each link offers further information on crucial aspects of engineering and process design that complement and enhance your knowledge of P&ID diagrams.

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.
If you are interested to know more about subjects like P&ID Symbols read our free introduction to these concepts in our article P&ID Diagram Basics - Part 1 - Symbols - How to read a P&ID.

P&ID Diagram Basics PART1 P&ID Diagram - Mindmap showing the different areas covered by this document

Piping and instrumentation diagram (P&ID) has a fundamental importance in process automation and process industry. It gives an overall view of the entire process loop of the facility. The Automation, Systems and Instrumentation Dictionary in its 4th edition defines what Piping and Instrumentation Drawing do: P&IDs show the interconnection of process equipment and the instrumentation used to control the process.

They are used as an element of communication between engineering, plant operation, maintenance and construction, since they show detail information of the plant relating to equipment, pipeline, instrumentation and common services. P&ID must be maintained accurately and updated.

The creation of the instrumentation diagram or p&id constitutes an important phase in the design of a process plant.

It is a diagram of the systems / equipment present in a section of the plant.

It shows the necessary graphical elements to execute, monitor and control specific processes.

The P&ID drawing does not describe the chemical reactions involved in the process nor does it describe procedures of the same.

It is an essential document in the process industry.

Any change made in the process must be reflected in the diagram, for example if an additional pump is installed, it is important because your safety and that of your collaborators could depend on this action.

From the point of view of the control, they show information related to the control loops, although it is not always possible to represent the entire control structure. To maintain clarity and save space in these diagrams, the complex logic is not shown, it is shown in the logic diagrams ISA-5.2-1976 (R1992).

From engineering point of view, Piping and instrumentation diagrams (P&IDs) are the foundation of the E/I&C (electrical/instrumentation and control) engineering package of deliverables.

A P&ID is a diagram that shows condensed information from multiple specialties.

This type of documents (the P&ID drawings) show, from the point of view of the process, different measures such as pressure, flow, level, etc. and their respective control schemes in relation to the elements of the plant and its interconnection pipes. These elements (mechanical equipment, piping, piping components, valves, equipment drivers and instrumentation and controls) are represented by symbols and labels. A standard set of symbols is used to prepare drawings of control systems and processes. Symbols have been developed to represent all of the components used in industrial processing, and have been standardized by ANSI and ISA.

The process equipment is shown using basically symbols such as bubbles (circles or balloons) and others to indicate valves, actuators, flow meters, etc

These symbols are assembled on the drawing in a manner that clearly defines the process.

The symbols used in these diagrams are based on the following standards :

Standard ANSI / ISA-5.1-1984 (R1992) Symbols and identification of instrumentation, this standard was developed by ISA-The Instrumentation, Systems and Automation Society (ISA) and the American National Standards Institute (ANSI).
Standard ISA 5.31983, Graphic Symbols for Distributed Control/Shared Display Instrumentation Logic, and Computer Systems.

In some cases, the control schemes are too complex to be shown in a P and ID drawing, in this case, independent functional control diagrams are created.

P&ID Diagram Basiscs ISA vs SAMA symbology - compare two different loops

For the design of specific diagrams it is possible to use the ISA symbology or SAMA symbology (The Association of Scientific Apparatus Manufacturers withdrew and no longer supports its standards, they are available only for historical purposes and general information).

Although the P&ID is the overall document used to define the process, the first document developed in the evolution of a process design is often the PFD (Process Flow Diagrams). Once a PFD is released for detail design, the project scope has been established and P&ID development may commence.

1.1. What is P&ID?

Pronounced as P.N.I.D

There is no universal standard that specifies the meaning of the letters P&ID. Usually, for example, the P in P&ID can mean Pipe, Plant or Process. The I refers to Instrument or Instrumentation. The D is for Drawing or Diagram

P&ID Diagram Basiscs Multiple Pipes in a Pipe Rack with multiple colors and sizes

Although the final format is the same for all cases, the different names given to this type of diagram are shown below:

Pipe Diagrams and Instrumentation Diagram
Process and Instrumentation Diagram
Piping and Instrumentation Diagram

In this post, for simplicity, we will refer to the document by the acronym, P&ID.

