Common P&ID Symbols

P&ID symbols are standardized graphical representations used on Process and Instrumentation Diagrams to identify valves, instruments, equipment, and piping components. The primary standards are ANSI/ISA-5.1 (instrumentation symbols and identification) and ISO 14617 (graphical symbols for diagrams). Every EPC project includes a legend sheet defining the symbols used on that project’s P&IDs.

Symbol Category	Common Symbols	Standard
Gate valve	Two triangles meeting at a point	ISA-5.1 / ISO 14617
Globe valve	Two triangles with a vertical line through the center	ISA-5.1
Ball valve	Two triangles with a filled circle at center	ISA-5.1
Check valve	Triangle with a vertical line (flow direction arrow)	ISA-5.1
Butterfly valve	Two triangles with a line at 45 degrees	ISA-5.1
Plug valve	Two triangles with a filled rectangle	ISA-5.1
Control valve	Valve symbol with diaphragm actuator on top	ISA-5.1
Safety/relief valve	Angle body with spring or pilot symbol	ISA-5.1
Instrument (field)	Circle with ISA tag letters	ISA-5.1
Instrument (panel)	Circle with horizontal line (panel-mounted)	ISA-5.1
Instrument (DCS)	Circle with dashed horizontal line	ISA-5.1

ISA Instrument Letter Codes

Instrument tags on P&IDs follow a structured format. The first letter identifies the measured variable; subsequent letters identify the function.

First Letter	Measured Variable	Common Functions
F	Flow	FT (transmitter), FIC (indicator-controller), FE (element)
P	Pressure	PT, PIC, PSV (safety valve), PSH (high switch)
T	Temperature	TT, TIC, TE (thermocouple/RTD), TSH (high switch)
L	Level	LT, LIC, LAH (high alarm), LALL (low-low alarm)
A	Analysis	AT (analyzer transmitter), AIC
H	Hand (manual)	HV (hand valve), HC (hand controller)
S	Speed/Safety	ST (speed transmitter), SIS (safety instrumented system)
V	Vibration	VT (vibration transmitter), VSH (vibration high switch)

A tag like PIC-2105 means: Pressure (P), Indicating (I), Controller (C), loop number 2105. This tag appears both on the P&ID and on the physical instrument in the field.

Line Designation Format

Piping lines on P&IDs carry a line designation that encodes critical information:

Segment	Meaning	Example
Size	Nominal pipe size (NPS)	6”
Fluid code	Service type	HC (hydrocarbon), CW (cooling water)
Line number	Sequential identifier	1001
Pipe class	Material and rating designation	A1A
Insulation	Insulation type code	H (hot), C (cold), T (tracing)

A complete line designation reads: 6”-HC-1001-A1A-H. This tells the piping engineer the size, service, unique line ID, material class, and insulation requirement in a single string.

Common Mistake

Confusing valve symbols on P&IDs. A gate valve and a ball valve look similar but specify entirely different products with different costs, lead times, and applications. Always verify the symbol against the project legend sheet before generating material take-offs.

Equipment Symbols

Equipment	Symbol Description
Centrifugal pump	Circle with discharge arrow and suction line
Reciprocating pump	Circle with triangle (piston indicator)
Heat exchanger	Circle with internal lines (shell-and-tube)
Vessel/drum	Vertical or horizontal cylinder
Tank	Open-top or closed-top cylindrical shape
Compressor	Circle with triangular flow arrows
Filter/strainer	Hourglass or triangular mesh symbol

For the complete P&ID symbol reference, including actuator types, line modifiers, and control loop conventions, see the detailed guide. Accurate symbol reading is required for generating reliable procurement documents and construction work packages.

Read the full guide to pipe classes and specifications