A piping and instrumentation diagram (P&ID) is a detailed diagram in the process industry that shows the piping and process equipment together with the instrumentation and control devices.

A piping and instrumentation diagram (P&ID) is defined as follows:

1. A diagram that shows the interconnection of process equipment and the instrumentation used to control the process. In the process industry, a standard set of symbols is used to prepare drawings of processes. The instrument symbols used in these drawings are generally based on International Society of Automation (ISA) Standard S5.1
2. The primary schematic drawing is used for laying out a process control installation.

They usually contain the following information:

• Mechanical equipment
• Process piping, sizes, and identification
• Process control instrumentation and designation (names, numbers, unique tag identifiers)
• Interfaces for class changes
• Computer control system
• Identification of components and subsystems delivered by others

P&IDs are originally drawn up at the design stage from a combination of process flow sheet data, the mechanical process equipment design, and the instrumentation engineering design.

During the design stage, the diagram also provides the basis for the development of system control schemes, allowing for further safety and operational investigations, such as a Hazard and operability study (HAZOP). To do this, it is critical to demonstrate the physical sequence of equipment and systems, as well as how these systems connect.

P&IDs also play a significant role in the maintenance and modification of the process after the initial build. Modifications are red-penned onto the diagrams and are vital records of the current plant design.

They are also vital in enabling the development of;

• Control and shutdown schemes
• Safety and regulatory requirements
• Start-up sequences
• Operational understanding.

P&IDs form the basis for the live mimic diagrams displayed on graphical user interfaces of large industrial control systems such as SCADA and distributed control systems.