P&IDs and Loop Diagrams

Process and Instrument Diagrams (P&IDs), also called Piping and Instrumentation Diagrams and Loop diagrams are construction and documentation drawings that show the flow of the process and illustrate the instrumentation control and measurement instructions, wiring and connections to the process.

The sections or subsystems of the process that are usually shown are called loops.

A Loop diagram will detail the connections and pneumatics and wiring from the field device through any junction boxes or marshalling cabinet to the controller or computer interface which controls the process.

Instrument line symbols

All instrument lines are to be drawn fine in relation to the process piping lines.

Tagging Nomenclature

The following table illustrates standard tagging used for instrument identification.

Table 1 : Typical Tag Number

Tag	Description
TIC 114	Instrument identification or Tag Number
T    114	Loop identification
114	Loop number
TIC	Function identification
T	First letter (measured variable)
IC	Succeeding letters (modifier, readout or function)

Table 2: Expanded Tag Number

10-PHA-5A	Tag Number (note: hyphens are optional as separators)
10	Option Prefix
A	Option suffix

The top of the tag will identify the instrument type, and the bottom will identify the loop number. Note that the loop number is just a section or sub subsystem of the process.

General Instrument or Function Symbols

The figure below shows general instrument or function symbols.

Related: Instrumentation Diagrams

Identification Letters

The tables below shows the recommended tag identification letters for instruments and control element designations used in P&IDs and loop diagrams