What is a P&ID Drawing

A piping and instrumentation diagram, or P&ID, shows the piping and related components of a physical process flow. It’s most commonly used in the engineering field.


Function and purpose of a P&ID

P&IDs are foundational to the maintenance and modification of the process that it graphically represents. At the design stage, the diagram also provides the basis for the development of system control schemes, like Hazard and Operability Study (HAZOP).

For processing facilities, it’s a graphic representation of:

  • Safety and regulatory requirements
  • Key piping and instrument details
  • Control and shutdown schemes
  • Basic start up and operational information


When to use a P&ID and who would use them

P&IDs are a schematic illustration of the functional relationship of piping, instrumentation, and system equipment components used in the field of instrumentation and control or automation. They are typically created by engineers who are designing a manufacturing process for a physical plant.

These facilities usually require complex chemical or mechanical steps that are mapped out with P&IDs to construct a plant and also to maintain plant safety as a reference for Process Safety Information (PSI) in Process Safety Management (PSM). If something does go wrong, reviewing the P&ID is usually a good place to start. P&IDs are invaluable documents to keep on hand, whether they’re used to streamline an existing process, replace a piece of equipment, or guide the design and implementation of a new facility. With the record they provide, changes can be planned safely and effectively using Management of Change (MOC).

P&IDs are used by field technologists/technicians, engineers, and operators to better understand the process and how the instrumentation is interconnected. They can also be useful in training workers and contractors.


Limitations of a P&ID

Since P&IDs are graphical representations of processes, they have some inherent limitations. They can’t be relied on as real models because they aren’t necessarily drawn to scale or geometrically accurate. There’s also no generally accepted universal standard for them, so they may look different from company to company — or even within the same company — based on internal standards, the type of software system being used, and the preferences of the creator. That’s why it’s important to design and review the documentation that gets down to the real nuts-and-bolts of support documents.


What should a P&ID include?

While there are no exact standards for the way P&ID should be drawn, there have been standards suggested by the Process Industry Practice (PIP), a consortium of process industry owners and engineering construction contractors who serve the industry. PIC001: Piping and Instrumentation Diagram Documentation Criteria details what a P&ID should contain:

  • Mechanical equipment with names and numbers
  • All valves and their identifications
  • Process piping, sizes and identification
  • Miscellaneous – vents, drains, special fittings, sampling lines, reducers, increasers and swagers
  • Permanent start-up and flush lines
  • Flow directions
  • Interconnections reference
  • Control inputs and outputs, interlock
  • Seismic category
  • Interfaces for class changes
  • Quality level
  • Annunciation inputs
  • Computer control system input
  • Vendor and contractor interfaces
  • Identification of components and subsystems delivered by others
  • Intended physical sequence of the equipment
  • Equipment rating or capacity

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