What is Piping and Instrumentation Diagram (P&ID)?

A piping and instrumentation diagram (P&ID) is a drawing in the process industry. A P&ID shows all piping, including the “physical sequence of branches, reducers, valves, equipment, instrumentation and control interlocks.” A P&ID is used to operate the process system, since it shows the piping of the process flow along with the installed equipment and instrumentation.

P & IDs play a key role in maintaining and modifying the process they describe, because it is important to demonstrate the physical sequence of equipment and systems, including how these systems connect. In terms of processing facilities, a P&ID is a visual representation of key piping and instrument details, control and shutdown schemes, safety and regulatory requirements, and basic start-up and operational information.

A P&ID should include the following:

  • Instrumentation and designations
  • Mechanical equipment with names and numbers
  • All valves and their identifications
  • Process piping, sizes, and identification
  • Vents, drains, special fittings, sampling lines, reducers, increasers, and swaggers
  • Permanent start-up and flush lines
  • Flow directions
  • Interconnections references
  • Control inputs and outputs, interlocks
  • Interfaces for class changes
  • Computer control system
  • Identification of components and subsystems delivered by the process

A P&ID should NOT include the following:

  • Instrument root valves
  • Control relays
  • Manual switches
  • Primary instrument tubing and valves
  • Pressure temperature and flow data
  • Elbow, tees and similar standard fittings
  • Extensive explanatory notes

A P&ID involves various symbols to represent all of the included parts, components, and information. Their symbology is defined on separate drawings referred to as “lead sheets” or “legend sheets.” Lead sheets should be customized to each company’s process plants, though in general, the P&IDs are based on a core set of standard symbols and notations. The most important part of the lead sheets is that they are organized logically so that it is possible to easily locate the symbols and tags. While it’s a good practice to have lead sheets for the major equipment in a factory, it may not be necessary because this major equipment already should be tagged and named with general specifications for identification purposes.

Letter and number combinations appear inside each graphical element and letter combinations are defined by the ISA standard. Numbers are user assigned and schemes vary. While some companies use sequential numbering, others tie the instrument number to the process line number, and still others adopt unique and sometimes unusual numbering systems. The first letter defines the measured or initiating variables such as Analysis (A), Flow (F), Temperature (T), etc. with succeeding letters defining readout, passive, or output functions such as Indicator (I), Recorder (R), Transmitter (T), etc.

Below are some piping and instrumentation diagram symbols with letters.


Because a P&ID contains such important information, it is critical to the workings of the process industry that the process plants apply tags or labels to keep track of all of the equipment, piping, valves, devices, and more. Those labels must match the symbology and should not fail, so that the plant’s operations run smoothly and efficiently. That’s why the unique identifiers involved in the P&ID, tagging, and labeling process are critical.

The P&ID and tags ensure that even collections of similar objects have unique tags so that identical valves, pumps, instruments, etc., can be uniquely identified
The P&ID and tags make it possible to assemble the process plant in a structured manner so that additions, deletions, changes, etc., are possible from a whole-unit scale down to a single valve on a pipe at any location.

The P&ID and tags contain scores of metadata that provides, or links to, more details including specifications, materials of construction, data sheets, etc.
Best Practices for Tagging Equipment When Considering P&ID.

Using a numeric-only system for tagging equipment is the best way for process industries to avoid the problems with labeling by abbreviated names. Structured tag systems are more intuitive for every team that deals with the equipment, including developers, operators, and maintenance. The equipment tag format should be a series of three numbers, beginning with an area number, followed by an equipment type code, and then ending with a unique sequence number.

Area numbers represent an area that may be determined by the physical, geographical, or logical grouping location by the plant site
Equipment types are fairly straightforward, but if equipment has multiple functions, users should determine how to select the most suitable equipment type code.

Sequence numbering is the consecutive numbering of similar equipment in any given area, and it’s important to being the sequence at 01 so that all equipment can have it’s own sequence number.

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