The Piping and Instrumentation Diagram (P&ID) is the definitive engineering document for a process plant. Every instrument, control loop, safety function, and equipment item is represented on the P&ID. The ability to read P&IDs fluently is a core skill for any automation or instrumentation engineer.

P&ID SYMBOL LEGEND

Common ISA 5.1 Tag Prefixes

FIC
Flow Indicating Controller

PIC
Pressure Indicating Controller

TIC
Temperature Indicating Controller

LIC
Level Indicating Controller

AIC
Analyzer Indicating Controller

CV
Control Valve

Tag Format:  F I C - 1 0 1  = First letter (measured var) + Middle letters (function) + Loop number

P&ID Standards

P&IDs follow ANSI/ISA-5.1 (Instrumentation Symbols and Identification) in North America, or ISO 10628 in Europe. The symbols and tag naming conventions come from these standards, though every plant also has its own legend and notes that extend the standard.

Instrument Tag Format

The standard ISA-5.1 instrument tag has the format:

[Loop ID] [Function Letters] [Loop Number]

For example: FIC-101 = Flow (F), Indicating (I), Controlling (C), Loop 101.

Common first-letter codes: F=Flow, P=Pressure, T=Temperature, L=Level, A=Analysis, H=Hand.

Subsequent letter codes: I=Indicating, R=Recording, C=Controlling, T=Transmitting, A=Alarming, S=Switch/Shutdown.

Instrument Bubble Symbols

  • Circle: Field-mounted instrument
  • Circle with horizontal line through middle: Panel-mounted (behind panel)
  • Circle with double horizontal lines: DCS/computer function
  • Hexagon: PLC function
  • Diamond: Analyser

Line Types

  • Solid line: Process piping
  • Dashed line: Instrument signal (pneumatic or electrical)
  • Dotted line: Software/data link
  • Chain-dashed line: Capillary or mechanical link

Control Loop Example

A basic flow control loop on a P&ID shows:

  1. FT-101: Flow Transmitter (field-mounted, measuring the flow)
  2. FIC-101: Flow Indicating Controller (DCS function – receives signal, displays flow, outputs control signal)
  3. FCV-101: Flow Control Valve (final element receiving the controller output)
  4. A line from FT-101 to FIC-101, and from FIC-101 to FCV-101

Safety Instrumentation on P&IDs

SIS elements are distinguished from basic process control by line style or colour (often yellow in digital P&IDs). The SIS tag suffix or prefix (typically XS, HS, or SIS prefix) identifies safety loops. ISA-84 requires SIS elements to be clearly identifiable and segregated on P&IDs.