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A Piping and Instrumentation Diagram (P&ID) is a detailed schematic that shows the piping, process equipment, valves, instrumentation, and control systems within a process facility and how these components are interconnected.
P&IDs are fundamental engineering documents in oil and gas, LNG, chemical and process, utilities, pharmaceutical, mining, and other asset-intensive industries. They are used throughout engineering, construction, commissioning, operations, maintenance, and facility modification.
Unlike drawings that represent the physical layout of a facility, a P&ID primarily shows the functional relationships between equipment, piping, instruments, and control systems. It uses symbols, lines, tags, and identification conventions to communicate how the process is configured and controlled.
For owner-operators and EPC contractors, the P&ID is also closely connected to structured asset information. Equipment and instruments shown on the drawing are typically identified by tags that should correspond with authoritative records such as the Master Tag Register (MTR).
A P&ID provides detailed information about how a process system is configured, interconnected, monitored, and controlled.
Depending on the project and drawing standard, a P&ID can show:
This is one of the main differences between a P&ID and a higher-level process diagram: the P&ID contains the detailed piping, instrumentation and control relationships needed to understand how the system operates. Lucidchart's high-ranking guide similarly emphasizes mechanical equipment, valves, piping identification, flow direction, controls and interlocks.
P&ID symbols are graphical representations used to identify equipment, piping, valves, instruments, and control functions on a Piping and Instrumentation Diagram.
Instead of showing each component realistically, P&IDs use standardized or project-defined symbols so complex process systems can be communicated consistently.
Common categories include:
Equipment symbols can represent:
The symbol identifies the equipment type, while a unique equipment tag identifies the individual asset.
Different symbols distinguish different types and functions of valves, such as:
Instrumentation symbols represent devices used to measure, indicate, transmit, monitor, or control process variables.
These can include instruments associated with:
Letter combinations are commonly used to communicate the measured variable and function of an instrument. Established instrumentation conventions such as ISA standards help create a consistent way of representing this information.
Different line types can represent different connections, including:
Because conventions can vary between companies and projects, the legend and applicable standards for the specific drawing should always be checked when interpreting a P&ID. P&IDs are schematic rather than scale or geometrically accurate drawings, and company conventions can differ.
Reading a P&ID means following the process through the drawing while interpreting its equipment, piping, symbols, tags, instruments, and control relationships.
A practical approach is:
The objective is not simply to understand where the process flows, but also which equipment is involved, how it is isolated, what is measured, and how the process is controlled.
P&IDs use engineering standards together with company- and project-specific conventions.
ANSI/ISA-5.1 is an important reference for instrumentation symbols and identification. It provides conventions for representing instrumentation, control, and automation functions on engineering diagrams.
However, organizations may also apply their own engineering specifications, numbering systems, drawing practices, and symbol conventions.
For this reason, engineers and operators should use the legend and applicable engineering standards for the specific project rather than assuming that every P&ID uses identical conventions.
A Process Flow Diagram (PFD) and a Piping and Instrumentation Diagram (P&ID) represent different levels of process information.
A PFD provides a higher-level overview. It typically shows major process equipment, primary process streams, and the overall relationship between the major parts of the process.
A P&ID provides substantially more detail about how the process is connected, instrumented, monitored, and controlled.
A P&ID typically adds information such as:
In simple terms:
PFD = how the overall process flows
P&ID = how the equipment, piping, instrumentation, and controls are interconnected
This distinction is also reflected by Lucidchart, which characterizes a PFD as the more conceptual, less detailed view and the P&ID as the more fully developed diagram.
A P&ID is detailed, but it is not intended to contain every engineering detail about the facility.
Information typically maintained in other engineering documents can include detailed physical dimensions, exact equipment locations, fabrication details, extensive material specifications, and other information that would make the P&ID unnecessarily difficult to read.
P&IDs are also generally not drawn to scale and should not be treated as accurate representations of the physical distance or geometry between components.
This is why P&IDs need to remain connected with the broader engineering document and asset information environment.
A P&ID is not only a drawing. It also contains identifiers that connect the graphical representation of the process with structured information about the underlying assets.
Every tagged equipment or instrument item shown on a P&ID should correspond with an authoritative equipment or tag record.
