P&ID Equipment Symbols with Meanings
The process equipment symbols you see around instruments on a P&ID: compressors and pumps, drums and columns, mixers and filters, heat exchangers, cooling towers, tanks, motors and turbines.
Centrifugal compressor
Compressor: a trapezoid that narrows from suction to discharge, drawn with inlet and outlet lines.
Reciprocating compressor
Piston compressor: a cylinder drawn as a rectangle with a piston line, often marked with the letters RC on the project legend.
Positive displacement pump
Rotary or gear pump: a circle with two touching inner circles (gear pump style); legends vary.
Horizontal drum or vessel
Horizontal pressure vessel with dished heads, used for separators, reflux drums and accumulators.
Column or tower
Distillation or absorption column: a tall vessel with dished heads; dashed lines show trays or packing.
Agitated vessel (mixer)
Vertical vessel with an agitator: a shaft from a top mounted motor with impeller blades inside.
Filter or strainer
Inline filter: a vessel or box with a dashed diagonal showing the filter element; Y strainers are shown as a small Y on the line.
Fan or blower
Fan: a circle with blades drawn as two crossing curves, or a circle with a tangential outlet for a centrifugal blower.
Plate heat exchanger
Plate exchanger: a rectangle with closely spaced vertical lines for the plates and four nozzles.
Air cooled exchanger (fin fan)
Air cooler: a tube bundle drawn as a rectangle with a zigzag tube line and a fan below or above.
Cooling tower
Cooling tower: a trapezoid or hyperbolic outline with a fan at the top for induced draft towers.
Storage tank (cone roof)
Atmospheric storage tank: a rectangle with a shallow cone roof; a floating roof tank shows the roof inside the shell.
Shell and tube exchanger
Shell and tube exchanger: a horizontal shell with a channel head and tube passes shown as a line through the shell.
Electric motor
Motor driver: a circle with the letter M, connected to the driven equipment by a shaft line.
Steam turbine
Turbine driver: a trapezoid that widens from inlet to exhaust (the reverse of a compressor).
Flame arrester
Flame arrester: a box or two lines across the pipe with crossed hatching, tagged FA on many legends.
How Equipment Symbols Are Drawn
Equipment symbols are not part of ISA 5.1, which covers instruments. Most companies base them on ISO 10628 or their own engineering standard, so shapes differ between projects. Equipment carries its own tag, for example P 101A for a pump, C 201 for a compressor, E 301 for an exchanger and V 401 for a vessel, separate from the instrument loop numbers.
Frequently Asked Questions
How is a compressor shown on a P&ID?
Usually as a trapezoid that narrows from the suction side to the discharge side.
Are equipment symbols in ISA 5.1?
No. ISA 5.1 covers instrument symbols and identification; equipment symbols follow ISO 10628 or company standards, shown on the project legend sheet.
What do letters like P, E and V mean on equipment tags?
Common equipment prefixes: P pump, C compressor, E exchanger, V vessel or drum, T tank or tower. The project numbering procedure defines them.
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