Gate valve
The general on/off valve. Fully open or fully closed, not for throttling.
P&ID symbols
A P&ID (piping and instrumentation diagram) shows how a process plant is piped and controlled: every pipe, valve, pump, tank and instrument, and the signals between them.
Instrument symbols and tags follow ISA-5.1 in most of the world. Equipment shapes vary a little between companies, so every P&ID set starts with a legend sheet; the shapes below are the common ones.
Free. In draw.io, open the library with File › Open Library from › Device. Each symbol below also downloads as SVG, or copies straight into draw.io.
Valves
A valve is two triangles meeting at their tips; what sits in the middle tells you the type. Valves are drawn open unless marked otherwise.
Valve, normally closed
Filled triangles mean the valve is normally closed, for example a drain or bypass.
Globe valve
Used to throttle (regulate) flow. Most control valves are globe valves.
Ball valve
A quarter-turn on/off valve with a bored ball inside.
Butterfly valve
A disc that turns in the pipe. Light and compact, common on large water and air lines.
Check valve
Also: non-return valve
Lets flow pass one way only, in the direction of the arrow. Stops backflow into a pump.
Plug valve
A quarter-turn valve with a tapered plug; often used on slurries and gas.
Needle valve
Fine control of small flows, often on instrument lines.
Angle valve
Inlet and outlet at 90 degrees, so the valve also turns the pipe.
Three-way valve
Three ports: splits one flow into two, or picks between two.
Pressure relief valve
Also: safety valve, PSV, PRV
Opens on its own when pressure gets too high. The zigzag is the spring that holds it shut.
Valve actuators
The symbol on top of a valve says what moves it.
Control valve
Also: diaphragm actuator, pneumatic valve
The dome is a diaphragm driven by air pressure. The controller moves the valve to any position, not just open or shut.
Motor-operated valve
Also: MOV, electric actuator
Opened and closed by an electric motor (M).
Solenoid valve
Snapped open or shut by an electric coil (S). Fast, on/off only.
Hand-operated valve
Also: handwheel
The T is a handwheel or lever: someone turns it by hand.
Pumps, compressors and fans
Centrifugal pump
The most common pump: a spinning impeller in a round casing. Liquid leaves through the outlet at the top.
Gear pump
Two meshing gears push a fixed amount per turn. Good for oil and other thick liquids.
Screw pump
A rotating screw moves the liquid. Gentle on viscous or solid-laden fluids.
Peristaltic pump
Also: hose pump, dosing pump
Rollers squeeze a hose; used to dose chemicals accurately.
Compressor
Raises the pressure of a gas. The narrowing lines show the gas being squeezed.
Fan
Also: blower
Moves air or gas at low pressure.
Tanks and vessels
Storage tank
Also: atmospheric tank, cone roof tank
A tank at atmospheric pressure, flat bottom and cone roof.
Floating roof tank
The roof floats on the liquid, which cuts vapour losses. Used for crude oil and fuels.
Vertical vessel
Also: pressure vessel
A closed vessel with rounded (dished) ends that can hold pressure.
Horizontal drum
Also: horizontal vessel
The same lying down; often a separator, an accumulator or a fuel tank.
Storage sphere
Also: Horton sphere
Holds liquefied gas such as LPG under pressure.
Mixing vessel
Also: agitated tank, reactor
A vessel with an agitator inside and a heating or cooling jacket around it.
Column
Also: tower, distillation column
A tall vessel with trays or packing inside, used to separate mixtures, for example by distillation.
Gas cylinder
Also: gas bottle
A portable bottle of compressed gas: nitrogen, CO2, oxygen.
Heat exchangers
Heat exchanger
Also: shell and tube
Moves heat from one fluid to another without mixing them. The zigzag is the tube bundle.
Plate heat exchanger
Thin plates stacked together; compact and efficient, common in food and HVAC.
Kettle reboiler
Boils liquid from the bottom of a column and sends the vapour back up.
Air cooler
Also: fin fan
Cools a process fluid with air blown over finned tubes.
Cooling tower
Cools water by evaporating part of it into the air.
