Piping and Instrumentation Diagram (P&ID) is a schematic representation of a process flow between all process units or equipment's in a plant. As the name suggests the graphical representation on a P&ID includes all piping connected between equipment's and the associated instrumentation necessary to design, construct and operate a plant facility.

The P&ID is one of the most important document produced by the ‘Process Department’, and usually requires substantial coordination and inputs from Piping and Instrumentation disciplines. P&ID forms the basis for all disciplines to progress with further detailed engineering on the project. The symbols used to represent process equipment items are usually the same as shown on Process Flow Diagrams or PFD's. PID's show the relationships between the major process system components. P&ID is neither a scaled drawing nor shows the relative locations or elevations of equipment's or process units with respect to each other though some client organizations may prefer to indicate the relative locations between the equipment's. 

All incoming and outgoing lines are identified at the ends of the P&ID in boxed arrows. The continuation P&ID numbers are indicated inside the boxed arrows. For Piping Discipline, the P&ID forms the basis for developing the detailed piping plans, hydrotest diagrams, preliminary bill of materials and many other detailed engineering activities. It is important that piping engineers work with the latest version of the P&IDs.

P&IDs being one of key documents undergoes several design reviews within the consulting organization and with the Owner. A P&ID is also subject to HAZOP and SIL review. Before the HAZOP review is carried out, the P&IDs should have gone through the necessary design reviews within the engineering organization and with the Owner and be at a ready for Design stage. The HAZOP review changes are documented and actions are monitored through a HAZOP actions tracking register.

Main Features of Piping and Instrumentation Diagram (P&ID)

The following are the main features of PID's:

  • All process streams and utility streams are shown with flow directions. It is recommended to place the arrows at the entry and exit of equipment and at the end of line segment. If the flow is bi-directional, the arrows may be shown at both ends of the line or at the center of the line.
  • Lines should enter a P&ID from the left side end and exit the P&ID at the right side end. Continuation drawing numbers should be shown at the ends of the line. The source and destination description should be shown above to below the continuation drawing numbers.
  • Line number for a line connecting two equipment's should be unique even if it includes more than one line size. However if there is a change in piping material class such as downstream of an isolation valve, the downstream line should be provided with a different line number. If the flow splits and has a branch connecting to another location, the branch should have a different line number.
  • Lines requiring slope or no pockets are clearly identified. The slopes are indicated as 1:100 or 1:500 depending on the process requirement. A slope of 1:100 means that the line should fall by 10mm for every horizontal run of 1000mm. Similarly 1:500 means that the line should fall by 2mm for every horizontal run of 1000mm. This means 1:100 lines are more steeper than 1:500 sloped lines. Sometimes the slope may be indicated as a percentage.
  • All line sizes with the line serial numbers, fluid codes, area codes, piping material class specifications or designations, piping class breaks, tie-in points, insulation and heat tracing requirements are shown. All fluid codes or service codes used in the PIDs are elaborated in the legend sheet.
  • Insulation symbol such as heat conservation insulation or personnel protection insulation is shown only on lines that are to be insulated. Insulation shall be designated by a symbol and code letter.
  • All valves are shown including process vents and drains. This includes control valves, emergency shutdown valves and other actuated valves. Valves with special requirements such as locking arrangement are identified. Actuated valves such as motor operated valves, pneumatically or hydraulically operated valves are differentiated from the manual valves by addition of the applicable actuator over the valves. The symbol for the valve remains same as the manual valve. In general, a control valve is shown with a diaphragm actuator and an on-off valve is depicted with cylinder and piston actuator.
  • All valves that are required to be locked open or locked close or interlocked with other valves should be identified on the P&ID with the appropriate legend.
  • All reducers, branches, blind flanges, insulating gaskets, spectacle blinds with their normal operating position (open or closed), removable spools, orifice plates, rupture disks, special items such as strainers, steam traps, flame arrestors, barred tees etc. are shown.
  • Hydrostatic test vents and drains are usually not shown but may be updated on the P&ID's by some Organizations during the as-built stage.
  • All spare connections required for the future are shown clearly indicating the purpose of the future connections.
  • All instruments and controls are shown with their tag numbers.
  • All equipment's are shown with their tag numbers, their rated capacity, overall dimensions, design pressures and design temperatures. Equipment details indicated in the PIDs should be consistent with the equipment list.
  • To avoid clutter on the PID, the equipment tag numbers and relevant details are tabulated on the top section or bottom section of the drawing.
  • All equipment and instrument details indicated in the PIDs are consistent with their respective data sheets.
  • All pipelines are shown with their line sizes and design parameters.
  • Legend of all equipment's and instruments are shown either on the same sheet or on a separate common sheet depending on the size and complexity of the project. The legend sheet usually includes all the abbreviations used in the P&ID. For e.g. LO for Locked Open Valves or LC for Locked Closed Valves. All typical piping connections and special items shown in the PIDs are described in the legend sheets. Similarly, all instrument abbreviations used in the PIDs are described in the legend sheet.
  • Special engineering notes are provided usually on the top right hand side of drawing. The notes should cover specific or unique project requirements or specifications.