2.What is P&ID used for?

The objective of the P and ID diagram is to show the necessary information in order to understand the relationship between the conceptual design of the process, developed through the PFD and HMB diagrams, with reality.

P&ID Diagram Basics - Old Chemical Plant in B/W

2.1. At design stage:

Defines the design basis for Piping and Instrumentation disciplines.It is used as a document that defines the base process, from which all the engineering, manufacturing, construction and operation will be developed.

P&ID Design

Serves to show and agree the operating and maintenance features between Engineer and Owner or between Engineer and Vendor.
Serves to show the interface with equipment/package vendors.
Perform HAZOP design/review. It is used as a reference document for the management of process safety information (PSI) in Process Safety Management (PSM).It allows obtaining the necessary information to be able to analyze the hazards of the process, the potential failures and the existing safeguards in order to minimize the greatest number of errors in operation, design or humans.

2.2. During operation:

Reference diagram for operator, work permit, plant modifications etc.

P&ID During Operation

Process technicians use P&IDs to identify all of the equipment, instruments, and piping found in their units.
New operators or technicians use these drawings during their initial training period. Knowing and recognizing these symbols is important for a new operator/technician.
Train Process Operators. Part of the training would pose situations and require the operators to be able to describe what specific valve should be changed, how it should be changed, and what to observe in order to monitor the effects of the change.
Support document in the development of operation and maintenance procedures.
Development of start-up/stop procedures where the plant is not under the influence of the installed process control systems.
During SAT, as a checklist against which each item in the plant is checked.
Instrumentation and Control Systems Maintenance. The process and instrumentation or piping and Instrumentation diagram (P&ID) is probably the document most used by instrumentation and control system maintenance personnel.
Instrumentation engineering. The P&ID diagram is the main and almost ideal document to synthesize all required measurement and control functions.

The P&ID is the last stage of the design of the plant, and therefore serves as a guide for those who will be responsible for the final design and construction.

Mechanical engineers and civil engineers will design and install pieces of equipment.
Instrument engineers will specify, install, and check control systems.
Piping engineers will develop plant layout and elevation drawings.
Project engineers will develop plant and construction schedules.

3.Owner

The P&ID is the daughter of the Process Flow Diagram (PFD), a mechanical discipline product, and the heat and material balance (HMB) sheet, a process engineering product.

They are usually constituted by process engineers and control engineers during the design and construction of the plant, with the process engineers being the persons authorized to validate the changes.

The owner of the P&ID is usually the Process Engineer or Operation Responsible.

P&ID Owner

This individual or group coordinates the information that is placed in the diagram to ensure that it is consistent with the purpose of the document.

This coordination is required to regulate the information provided by the different departments towards this document.

Below is a list of some of the different specialties within the engineering that provide information for it to be reflected in this diagram:

Process Engineering: diagram content, process data, vendor equipment depiction.
Mechanical Engineering: pipe ID (identification, not inside diameter) numbers, equipment numbers and labels, graphical depiction of equipment and piping.
Instrumentation Engineering: instrument numbers; graphical presentation of instruments and instrument wiring/tubing, Type of Instruments.
Controls/Systems Engineering: control logic depictions, Communications among Control Systems.
Electrical Engineering: motor ratings, motor wiring depictions.

Usually the creation and maintenance of this type of diagram is the responsibility of the mechanical department.

4.What is P&ID used for?

4.1 What is included in a P and ID?

A P&ID diagram is a complex representation of the various production units found in a plant.

It is used by people from different disciplines.

The main users of the document after the start-up of the plant are the process technicians and instrument and electrical, mechanical, safety, and engineering personnel.

This section will aid in solving the long existing and continuing problem of confusing information on PIDs. The fact that there is confusion can be understood because there really is no universal, national or international, multidiscipline standard that covers the development and content of P&ID and specifies what information should be included on a P and ID or even, for that matter, the meaning of the letters P and ID.

