P&ID Symbols and Meanings: The Complete Legend

What is a P&ID and why the symbols matter

A piping and instrumentation diagram, or P&ID, shows how a process is put together: the equipment, the piping that connects it, the valves that control the flow, and the instruments and control loops that run it. It does not show real lengths or scale. It shows logic, unlike a process flow diagram, or PFD, which stays high level. The whole thing is drawn in symbols, so reading a P&ID is reading its symbols and their meanings. If you are learning how to read a P&ID, this guide is a plain-language legend of the common piping and instrumentation diagram symbols and their meanings, and how to use them. For an estimator the P&ID is where the valve counts, the instrument counts, the equipment and the line list come from, so the symbols are not trivia, they are the takeoff.

The four symbol groups

Everything on a P&ID falls into one of four groups, the four types of P&ID symbols you will meet. Equipment symbols show the vessels, tanks, columns, pumps, compressors and heat exchangers, drawn as recognizable schematic shapes. Line symbols show the piping and the signals, with different line styles for a process pipe, a pneumatic signal, an electric signal and a data link. Valve symbols show the valve types, drawn as the bowtie and its variants. Instrument symbols, the bubbles and tags, show what is measured and controlled and where the instrument sits. Learn to sort a symbol into its group first, then read the symbol and its meaning.

Line and signal symbols

These P&ID line and signal symbols separate the piping from the signals at a glance. The process piping is drawn as a heavy solid line, and the line number tells you its size, service and class. The signal lines are thinner and coded by pattern. A connection to the process, the impulse line, is a thin solid line. A pneumatic signal is a line with double crosshatch marks. An electric signal is a dashed line. A software or data link inside a control system is a line with small circles or dots. A capillary is a line with crosshatch. The pattern tells you what kind of signal it is, which tells you the instrument type feeding it, so each line style has a clear meaning.

Equipment symbols

Equipment is drawn as a simplified picture of the real thing. A vessel or drum is a capsule or cylinder, a column is a tall vessel with internals shown, a pump is a circle with a triangle, a compressor has its own outline, and a heat exchanger is a shell with tubes or a circle with the flow paths. Each piece carries an equipment tag, for example a pump P-101 or an exchanger E-201, and often a duty or size note. You read the shape for what it is and the tag for which one it is.

Valve symbols

Valves are drawn as a bowtie, two triangles meeting at a point, with the type shown by what sits in the middle or on top. These are the common P&ID valve symbols you will see across projects.

A control valve carries an actuator symbol on top that tells you how it is driven: a dome or bell for a diaphragm actuator, a rectangle for a piston, a circle with M for a motor, and a small box with S for a solenoid. The actuator matters for the count, because a control valve with its actuator, positioner and tubing is a different item from a plain manual valve. The control valve symbol can vary slightly by project, so always confirm it in the drawing legend.

Instrument bubbles and how to read them

An instrument is a circle, called a bubble. The line through the bubble tells you where it sits and who can reach it.

The tag inside the bubble is the other half. Read it and you know what the instrument does. These are commonly called P&ID instrument tags on drawings and in specifications.

The instrument tag, decoded

Instrument tags follow ANSI/ISA-5.1. The first letter is the measured or initiating variable, and the letters after it are the function.

So FT is a flow transmitter, FIC is a flow indicating controller, LG is a level gauge, LSH is a level switch high, PT is a pressure transmitter, TIC is a temperature indicating controller, and PSV is a pressure safety valve. Add the loop number and you have the full tag, for example FIC-101. Read the first letter for what it senses and the rest for what it does with it.

Reading a P&ID for a takeoff

For estimating, the P&ID is the bid-stage source. It does not carry lengths, but it carries the counts. Off a P&ID you take the valve count by type and size, the instrument count by type off the bubbles and tags, the equipment list off the equipment symbols, the specialty items, and the line list with the size, service and class for each line. You itemize the high-value items, the valves, instruments and equipment, straight from the symbols and the schedules, and you carry the line list into the isometric stage for the lengths and bulk material. Miss a control valve station or read a spec break wrong and the count is off before pricing starts.

Common mistakes reading P&ID symbols

The usual ones: reading a control valve as a plain valve and missing the actuator, positioner and tubing that go with it, misreading the bubble line and putting an instrument in the wrong location class, reading only the first letter of a tag and missing the function, and trusting a symbol from memory instead of the drawing legend, which is where the project defines its exact set. Signal lines get skimmed too, and the signal type tells you the instrument type.

Where theTakeoff fits

theTakeoff reads P&IDs and pulls the counts off the symbols and the tags. It lists the valves by type and size, the instruments by type off the bubbles, the equipment, and the line list with each line's size, service and class, and every count links back to the symbol on the drawing. The estimator checks the count against the sheet instead of building it by hand, and owns the number that goes in the bid.