What a P&ID will not show

A P&ID is not intended to show the following:

  • Instrument tubing and valves. Instrument hook-up drawings are prepared to show these details.
  • P&IDs will show the reducers but is not intended to show elbows, type of branch connections (e.g. it's a reducing tee or olet branch or set-on branch). These details will be available in the piping material specification.
  • P&ID will not show the pipe routing, pipe lengths and pipe fittings. Piping drawings such as General Arrangement Drawings (GADs) and Isometrics will show these details.
  • P&ID will not show pipe support locations. 
  • In most organizations, the Vendor package P&IDs are shown as blank boxes with tie-in connections outside the boxes. Reference is provided to the Vendor P&ID for the details inside the package. The scope of work agreement may include a requirement to make the Vendor P&IDs on the Organizations standard template.

Piping and Instrumentation Diagram (P&ID) Abbreviations

Following are the commonly used abbreviations in P&IDs.

Abrreviation Description
AG Above Ground
ATM Atmosphere
BL Battery Limit
CSO Car Seal Open
CSC Car Seal Closed
DCS Distributed Control System
DP Design Pressure
DT Design Temperature
ESD Emergency Shut Down
FL Fail Locked or Last
Valve to remain in last (locked) position in case of a malfunction such as loss of signal or power or instrument air supply.
FC Fail Closed
FO Fail Open
FV Full Vacuum
HS Hand Switch
HC Hose Connection
LC Lock Closed
LO Locked Open
MOV Motor Operated Valve
MAX Maximum
MIN Minimum
NNF Normally No Flow
PLC Programmable Logic Controller
SG Specific Gravity
SO Steam Out
TSO Tight Shut Off
UG Under Ground
W/O Without

Piping and Instrumentation Diagram (P&ID) Symbols

Below are provided example of symbols used in Piping and Instrumentation Diagrams. These examples are generic and different Organizations have their own versions.

P&ID General Symbols for Piping

Diagram below shows typical PID Symbols for various piping components such as reducers, spectacle blinds, sight glass, hose connections, blind flanges etc.

General Piping Symbols
Examples of General Piping Symbols on P&IDs

P&ID Symbols for Piping Valves

Diagram below shows typical PID Symbols for various types of valves. All valve symbols are represented in same size on P&IDs. Valves sizes may not be indicated on the P&ID if the valve size is same as line size.

Symbols for Piping Valves
Examples of Symbols for Piping Valves

P&ID Symbols for Instrumentation

Diagram below shows typical PID Symbols for Instrument signals and actuators for Control valves. Motor Operated Valves are also used for on-off valves in addition to control valves.

Symbols for Instrumentation
Examples of Symbols for Instrumentation

Equipment and Tank Symbols

Diagram below shows PID Legend for Vessels, Distillation Columns, Fixed Cone Roof Tanks, Floating Roof Tanks and Dome Roof Tanks.

Legend for Equipments and Tanks
Examples of Symbols for Equipments and Tanks

Heat Exchangers Symbols

Diagram below shows PID Legend for various types of heat Exchangers such as Shell and Tube Heat Exchangers, Plate Type Heat Exchangers, Air Cooled Exchangers and Fired Heaters.

Legend for Heat Exchangers
Examples of Symbols for Heat Exchangers

Pumps and Compressors Symbols

Diagram below shows PID Legend for Centrifugal Pumps, Reciprocating Pumps, Centrifugal Compressors and Reciprocating Compressors

Legend for Pumps and Compressors
Examples of Symbols for Pumps and Compressors

Example of Piping and Instrumentation Diagram

Piping and Instrumentation Diagram
Example of Piping and Instrumentation Diagram

Example of Piping and Instrumentation Diagram Legend Sheet

Process and Instrumentation Diagram (PID)
Example of Process and Instrumentation Diagram Legend Sheet