For example, a pump tag on a P&ID can connect the drawing to:
A governed Master Tag Register (MTR) provides the authoritative reference for these identifiers.
This creates an important relationship:
P&ID → tag → equipment record → technical data → documentation
Maintaining this relationship allows engineering, maintenance, and operations teams to move between the drawing and the structured information describing the physical asset.
P&IDs change throughout the lifecycle of a facility.
During engineering and construction, revisions may result from design development, vendor information, engineering changes, construction modifications, commissioning findings, or approved changes.
Changes continue during operations as equipment and process systems are modified.
This makes revision and status control essential.
Controlled P&ID management should make it possible to determine:
If outdated P&IDs remain in circulation, engineering, maintenance, or operations personnel can potentially make decisions using information that no longer represents the approved facility configuration.
For that reason, P&ID revision control is more than an administrative document-management activity; it supports reliable engineering and operational decision-making.
An as-built P&ID represents the process system as it was actually constructed or subsequently modified rather than only showing an earlier design state.
Differences can emerge between the engineering design and the final installed configuration during construction and commissioning.
These changes need to be incorporated into controlled as-built documentation so the drawings available to operations accurately reflect the facility being operated.
Final as-built P&IDs therefore form an important part of the Document Handover Package transferred from the project organization or EPC contractor to the owner-operator.
P&IDs are also important during Management of Change (MOC).
When an approved change modifies piping, equipment, instrumentation, control logic, or another element represented on the P&ID, the affected engineering documentation needs to be evaluated and updated accordingly.
This creates a lifecycle relationship:
Proposed change → MOC → implementation → P&ID update → as-built status
Lucidchart likewise identifies P&IDs as important references for process modification and Management of Change.
Keeping this loop controlled helps prevent the documented plant configuration from gradually diverging from the physical facility.
P&IDs remain relevant long after engineering design is complete.
They support activities such as:
P&IDs are therefore not simply project deliverables. They become long-lived engineering records used by multiple disciplines throughout the operating life of the facility. High-ranking P&ID guidance similarly connects them with construction, maintenance, process safety, operations, modification, and MOC.
Sharecat supports P&IDs as part of the wider asset information environment, rather than treating drawings as isolated files.
Equipment and instrument tags referenced on P&IDs can be connected to governed records in the Master Tag Register (MTR), helping maintain consistency between engineering drawings and structured asset data.
Revision and status information helps teams control the relevant drawing versions, while relationships between documents and tags provide traceability between the P&ID and the equipment it represents.
During project handover, these relationships help connect final as-built drawings with the asset information that operations and maintenance teams will use after takeover.
The objective is to maintain a controlled relationship between:
P&ID → revision → equipment tag → asset data → associated documentation
This reduces the risk of P&IDs becoming isolated documents that are technically stored but disconnected from the governed asset information needed to understand and trust them.
P&ID stands for Piping and Instrumentation Diagram. It is a detailed schematic showing piping, process equipment, valves, instrumentation, and control relationships within a process system.
P&ID symbols are graphical representations used for equipment, piping, valves, instruments, signals, and control functions. Instrumentation identification commonly follows established conventions such as ANSI/ISA-5.1.
A P&ID can include process equipment, piping, valves, instrumentation, control loops, flow directions, equipment and instrument tags, vents, drains, interlocks, and connections between systems.
Start with the drawing legend, identify the major equipment, follow the process piping and flow direction, interpret the valves and instrumentation, read the equipment and instrument tags, and then follow the control and signal relationships.
A PFD provides a higher-level representation of the process and major equipment. A P&ID provides significantly more detail about piping, valves, instrumentation, equipment identification, and control relationships.
No. P&IDs are schematic diagrams and generally do not represent the actual physical dimensions, distances, or geometrical arrangement of equipment and piping.
Tags uniquely identify equipment and instruments shown on the drawing. They create the connection between the graphical P&ID and structured information such as equipment records, technical attributes, documentation, and maintenance information.
P&IDs change as engineering designs and physical facilities change. Revision control helps ensure users can identify the current approved drawing and distinguish it from superseded information.
An as-built P&ID is an updated drawing intended to represent the process system as it was actually constructed or subsequently modified.
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