What is a P&ID? How to read the symbols, tags and lines

A P&ID (piping and instrumentation diagram) is a schematic drawing of a process plant that shows all the equipment, piping, valves and instruments, and how the measurement and control loops connect them. It is not a map: a P&ID shows how the plant works, not where everything sits. For maintenance teams it is the drawing you use to plan isolations, trace faults and record changes, provided it matches what is actually installed.

What a P&ID is, and what it is not

A plant is described by several kinds of drawing, and they are easily mixed up. The process flow diagram (PFD) is the high-level view: the main equipment, the streams that connect it and design conditions such as flow, pressure and temperature. The P&ID goes a good deal further. It adds the hand valves, the line numbers, the field instruments with their tags and the signal connections to the control system.

What a P&ID leaves out matters just as much: the wiring, the control software and the physical layout. For those you need the loop diagram, the functional description of the controls and the plot plan.

Be careful with the abbreviation PID. Some people use it for the drawing, but in control engineering PID stands for a control algorithm (proportional, integral, derivative), as in a PID controller. In work orders and document titles, write P&ID.

What you will find on a P&ID

A well-drawn P&ID shows at least:

• Equipment with its own number, such as tanks, vessels, pumps, compressors and heat exchangers, often with a box of design data.
• Pipelines with a line number and flow direction. The line number encodes the key properties of the pipe. In the made-up line number DN80-W-1003-A1, DN80 is the nominal diameter, W the fluid code, 1003 the sequence number and A1 the piping class, which sets the material and pressure rating among other things. An insulation or tracing code sometimes follows. The format differs from project to project.
• Valves and fittings: hand valves, check valves, control valves with actuators, safety valves, blinds, drains and vents.
• Instruments with a tag: transmitters, local gauges, switches and analysers.
• Signal lines and control loops: how a measurement drives a valve or actuator through a controller.
• Safeguards: alarms, switches, interlocks and safety valves, with a reference to the logic diagram or cause-and-effect chart where the logic is kept.
• References and notes: arrows to the continuation sheet and notes on special operating conditions.

P&ID symbol standards: ISA-5.1, ISO 10628 and the legend

For instrument tags and symbols, ISA-5.1 is the most widely used standard, and also the oldest one published by the International Society of Automation. The current edition is ANSI/ISA-5.1-2024, now titled Instrumentation and Control – Symbols and Identification, with two technical reports on identification and graphic symbols alongside it. The same series includes ISA-TR5.1.04-2026, on the content of PFDs and P&IDs.

For the diagrams themselves and the symbols for equipment and fittings, the chemical and petrochemical industry uses ISO 10628-1 (classification, content and drafting rules for flow diagrams) and ISO 10628-2 (graphical symbols). ISO 10628-2 applies the broader ISO 14617 symbol series; for electrotechnical diagrams it refers to IEC 60617. In power stations that use KKS, every system, pump and valve on the P&ID also carries a code from the Kraftwerk-Kennzeichensystem. How those codes are built is explained in KKS coding.

Above all, your own project legend comes first. How control is shown on a P&ID varies between engineering contractors, and many depart from the standard in the details. That is why every set of P&IDs comes with a symbols sheet (legend or lead sheet). Read it first, especially on drawings from a foreign supplier.

Instrument tags: how to read a tag such as LIC-101

Every instrument on a P&ID carries a tag: a few letters and a loop number. The letters describe the function, the number the loop it belongs to. Under ISA-5.1 the first letter is the measured variable, for example F (flow), L (level), P (pressure), T (temperature) or A (analysis). The letters that follow say what happens to that measurement: I (indicate), T (transmit), C (control), A (alarm), S (switch) or V (valve). All the instruments that work together on one task share a loop number.

The diagram below shows a small P&ID with one level control loop and the instruments around a pump.

Watch the position of each letter. The same letter means something different in first place than further along: a TT is a temperature transmitter, and in LAL the first L means level and the last one low. With S, the position does not help: in PSV the S stands for safety, in LS and PSH for a switch. So read such a tag as a whole. Large plants often put an area number in front of the letters, and add a suffix letter (A, B, C) after the loop number when one loop has several identical measurements.

The shape around the tag tells you where to find the instrument. A plain circle is a field-mounted instrument, such as LT-101 and PI-102. A horizontal line through the circle means the operator can read or adjust the function, usually from the control room, as with LIC-101. ISA-5.1 draws a function shown on the control system's screens as a circle inside a square, although not every drawing office follows that convention; the example diagram also shows LIC-101 as a circle with a line. A temperature measurement often consists of a TE (the element or sensor) and a TT (the transmitter); a thermometer read locally is a TI. On a boiler steam drum you may find level glasses (LG), level transmitters (LT) and level switches (LS) side by side.