All control function directly connected to the process shall be shown on P&ID.

The P and ID diagram is a graphic index of all the main process equipment, and all the main sections of pipe instruments, and shows their functional relationships.

P&ID typically show the following types of information:

4.1.1. The process / piping part includes:

Major and minor equipment present in process/plant such as vessels, drums, pumps, reactors ... showing
- Name and Number
- Location
- Capacity
- Pressure
- Liquid Level
- Usage
- Spare units
- Parallel units
- Summary details of each unit
All piping and transport equipments for process and utility fluids. Including
- Specification of Materials of construction
- Identification of the pipe
- Flow directions
- Size of the pipe, Schedule (thickness)
- Insulation (thickness and type). Information relative to insulation (hot or cold) and tracing of pipes or equipments.
- Safety devices.
- Include Drains, Sample Connections

4.1.2. The Electrical & Instrumentation part includes:

All motors, giving voltage, power, and other relevant information like buttons used to control them.
All required function to control the process like Indicators, Recorders, Controllers , including:
- Measurement technology (graphical symbols of instruments)
- Location of instrument
- Major function
- Process control loop number
- Range
- System used to perform automation task (local instrument, DCS, PLC...)
All control valves, giving
- Type of control
- Type of valve
- Type of valve action
- Fail-save features
- Actuator technology (graphical symbols of valves)
- Communication link between instruments systems and actuators (electric, digital pneumatic/hydraulic).
- Flow and pressure information
Self controlled valves
Main instruments required to perform these functions (Essential items to understand the functionalities required).
All safety valves and pressure regulators, giving temperature and operating ranges
Interlocks and safety related functions

According to plant size, it will be represented on one to many P&I diagrams. Generally, plants are divided in several areas, each area corresponding to one or more P&IDs diagrams. It is recommended to extend the P&ID approach for other parts or system than Process.

Fire and Gas detection systems: P&I based on Layout drawings showing gas, fire or smoke detectors, emergency button ...
Fire protection systems
Package equipments
Analyzers

4.2. Conventions in Constructing Piping and Instrumentation Diagrams

There are no universal standards that address the format to be used in developing P&IDs. The format used by most design groups has been developed over many years. However, here are a few guidelines that serve as a simplistic de facto standard:

The devices are represented as icons. An item is shown only in one P&ID.
Normally the plant layout of the equipment as well as the PFD is shown from left to right.
The process flows from the left of the P&ID to the right.
P&IDs are developed as D size sheets (22" x 34") or larger, but should be legible when reduced to B size (11" x 17") for ease of use in the office and in the field.
P&IDs should show sufficient information to define the process without crowding. One to three pieces of equipment with auxiliaries is normally sufficient for one P&ID.
P and ID diagrams do not usually use colors to indicate or convey their meaning. Colors are used at the design level in a monitor but are usually printed in black and white.
To reduce clutter, a typical detail can be used for repeated components (see the typical drain on Figure 2-21).
When piping gets complex, auxiliary P&IDs are used.
Add notes for understanding and clarity.
Show relative sizes of equipment, but do not include specific elevations or dimensions. They do not show the scale of the elements or their geometry. They also do not show information regarding the orientation of the team or its actual location.
Every set of P&IDs should include a legend sheet, or sheets, to define the symbols and abbreviations used.
The free space on a P&ID should facilitate addition of future process changes; it is best not to start with congested P&IDs.
The pneumatic piping is shown where the information is important to communicate the functionality of the controls, and the information of the electrical wiring is presented (in the form of dotted lines) to communicate the functional intention to the electrical and instrumentation disciplines.

Download our free PDF file about this article!

Rating: 4.4957 - 694 reviews

We have prepared this complete PDF so that you have all the information we give in this article and you can share it, discuss it with your colleagues, and use it professionally. Having the content in PDF format allows you to access it offline, print it easily, and keep it well-organized for future reference.

