P&ID symbols and how to read a P&ID: lines, valves, instrument bubbles, tags and loops
What a piping and instrumentation diagram shows and leaves out, the line types, valve and equipment symbols, the ISA-5.1 instrument bubbles and what each shape says about where the function lives, how to read an instrument tag, how to follow a P&ID from the source, how it differs from a PFD, and what keeps it true. With an example drawing.
Key takeaways
- A P&ID shows every line, valve, piece of equipment and instrument in a process system, with how it is controlled and protected. It is the working drawing for isolation, modifications, commissioning and control system configuration.
- The symbols come from ANSI/ISA-5.1 for instruments and from ISO 10628-2 or a site's own legend for equipment and valves. A drawing without a legend sheet is read by guesswork.
- Every instrument has a tag in two parts. The letters say what it measures and what it does: PT is a pressure transmitter, TIC a temperature indicating controller. The number ties it to its loop.
- The shape of the bubble says where the function lives: a plain circle in the field, a circle in a square in the DCS, a diamond in a square in a PLC. The line type says how the signal travels.
- A P&ID is read by following one line at a time from the source, and it is only as useful as it is current. The redline from every modification has to reach the drawing.
A fitter is asked to isolate a lube oil filter for a change-out. The P&ID shows the filter, the valves either side of it, the drain and the differential pressure transmitter across it, and the isolation is written up from the drawing in a few minutes. Or the filter has a bypass that was added in a shutdown three years ago and never drawn, and the isolation leaves it open. The P&ID is the drawing every isolation, modification and control change is planned from, and reading it is a skill most people in a plant pick up by being shown.
This guide sets out what a P&ID shows, the symbols on one and where they come from, how instrument tags and bubbles are read, how the drawing is followed from the source, how it differs from a process flow diagram, and what keeps it true. The example drawing below is the crusher lube oil circuit used across this site. It sits beside the single line diagram guide, which is the electrical equivalent, and the control system design documents guide, which covers the documents built from the P&ID.
What the drawing shows
A P&ID shows a process system as it is connected and controlled. Every piece of equipment is drawn with its tag. Every line is drawn with its number, which carries its size, service and piping specification. Every valve is drawn with its type and, where it matters, its normal position. Every instrument is drawn with its tag, and the signal lines show where each measurement goes and what it acts on. Interlocks and trips are marked, with a reference to the logic drawing or the cause and effect matrix that defines them.
What it does not show is layout. A P&ID is not to scale, and two items side by side on the drawing may be 50 metres apart on the plant. Physical arrangement belongs on the general arrangement and isometric drawings. The P&ID answers a different question: what is connected to what, and what controls it.
The example above carries what a P&ID has to carry. The tank with its level transmitter, alarmed low in the DCS. Duty and standby pumps, each with a check valve so the standby does not run backwards. The pressure gauge on the discharge. The filter with a differential pressure transmitter across it, alarmed high, which is how the blockage is found before it starves the bearings. The cooler with a temperature loop that moves the cooling water valve, marked to fail open. And the flow transmitter that feeds the low flow trip, with the trip defined on the cause and effect matrix it references.
Lines and connections
The line type is the first thing to read, because it says whether you are looking at process fluid or a signal. Heavy solid lines are the main process. Thin solid lines connect instruments to the process. Dashed lines are electrical signals, which on a modern plant means most of the instrument signals. Older plants have pneumatic signal lines, drawn with double cross strokes, running to control valves and positioners. A line with small circles is a data link, such as a fieldbus or a software connection between functions.
Every process line carries a line number. A common format is size, service, sequence number and piping specification, so 40-LO-303-A1 is a 40 mm lube oil line, number 303, to piping class A1. The line number ties the drawing to the line list, the isometrics and the piping specification, and it is how a modification finds the materials and pressure rating of the line it is cutting into.
| Line | How it is drawn | What it carries |
|---|---|---|
| Major process line | Heavy solid line | The main process flow, labelled with a line number |
| Minor process line | Thinner solid line | Utilities, drains, vents and small bore connections |
| Instrument process connection | Thin solid line to the instrument | Where the instrument takes its measurement from the process |
| Electrical signal | Dashed line | A wired signal, usually 4 to 20 mA, between an instrument and the control system |
| Pneumatic signal | Solid line with double cross strokes | Instrument air signal, common on older control valves and positioners |
| Data link | Solid line with small circles | A fieldbus or network link, or a software link between functions in the control system |
| Capillary | Solid line with crosses | A filled system, such as a remote seal on a pressure or level instrument |
| Off-page connector | Arrow or flag with a drawing number | Where the line continues on another sheet, and which one |
Valves and equipment
Most valve symbols start from two triangles point to point, and the detail inside says the type: plain for a gate valve, a dot for a globe, a circle for a ball. A check valve has one side filled, with the filled side on the outlet. A control valve has an actuator drawn above the body and a tag, and its fail position is written beside it: FO fails open, FC fails closed, FL fails last position. The fail position matters to anyone isolating or commissioning the valve, because it says what the valve does when its air or signal is removed.
