Basics of Piping and Instrumentation Diagrams (P&IDs)

• The term P&ID stands for Piping and Instrumentation Diagram or Drawing.
• In other words it is also called a process and instrumentation diagram or simply a P&I diagram or drawing.
• P&IDs are commonly called engineering flow drawings or mechanical flow diagrams.
• These P&IDs are most used in the process industries to show the process flow and other installed equipment and instruments.
• These P&IDs also show the interconnection of process equipment and the instrumentation used to control the process.
• P&IDs are schematic diagrams that include both details of engineering and design of the process plants.
• These Piping and Instrumentation Diagrams play an important role in the design and engineering of process plants and piping systems.
• These P&IDs also play an important role in plant maintenance and modification.
• These P&IDs demonstrate the physical sequence of equipment and system as well as how they all connect.
• A P&I diagram can be used by every discipline of those who are involved in the designing, engineering, and construction of process plants.
• These P&ID documents are used as a design reference for piping and process plant layout for the purpose of checking the engineering and design documents and drawings that are associated with the project.
• P&IDs are also used in purchasing materials and for generating a material bill for procurement and construction of the project.
• The P&ID provides the basis for the development of the project during the design stage to allow for further safety and operational investigations such as hazards and operability studies.

The P&ID is a pictorial representation of:

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

P&IDs typically contain the following information:

• All the equipment and its specifications are usually presented in the form of a table.
• All piping and line specifications.
• All piping system components such as fittings, flanges, and valves with their specifications.
• All instrumentation and control components.
• Flow directions.
• Information on process variables such as pressure and temperature.
• Material specifications.
• Specialty items such as strainers.
• Control Input and Output, Interlocks, and Alarm systems.
• Interconnections references.
• Computer control system input
• Identification of components and subsystems delivered to others.

ISA P&ID Symbols:

• ISA standard S5.1: Instrumentation Systems and Automation Society.
• In every process industry, most of the piping and instrumentation diagrams are abbreviated as P&ID.
• These are made by using a standard set of instrument symbols based on ISA.

Now let us study how these ISA standard sets of instrument symbols are used for developing P&ID drawings.

An ISA standard S5.1 defines four sets of general symbols to identify the instruments that are used in a piping and instrumentation diagram (P&ID) mainly:

1. Discrete Instruments
2. Shared Control or Shared  Display
3. Software Computer Function.
4. Programmable Logic Controllers.

How do we classify instruments by symbols used in P&IDs?

These sets of distinct symbols used in P&IDs are grouped into three location categories namely:

1. Central control room is the primary location.
2. The local panel body in the field or process plant is called the auxiliary location.
3. Field mounted.

How are the instruments on P&ID drawings indicated?

Indication of instruments in a P&ID

• Circular symbols called circles on P&ID drawings indicate an individual instrument.
• Shared control or shared display elements are circles surrounded by a square.
• A circle surrounded by a square on P&ID drawings indicates a shared control or shared display element.
• A hexagon symbol on P&ID drawings indicates a software computer function.
• A diamond symbol inside a square on P&ID drawings indicates a programmable logic controller (PLC) function.

What does a horizontal line inside graphical elements indicate?

• Adding a single horizontal bar or line inside any of these four graphical elements indicates the function resides in the primary location, usually in a central control room.
• Adding the double horizontal line inside any of these four graphical elements indicates the function resides in an auxiliary location.
• Adding a dashed horizontal line inside any of these four graphical elements indicates the function resides behind a panel board or in some locations that are inaccessible to the operator.
• The absence of a horizontal line inside any of these four graphical elements indicates that the device or instrument is field-mounted.

A brief description of these graphical elements used in piping and instrumentation diagrams is shown in the below table.

	Central control room		Auxiliary Location
	Operator Access	Behind panel or inaccessible to operator	Access to Operator	Behind panel or inaccessible to operator	Field Mounted
Discrete Instruments
Shared Control or Shared Display
Software Computer Function
Programmable Logic Controllers

• A combination of letters and numbers appears inside a symbol in each graphical element.
• The ISA standard defines the combination of letters and numbers to indicate the instrument’s functionality.
• These letters and numbers are assigned by the user. But some companies or project teams use sequential numbering.
• The first letter in the tag number defines the process parameter or measured variables such as Analysis (A), Flow (F), Level (L), Pressure (P), and Temperature (T).
• The second letter with succeeding letters defines the readout, passive, or output functions such as Indicator (I), Recorder (R), Transmitter (T), Controller (C), and so on.