Line types: process, signal and data

ISA-5.1 also gives each type of line a meaning, including:

• Process line: a solid line for the pipe itself, with arrows for the flow direction.
• Instrument connection: a solid line between the process pipe and the instrument.
• Pneumatic signal: a line with pairs of short diagonal slashes.
• Electric signal: a dashed line, for both analogue (4-20 mA) and on/off signals.
• Data link: a line with small circles, for communication between devices and functions in the control system.

The example diagram uses only process lines, instrument connections and electric signals. Control valves often show their fail position as well: FC (fail closed) if the valve closes on loss of air or signal, FO (fail open) if it opens. LV-101 is FC, so the feed stops if the control loop fails.

How to read a P&ID in seven steps

Whether you are tracing a fault or preparing a job, this order helps:

1. 1Check the title block and revision. Drawing number, sheet, revision and status: is this the current version, and is it the as-built? Managing revisions is covered in technical document control.
2. 2Read the legend. Symbols, abbreviations, the line number format and this project's tagging convention.
3. 3Follow the main flow. Start at the feed and follow the fluid through the plant. Watch for references to continuation sheets at the edges.
4. 4Identify the equipment. Note the number and design data of each tank, pump or heat exchanger.
5. 5Trace the control loops. Measure, compare, correct: in the example LT-101 measures, LIC-101 compares the reading with the setpoint and LV-101 adjusts. Check the fail position too.
6. 6Find the safeguards. Safety valves (often with their set pressure), alarms, switches and interlocks. If the logic is kept elsewhere, the P&ID refers to that document.
7. 7Check line numbers and specifications. Where does the piping class change, and where are the drains, vents and blinds? Those are the points you need to isolate and open up safely.

How maintenance teams use the P&ID

Isolation and lockout

The P&ID is the main tool for planning an isolation. In its guidance on the safe isolation of plant and equipment (HSG253), the UK's Health and Safety Executive describes checking isolations against the current P&ID, marking them up on a copy and walking the plant to confirm that the installation matches the drawing. If it does not, the isolation is reassessed and the P&ID corrected. The guidance cites an incident during a heat exchanger replacement: a vent line to a second reactor was missed because nobody had referred to the P&IDs, and opening the line released a substantial amount of hydrogen bromide. How isolation, lockout and last-minute risk assessments fit together is covered in safe maintenance with LMRA and LOTO.

Fault-finding and work preparation

When an operator reports that a valve is not controlling properly, the P&ID gives you its tag, the loop it belongs to and the measurement that drives it. That also tells you which tag belongs on the work order. During work preparation, an extract of the P&ID with the isolation points goes into the work pack, and in a root cause analysis the drawing shows which other parts of the loop could be behind the problem.

HAZOP and changes

In a HAZOP study, a team works through the P&ID section by section with set guide words such as 'no', 'more' and 'less', looking for deviations with hazardous consequences. Every later change therefore has to reach the drawing as well. In the United States this is a legal requirement for processes involving highly hazardous chemicals: OSHA's process safety management rule (29 CFR 1910.119) lists P&IDs as required process safety information and requires that information to be updated when a change affects it.

Linking tags to the asset register

Tags on the P&ID and codes in your maintenance system must point to the same thing. A tag such as PI-102 belongs to a place in the process, not to the physical instrument fitted there today: replace the gauge and the tag stays the same. That is the distinction between position and asset that a good asset register makes. In asset management software those are two separate levels.

Keeping P&IDs up to date: from redline to as-built

A P&ID is only useful if the drawing is right. Plants change all the time: an extra drain, a replacement valve with a different fail position, a temporary hose connection that never gets removed. Three habits keep the drawings in step with reality:

• Every change through a set procedure. Even a small modification goes through management of change and is only closed once the P&ID has a new revision.
• Incorporate redlines quickly. Technicians mark the differences they find in red on a copy. Give each drawing set one owner who incorporates them and issues the new revision. HSE advises formally revising and reissuing a corrected drawing at the earliest opportunity, and checking whether other P&IDs are also wrong.
• Walk the plant regularly. Before a shutdown, compare the P&IDs you will use for isolations with the plant itself.

You can see how drawings, their revisions and linked assets are managed in practice under technical document management.

Common mistakes with P&IDs

• Trusting the drawing without looking. An out-of-date P&ID shows a plant that no longer exists. Always walk the system when isolating.
• Using an old printout. The copy in the cabinet or the shift supervisor's folder is often a revision behind. Take the drawing from the central system.
• Reading letters in the wrong position. A T is a temperature in first place and a transmitter further along. Mix them up and you look for the wrong instrument.
• Assuming every supplier uses the same symbols. A package from a foreign supplier may come with its own legend.
• Reading the P&ID as a layout. A valve drawn on the left may sit on the other side in the field. Use the plot plan or the isometric to find it.
• Tags that appear nowhere else. If the tag on the drawing, the label in the field and the code in the maintenance system differ, the history ends up scattered.