We consider that it is better to have a well-formatted text that includes all the key concepts explained in this publication, ready to share or save for later use. A PDF ensures that the layout remains intact across different devices and makes it easier to annotate or highlight important points.

Information and Definitions

Alarm Systems

Indications of alarms that warn operators of abnormal conditions. These are often displayed on P&IDs with specific symbols to signify critical warnings for safety and maintenance.

Control Loops

Diagrams showing how sensors, controllers, and actuators work together to control a process variable. Control loops are critical for automating and maintaining system stability.

Equipment

The major physical components within the process, such as pumps, compressors, heat exchangers, and tanks. Each piece of equipment is represented by a standardized symbol and unique identification code.

HMI (Human-Machine Interface)

The interface that operators use to interact with the system, often represented in the P&ID to show monitoring and control points accessible to the operator.

Instrumentation

Devices that measure, monitor, and control process variables such as pressure, temperature, flow, and level. Instruments are represented by unique symbols in the P&ID to show their function and connection in the system.

Interlocks

Safety features that prevent certain actions unless certain conditions are met, represented on the P&ID to prevent accidents and maintain system integrity.

Isometric Drawings

While not part of P&IDs, isometric drawings are often referenced as 3D representations that provide more detail on the physical layout of the piping system.

Line Designation or Tagging

Alphanumeric codes that uniquely identify pipes, instruments, and equipment. Line tags indicate specific details about the material, pressure, and size of pipes within the P&ID.

Loop Numbers

Identification numbers used to categorize and locate control loops, each corresponding to a specific measurement or control function within the process.

Piping

The network of pipes that carry fluids from one piece of equipment to another. In P&IDs, piping is denoted with lines that may vary in thickness or style to show different types or sizes.

PLC (Programmable Logic Controller)

A digital control device that uses programmed instructions to automate processes. P&IDs display the interface points of the PLC with the equipment and instrumentation it controls.

Process Flow

The sequence of operations or path that materials follow through the system, represented by lines, symbols, and annotations on a P&ID to indicate direction and flow paths.

Process Safety

Representations of safety mechanisms such as emergency shutdown systems (ESD) and other critical safeguards that protect personnel and equipment.

Process Variables

Key measurable factors like temperature, pressure, flow rate, and level that influence and describe the process's operation. These are monitored and controlled through instruments represented on the P&ID.

Safety Relief and Pressure Relief Valves (SRV/PRV)

Valves designed to release excess pressure from a system to prevent failure or explosion, represented with specific symbols to show their critical role in safety.

Setpoints

The predefined values of process variables (e.g., temperature or pressure) that the control system aims to maintain, ensuring consistent operation.

Signal Types

Indications of the type of signal (e.g., electrical, pneumatic, or digital) used to transmit data between instruments and control devices within the P&ID.

Symbols and Legends

These are standardized shapes and icons that represent equipment, instrumentation, and controls in the P&ID. The legend explains each symbol used, ensuring consistent interpretation of the diagram.

Valves

Mechanical devices that control the flow of fluids within the system. Valves can be shown as open, closed, or controlled automatically, and are represented by specific symbols for each type (e.g., gate, ball, or butterfly valves).

P&ID Symbol Diagram Basics References

1 MEIER,F.A. (2004) Instrumentation and Control Systems Documentation

2 SAMSON (2013) Terminology and Symbols in Control Engineering

3 ANDREW,W.G. (1974) Applied Instrumentation in the Process Indust Resource Material William G. Andrew & H. B. Williams

4 ISA-5.1-1984-(R1992), Instrument Symbols and Identification

5 ISA-5.4-1991, Instrument Loop Diagrams

6 ANSI ISA-S5.5-1985,Graphic Symbols for Process Displays

7 DOUGLAS,O.J. (2005) Applied Technology and Instrumentation for Process Control

8 DUNN,W.C. (2006) Introduction to Instrumentation Sensors And Process Control

9 GOETTSCHE,L.D.(2005) Maintenance of Instruments and Systems

10 HUGHES,T. (2002) Measurement and Control Basics

11 ISA (2012) Successful Instrumentation and Control Systems Design

12 LOVE,J. (2007) Process Automation Handbook A Guide to Theory and Practice

Another calculators or articles that may interest you ...

