P&ID legends


As an Instrument Technician trainee you will use Piping and Instrument Diagrams or P&ID’s in your job.  You must be able to interpret the information and symbols shown on the drawing.For that you need the help of P&ID legends.

The P&ID’s on the job site will help you in five different ways:

  1. To identify major equipment used in the process system.
  2. To identify and trace the  flow of the product through the plant.
  3. To identify information  defining pipe sizes and allowable working pressures.
  4. To understand the process that is going on in the plant.
  5. To find the locations of control valves and other instruments where they can be easily seen for preventive maintenance.


Main Process Lines  are the major pipelines in a process system.  Most P&IDs will show these as heavy lines with arrows on the lines.  The arrows show the direction of flow.

Auxiliary (secondary)  Process Lines are pipes that  feed into the main process line or draw from the main process line. (ie shown in fig below)




Instrument legend





Piping on a  P&ID  is indicated by:

1)     Usage:     For example, process, drain, nitrogen, blowdown, etc.

2)     Line Number:  The identification number of the line on the plant.

3)     Size:        Usually in inches.

4)     Piping Class:    The piping specification, both material and pressure rating


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P&ID and Common Abbreviation


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