Instruments
Instruments are drawn as bubbles with a tag inside, such as PT-101. The line through the bubble says where the instrument is; the shape says what kind it is.
Field instrument
A plain circle: mounted in the plant, on or near the pipe. A local gauge or a transmitter.
Control room instrument
A line through the middle: in the main control room, where the operator can see it.
Local panel instrument
A double line: on a panel out in the plant, such as a compressor control panel.
Behind-panel instrument
A dashed line: in the control room but out of the operator's sight, for example inside a cabinet.
DCS function
Also: shared display, shared control
A circle in a square: a function of the distributed control system, shown on the operator's screen.
Computer function
A hexagon: a calculation done in a computer, such as a flow total or an optimizer.
PLC function
A diamond in a square: logic in a programmable logic controller, such as an interlock or trip.
Lines and signals
The line style says what travels along it.
Process line
Also: pipe
A heavy solid line is a pipe carrying the process fluid. An arrow shows the flow direction.
Electrical signal
A dashed line: a wired signal, such as 4-20 mA from a transmitter to the control system.
Pneumatic signal
A line with double slashes: an instrument air signal, often 3-15 psi to a valve.
Hydraulic signal
A line with L marks: a signal carried by hydraulic oil.
Capillary tube
A line with crosses: a sealed capillary, as on a filled temperature or pressure gauge.
Data link
Also: software link, fieldbus
A line with small circles: data between systems, such as a fieldbus or a link inside the control system.
Piping parts
Reducer
Joins a larger pipe to a smaller one.
Y-strainer
Catches dirt before it reaches a pump or valve. The leg holds the screen.
Spectacle blind
Also: figure 8 blind
A plate between two flanges. Turned one way it blocks the pipe completely; turned the other it lets flow through.
Flexible hose
A flexible section, for example at a loading point or to take up vibration.
Expansion joint
Also: bellows
Absorbs movement as the pipe heats up and grows.
Rotameter
Also: variable area flowmeter
A float in a tapered glass tube shows the flow rate.
Venturi meter
Measures flow from the pressure drop in a narrowed section of pipe.
How to read an instrument tag
The letters inside a bubble say what the instrument measures and what it does. The first letter is the measured value; the next letters are its functions. The number is the loop, so FT-101 and FIC-101 belong together.
| Letter | First letter (measures) | Next letters (does) |
|---|---|---|
| A | Analysis (pH, O2, gas) | Alarm |
| C | Control | |
| E | Voltage | Sensor (element) |
| F | Flow | |
| I | Current | Indicate |
| L | Level | Low (in alarms: LAL) |
| H | Hand (manual) | High (in alarms: LAH) |
| P | Pressure | |
| S | Speed | Switch |
| T | Temperature | Transmit |
| V | Vibration | Valve |
| Z | Position |
Examples
PT-101: pressure transmitter. TI-205: temperature indicator (a local gauge). FIC-110: flow indicating controller, usually in the DCS. LAH-300: level alarm, high. PSV-12: pressure safety valve.
Marked up a P&ID by hand?
Take a photo of a sketch or a red-lined print. Sketch2scheme turns it into an editable draw.io diagram you can clean up and share.
Questions
What is the difference between a P&ID and a PFD?
A process flow diagram (PFD) shows the main equipment and flows, often with temperatures and pressures. A P&ID adds every pipe, valve and instrument and how they are controlled. Plants are built and run from P&IDs.
What does a line through an instrument circle mean?
Where the instrument is. No line: in the field. One solid line: in the main control room. Two lines: on a local panel. A dashed line: behind a panel, out of the operator's view.
Which standard defines P&ID symbols?
ISA-5.1 for instruments, tags and signal lines. Equipment and valve shapes follow ISO 10628 or company standards, which is why each P&ID set comes with its own legend sheet.
Can I draw P&IDs with these symbols for free?
Yes. Download the draw.io library, open it in draw.io (free, in the browser or on desktop) and drag the symbols onto your drawing.
More symbol charts
Symbols are drawn with draw.io (diagrams.net), which is open source. Use them freely in your own drawings, documents and lessons.