1 P&ID Diagram Basics 2 - International Standards - Learn standard symbols in global instrumentation practices.

2 P&ID Diagram Basics 3 - Functional Identification - Master functional identification conventions in P&IDs.

3 Introduction to Hazop Analysis - Comprehensive guide to HAZOP studies and process safety risk analysis.

4 Pressure Measurement Fundamentals - Explore pressure sensing principles and instrumentation types.

5 Molecular Weight of Common Gases - Reference table for gas molecular weights and properties.

6 Orifice Plate Calculator - Size - Precise tool for orifice diameter calculations.

7 Flow Rate Calculator - Calculate flow rates for liquids and gases through pipes.

8 Orifice Plate Calculator - Pressure Drop - Determine pressure losses across orifice plates.

Frequently Asked Questions

Q1 What is the purpose of a P&ID Symbol Diagram?

A1 The primary purpose of a P&ID Symbol Diagram is to represent the essential components, equipment, and instrumentation of a process system or industrial plant. It provides a visual representation of how these elements interact, the flow of materials, and the control systems involved. P&ID diagrams are crucial for engineers, operators, and maintenance personnel to understand and communicate the design, operation, and maintenance of the system. They play a critical role in process engineering, safety, and regulatory compliance.

Q2 Who owns the responsibility for creating and maintaining P&ID diagrams?

A2 Typically, the responsibility for creating and maintaining P&ID diagrams falls under the domain of process engineers, design engineers, or drafting professionals. These experts work closely with the project team and relevant stakeholders to ensure that the P&ID accurately reflects the process system. It is essential to keep P&ID diagrams up to date as any changes or modifications to the system occur to maintain the document's accuracy and usefulness. In many organizations, P&ID ownership may be assigned to the engineering department or a dedicated documentation team.

Q3 What are the essential contents of a P&ID Symbol Diagram?

A3 P&ID diagrams contain various symbols, lines, and annotations. The key components include process equipment (such as pumps, vessels, and valves), pipelines, instrumentation and control devices (like sensors, transmitters, and controllers), safety devices, flow directions, and connection points. Additionally, P&IDs may include auxiliary information like labels, equipment tags, line numbers, and legends to ensure clarity and understanding. The use of standardized symbols and conventions is essential to make P&ID diagrams universally comprehensible.

Q4 How are P&ID diagrams used in the engineering and industrial sectors?

A4 P&ID diagrams are indispensable tools in the engineering and industrial sectors. They are used for various purposes, including design, construction, operation, and maintenance of process systems. Engineers use P&IDs during the design phase to plan and optimize systems, while operators rely on them for day-to-day monitoring and troubleshooting. Maintenance teams refer to P&IDs to identify equipment locations and understand how to service them. Furthermore, P&IDs are valuable for safety assessments and regulatory compliance, helping organizations maintain a safe and efficient operational environment. Overall, P&ID diagrams are crucial documents that facilitate effective communication and decision-making across the lifecycle of a process system.

Q5 How do P&ID diagrams differ from other engineering diagrams, like process flow diagrams (PFDs)?

A5 P&ID diagrams and Process Flow Diagrams (PFDs) serve distinct purposes in industrial engineering but complement each other. While PFDs provide a broad overview of a process, showing the general flow of materials, basic equipment, and operating conditions, P&ID diagrams focus on detailed equipment layout, instrumentation, and control information. PFDs typically cover the "what" and "how" of the process at a high level, showing material flows and essential equipment but leaving out specifics. In contrast, P&ID diagrams go into the "how" at a micro-level, showing each piece of equipment, valves, and their connections, as well as control mechanisms like pressure and temperature control. A critical difference is that P&ID diagrams include detailed information on instrumentation and control devices, specifying their placement and function within the system. This makes P&IDs crucial for maintenance, troubleshooting, and safety, as they allow engineers and technicians to see the exact location of sensors, valves, and other control components. Additionally, P&IDs are more regularly updated than PFDs as they reflect specific aspects of construction and operation. Therefore, while PFDs provide a general understanding of the process, P&ID diagrams are indispensable for those who need detailed operational insights, helping to ensure both safe and efficient operations.