Equipment symbols vary more between legends than instrument symbols do, which is why the legend sheet matters. Pumps, tanks, heat exchangers and filters have recognisable conventional shapes, and each carries a tag that matches the asset register and the label on the plant.
| Symbol | How it is drawn | What it tells you |
|---|---|---|
| Gate valve | Two triangles point to point | Isolation, fully open or fully closed |
| Globe valve | Two triangles with a dot at the centre | Throttling by hand |
| Ball valve | Two triangles with a circle at the centre | Quick isolation, a quarter turn |
| Butterfly valve | Two triangles with a line through, or a disc on a stem | Isolation or throttling on larger lines |
| Check valve | Two triangles with one filled, or an arrow | Flow in one direction only. The filled side is the outlet |
| Control valve | Valve body with an actuator above, and a tag | Positioned by the control system. Fail position marked FO, FC or FL |
| Relief valve | Angle body with a spring above | Opens on overpressure. Set pressure on the drawing or the datasheet |
| Pump | A circle with a tangential outlet | Tagged, duty and standby shown as A and B |
| Filter or strainer | A box or a line with a dashed diagonal | Often with a differential pressure instrument across it |
| Heat exchanger | A circle or a shell with a line or zigzag through it | Hot and cold sides each with their own lines |
Instrument bubbles and where the function lives
Every instrument on a P&ID is a bubble with its tag inside, and the shape of the bubble says where the function is. ANSI/ISA-5.1 sets the shapes. A plain circle is a discrete instrument in the field. A line across the circle puts it in a control room. A circle inside a square is a shared display and control function, which is the DCS. A diamond in a square is a PLC function. The shape is a practical guide: a controller drawn as a circle in a square is changed in the DCS configuration, not on a panel.
| Shape | What it means | Example |
|---|---|---|
| Circle, no line | A discrete instrument mounted in the field | A pressure gauge on the line, a local transmitter |
| Circle, solid horizontal line | A discrete instrument in the control room, normally accessible to the operator | A panel indicator or controller |
| Circle, dashed horizontal line | A discrete instrument behind a panel, not normally accessible | A relay or converter in a cabinet |
| Circle inside a square | A shared display and shared control, usually the DCS | A controller configured in the DCS and shown on the operator screen |
| Diamond inside a square | A function in a PLC | An interlock or sequence in the PLC |
| Hexagon | A computer function | A calculation in a supervisory system or historian |
| Diamond | Interlock or binary logic, with a reference to the logic drawing | A trip or permissive, detailed on the cause and effect matrix |
The bubble's shape says where to find the function. The letters say what it is. The number says which loop it belongs to.
Reading an instrument tag
An instrument tag has letters and a number. The first letter is the measured variable, sometimes followed by a modifier such as D for differential. The succeeding letters are the functions, read in order: indicate, record, control, transmit, alarm, switch, valve. H and L at the end of an alarm or switch say high or low. So TIC-303 is a temperature indicating controller in loop 303, TV-303 is the valve it moves and TT-303 the transmitter that measures. Everything in a loop shares the loop number, which is how the drawing, the IO list, the loop diagram and the DCS configuration all find each other.
The table lists the first letters most often met on mining and process plant with typical tags. The full table in ISA-5.1 is longer and has user-defined letters that each owner assigns, which is another reason to read the legend sheet first.
| First letter | Measured variable | Typical tags |
|---|---|---|
| F | Flow | FT flow transmitter, FIC flow indicating controller, FSL flow switch low |
| L | Level | LT level transmitter, LAH level alarm high, LG level gauge |
| P | Pressure | PI pressure indicator, PT pressure transmitter, PSV pressure safety valve |
| PD | Pressure, differential | PDIT differential pressure indicating transmitter across a filter |
| T | Temperature | TT temperature transmitter, TIC temperature indicating controller, TV temperature control valve |
| A | Analysis | AT analyser transmitter, with the property named beside it (pH, O2, conductivity) |
| S | Speed or frequency | ST speed transmitter on a conveyor pulley |
| V | Vibration | VT vibration transmitter, VAH vibration alarm high |
| W | Weight or force | WT weight transmitter on a belt scale |
| Z | Position | ZSO and ZSC, valve open and closed limit switches |
| H | Hand | HS hand switch, HV hand operated valve |
Following a P&ID from the source
A P&ID is read one line at a time in the direction of flow. In the example, start at the tank. The suction leaves the bottom through a gate valve and splits to the duty and standby pumps. Each discharge has a check valve, so the pump that is not running cannot be driven backwards, and the two join. The pressure gauge reads the common discharge. The filter is next, with its differential pressure transmitter. The cooler follows, with the oil through one side and the cooling water through the other, and the temperature loop after it. The flow transmitter is the last instrument before the oil leaves the sheet for the bearings, and the return comes back on the line across the top.
Then read the signals. Each dashed line from a transmitter goes to the function that uses the measurement. TT-303 goes to TIC-303 in the DCS, and the dashed line from TIC-303 goes to the actuator of TV-303. FT-304 goes to an interlock diamond whose note says the low flow trips the crusher, with the cause and effect matrix as the reference. That is the read an isolation, a modification or a fault-finding job needs: what is in the line, what measures it, and what acts on the measurement.
