p&id symbols | ISA Symbols and Loop Diagrams

p&id symbols, Piping and Instrumentation Diagrams or simply P&IDs are the “schematics” used in the field of instrumentation and control (Automation).

The P&ID is used to by field techs, engineers, and operators to better understand the process and how the instrumentation is inter connected.

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Most industries have standardized the symbols according to the ISA Standard S5.1 Instrumentation Symbol Specification.

  • Piping & Instrumentation Drawing (original)
  • Process & Instrumentation Diagram (also used)
  • Process Flow Diagram – PFD (simplified version of the P&ID)

Temperature Process

Using pictorial diagrams may be informative however it is not practical or CAD friendly especially in a multi-loop process.

temperature symbol

Building the P&ID

The P&ID will use symbols and circles to represent each instrument and how they are inter-connected in the process.

building the P&id

Tag Numbers

Tag “numbers” are letters and numbers placed within or near the instrument to identify the type and function of the device.

tag number

Tag Descriptors

1 . The first letter is used to designate the measured variable











2. The succeeding letter(s) are used to designate the function of the component, or to modify the meaning of the first letter.





 Tag Numbers

Tag “numbers” are letters and numbers placed within or near the instrument to identify the type and function of the device.

tag number 1

ISA S5.1 Identification Letters

First – Letter Succeeding – Letters
Measured or Initiating Variable Modifier Readout Function Output Function Modifier
A Analysis
C Control
D Differential
F Flow Rate Ratio
H Hand High
I Current Indicate
L Level Low
P Pressure, Vacuum
Q Quantity Totalizer
S Safety Switch
T Temperature Transmit
V Vibration Valve, Damper
Z Position Actuator



Instrument Location

The presence or absence of a line determines the location of the physical device. For example no line means the instrument is installed in the field near the process.

instrument location

Shared Displays/Shared Control

Some instruments are part of a Distributed Control System (DCS) where a specific controller or indicator can be selected from many others but shown in one location (like a terminal screen)

shared control_display

Summary of instrument type & location

summary of instrument

Piping and Connection Symbols

These symbols are used to identify how the instruments in the process connect to each other.

And what type of signal is being used. (electrical, pneumatic, data, etc.


Valve Symbols

valve symbols

P&ID Example

p&id example

P&ID Exercise

p&id exarsise

p&id exarsise solution

Process Flow Diagram – PFD

A PFD shows less detail than a P&ID and is used only to understand how the process works


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