Q6 What role do P&ID diagrams play in safety and compliance within industrial facilities?

A6 P&ID diagrams play a critical role in ensuring safety and regulatory compliance in industrial environments. They document the exact layout of equipment, piping, and instrumentation, providing a precise roadmap of the facility's process systems. This information is essential for identifying potential hazards, such as high-pressure lines, hazardous material flows, and emergency shut-off points, which are clearly indicated on the diagrams. Safety protocols depend heavily on P&ID diagrams because they offer real-time insights for emergency responses, such as isolation points for hazardous materials or systems under pressure. In case of an emergency, maintenance and operational teams can refer to P&IDs to locate critical valves or relief points quickly. Furthermore, compliance with industry regulations such as those mandated by OSHA or EPA often requires facilities to keep updated and accurate P&ID diagrams. These diagrams must reflect any system modifications, ensuring that safety audits and regulatory inspections are met with current documentation. Regulatory bodies frequently mandate P&IDs as part of safety documentation to verify that facilities are maintaining safe operation standards and that any risks are systematically managed. Properly maintained P&IDs help organizations avoid compliance issues and ensure that all personnel are aware of the exact layout and potential hazards in the facility, significantly improving overall safety and risk management.

Q7 How do P&ID diagrams facilitate collaboration between different engineering disciplines?

A7 P&ID diagrams act as a universal reference point, bridging gaps between different engineering disciplines involved in the design, construction, and maintenance of industrial systems. Engineers from various fields such as mechanical, chemical, electrical, and control engineering rely on P&ID diagrams to understand the complete system's design and its operational requirements. Each symbol and notation in a P&ID holds specific meaning, allowing interdisciplinary teams to quickly interpret information relevant to their fields. For example, mechanical engineers use P&ID diagrams to understand the equipment layout, piping connections, and physical requirements of the system. At the same time, control engineers look at P&IDs to determine the necessary control devices, such as sensors and control valves, and their relationship to the rest of the process. Chemical engineers, on the other hand, use these diagrams to assess the process conditions and material flows, which are critical for designing safe and efficient systems. By standardizing how process components and instrumentation are depicted, P&ID diagrams ensure that all disciplines are aligned, reducing misunderstandings and miscommunication. This alignment allows for smoother project workflows, fewer errors during construction, and an easier handover to operations teams. Ultimately, P&ID diagrams are an indispensable tool for ensuring that multidisciplinary teams work seamlessly towards shared goals within complex industrial projects.

Q8 What challenges are commonly faced when creating and updating P&ID diagrams, and how are they managed?

A8 Creating and updating P&ID diagrams is challenging due to the need for accuracy, adherence to standardized symbols, and consistent documentation across teams. A significant challenge is ensuring that every component of the process, including minor valves, sensors, and interconnecting pipes, is represented correctly. This requires detailed knowledge and coordination among various teams to prevent errors or omissions that could compromise safety and efficiency. As industrial systems evolve with upgrades or modifications, keeping P&ID diagrams current can be labor-intensive and costly. Every change in equipment, piping, or instrumentation requires an update to the P&ID, often managed through change control procedures to ensure accuracy. Failure to update these diagrams in real time can lead to misinterpretations during maintenance or operational troubleshooting, potentially causing operational downtime or even safety risks. Technology has increasingly aided this process with digital tools and databases that store and manage P&ID data, making updates more efficient and minimizing human error. These systems allow for real-time updates, automatic change tracking, and version control. Additionally, engineering standards such as ISA S5.1 provide clear guidelines for symbols and notation, simplifying the standardization of P&ID diagrams. By investing in both technology and rigorous processes, industries manage the complexities of creating and maintaining accurate, up-to-date P&ID diagrams.