p&id symbols | ISA Symbols and Loop Diagrams |

Table of Contents

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.

Read In Hindi

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.

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.

Tag Numbers

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

Tag Descriptors

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

Pressure

Level

Flow

Temperature

Indicator

Recorder

Controller

Transmitter

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

Indicator

Recorder

Controller

Transmitter

Tag Numbers

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

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

Examples

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.

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)