A P&ID beside a PFD
The process flow diagram comes first in design and shows how the process works, with the mass and energy balance. The P&ID is developed from it and shows everything needed to build, operate and maintain the plant. They answer different questions, and confusing the two is behind a lot of 'the drawing is wrong' complaints that are really 'I am reading the wrong drawing'.
| PFD | P&ID | |
|---|---|---|
| Purpose | How the process works: the flows, the main equipment and the mass and energy balance | How the plant is built and controlled: every line, valve, instrument and loop |
| Equipment | Main equipment only | All equipment including spares, drains, vents and small items |
| Lines | Main process streams, with stream numbers | Every line, with size, service, number and specification |
| Valves | Only those that change the process | Every valve, with type, size where it matters, and normal position |
| Instruments | Key controls only | Every instrument, tagged, with its location and its loop |
| Data | Flows, temperatures and pressures for the design cases | Design and set points referenced to datasheets and the instrument index |
| Used for | Design basis, process studies, operator training on the process | Isolation, modifications, commissioning, control system configuration, hazard studies |
Keeping it true
The P&ID is changed by every modification that adds a valve, moves an instrument, adds a bypass or changes a control. Each produces a redline and a reissued revision through management of change, and the change is not closed until the drawing in the control room and in the document system is the new revision. The document register lists the P&IDs by the systems they show, which is how a change to a system finds the sheets it has to reissue.
A P&ID also feeds the documents built from it: the instrument index, the IO list and cause and effect matrix, the line list and the isolation standards. A tag changed on the drawing and nowhere else is a tag that the IO list, the DCS and the CMMS now call something different.
Where P&IDs go wrong
- No legend sheet in the set, or a legend from a different project, so symbols are read by guesswork.
- Bypasses, drains and temporary connections added on the plant and never drawn.
- Instrument tags on the drawing that differ from the tag plates, the IO list and the DCS.
- Control valves with no fail position marked.
- Interlocks shown as a diamond with no reference to the logic that defines them.
- Line numbers missing, so nobody can tell the pipe class from the drawing.
- The issued-for-construction revision still in the control room years after start-up.
- PFDs used for isolation because they were the drawing that was easiest to find.
Common questions
What is a P&ID?
A piping and instrumentation diagram. It shows a process system's equipment, piping, valves, instruments and control loops, with each item tagged, and the interlocks and safety devices that protect it. It is not drawn to scale and does not show where things physically sit. It shows how they connect and how they are controlled, which is what operations, maintenance, controls and hazard studies all need.
What standard covers P&ID symbols?
ANSI/ISA-5.1 is the reference for instrument symbols and identification letters and is used in most of the world, including Australia. ISO 10628-2 sets graphical symbols for equipment, valves and piping on process diagrams, and IEC 62424 covers how process control functions are represented on P&IDs and exchanged with engineering tools. Many owners and engineering contractors publish their own legend sheets built on these. The legend sheet in the drawing set is the one that governs that drawing.
How do you read a P&ID?
Start with the legend sheet so the symbols are known. Find the source of the stream, often a tank or the inlet from another sheet, and follow one line at a time in the direction of flow. At each valve read its type and normal position. At each instrument read the tag letters to know what is measured and what is done with it, and follow the dashed signal line to see where the measurement goes and what it acts on. Off-page connectors give the sheet where the line continues.
What is the difference between a P&ID and a PFD?
A process flow diagram shows how the process works: the main equipment, the main streams and the flows, temperatures and pressures for the design cases. A P&ID shows how the plant is built and controlled: every line with its size and number, every valve, every instrument and every control loop. The PFD is the design basis. The P&ID is the drawing people work from.
What do the letters in an instrument tag mean?
The first letter is the measured variable: F flow, L level, P pressure, T temperature, A analysis, V vibration. A modifier can follow it, such as D for differential. The succeeding letters are the functions: I indicate, T transmit, C control, A alarm, S switch, V valve, E primary element. H and L after an alarm or switch mean high and low. So LAL is a level alarm low and PDIT a differential pressure indicating transmitter. The number after the letters is the loop number.
What does a circle in a square mean on a P&ID?
A shared display and shared control function, which in practice means a function configured in the distributed control system and shown on the operator's screen. A plain circle is a discrete instrument in the field. A circle with a horizontal line is a discrete instrument in the control room. A diamond inside a square is a function in a PLC, and a hexagon a computer function. The shape tells you where to look for the function when it needs changing.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
- ANSI/ISA-5.1-2024 Instrumentation and control symbols and identification (ISA)
- ISO 10628-2:2012 Diagrams for the chemical and petrochemical industry, part 2: graphical symbols (BSI (BS EN ISO 10628-2))
- IEC 62424:2016 Representation of process control engineering in P&ID and data exchange (IEC)
- IEC 60617 Graphical symbols for diagrams (database) (IEC)
- IEC 81346-1:2022 Industrial systems, installations and equipment: structuring principles and reference designations (IEC)
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