The short answer
How to Read a P&ID
A piping and instrumentation diagram (P&ID) is the drawing that shows every pipe, vessel, valve, and instrument in a process and how they connect. Its instrument symbols follow ISA-5.1: a tag such as FT-101 names the measured variable and the function, the bubble shape says where the device lives, and the line style says how signals travel. Read it loop by loop, not left to right.
Key points
- The tag letters follow ISA-5.1: the first letter is the measured variable, the rest are the functions performed.
- Bubble decoration says where a device is: field, control room panel, behind the panel, or in a shared control system.
- Line style distinguishes process piping from pneumatic, electrical, and software signals.
- A loop number ties a transmitter, its controller, and its final element together across sheets.
- Valve failure position, FC or FO, is on the drawing and it is a design decision, not a default.
What a P&ID is for
A process flow diagram shows what the plant does. A P&ID shows how it is instrumented and controlled. Every pipe with its size and material, every vessel with its nozzles, every valve with its type and failure position, and every instrument with a tag number is on it. It is the drawing a controls engineer reads first, because the control philosophy is written into it: what is measured, what is controlled, and what closes when something goes wrong.
A P&ID is not a wiring diagram and not a layout. It does not tell you where a device physically sits or how it is terminated. It tells you what the device is, what loop it belongs to, and how it relates to the process. The loop sheets, the schematics, and the panel drawings are built from it.
Reading a tag
Instrument tags follow ISA-5.1, Instrumentation Symbols and Identification. A tag has a letter code and a loop number. In FT-101 the letters say what the device is and the number says which loop it belongs to.
The first letter is the measured or initiating variable. The letters that follow describe functions, read in order. A modifier letter can qualify either part, which is where most confusion starts.
| Letter | Measured variable | Example tag |
|---|---|---|
| F | Flow | FT-101, flow transmitter |
| L | Level | LSH-205, level switch high |
| P | Pressure | PIT-310, pressure indicating transmitter |
| T | Temperature | TE-402, temperature element |
| A | Analysis | AIT-510, analyzer indicating transmitter, such as chlorine or pH |
| H | Hand | HS-120, hand switch |
| Z | Position | ZSC-118, position switch closed |
| S | Speed | ST-130, speed transmitter |
| Letter | Function | Meaning on the drawing |
|---|---|---|
| T | Transmit | Converts the measurement to a signal the control system can read |
| I | Indicate | Displays the value locally or on a screen |
| C | Control | Performs automatic control |
| S | Switch | Produces a discrete contact at a setpoint |
| E | Element | The primary sensing element, such as a thermocouple or an orifice plate |
| V | Valve | The final control element |
| Y | Relay, compute, convert | A solenoid, an I/P converter, or a calculation |
| A | Alarm | An annunciated condition |
| H, L, HH, LL | High, low, high-high, low-low | Modifiers on switches and alarms |
Reading the bubble
The shape around the tag says what kind of device it is, and the line through it says where it is. This is the part of the drawing that tells you whether a function lives in the field, on a panel, or in software.
- Circle
- A discrete instrument: a physical transmitter, switch, gauge, or single-loop controller.
- Square with a circle inside
- A function in a shared display and control system, which in most plants means the SCADA or DCS.
- Hexagon
- A computer function, used for calculations and software that are not part of the main control system.
- Diamond inside a square
- A function in a programmable logic controller.
- No line through the symbol
- Field mounted.
- Solid horizontal line
- Mounted on a control panel or in the control room, accessible to the operator.
- Dashed horizontal line
- Mounted behind the panel or otherwise not accessible to the operator.
- Double horizontal line
- Mounted on a local or auxiliary panel in the field.
Reading the lines
Process piping is drawn with a heavy solid line and carries a line number giving size, service, material, and often insulation. Everything else is a signal, and the line style says what kind.
- Solid line
- A process connection or a mechanical link.
- Dashed line
- An electrical signal, which in practice is a 4-20 mA loop, a discrete contact, or a control wire.
- Line with double slashes
- A pneumatic signal, usually 3-15 psi to a valve positioner.
- Line with small circles
- A software or data link inside the control system.
- Line with an L shape repeated
- A capillary tube, used on filled-system temperature and level instruments.
Tracing a loop
The loop number is the thread. Pick a transmitter, note its number, and find every other device with that number: the controller function in the SCADA bubble, the valve or drive it acts on, the switches that trip it, the alarms it raises. On a large drawing set the loop crosses sheets, and an off-page connector with a sheet reference carries it across. Follow the number, not the lines.
- 1
Start at the measurement.
Find the element and transmitter. Their tag tells you the variable and the loop number.
- 2
Find the controller.
Look for the same loop number with a C in the function letters, usually in a shared-display bubble. That is where the setpoint lives.
- 3
Find the final element.
The valve, drive, or motor the controller acts on. Note the failure position marked beside a valve: FC fails closed, FO fails open, FL fails last position.
- 4
Find the protection.
Switches and alarms carrying the same number with H, L, HH, or LL modifiers. These are interlocks and alarms and they will be in the control narrative.
- 5
Check the sheet references.
Off-page connectors carry the loop to other sheets. Follow every one before deciding you understand the loop.
What the drawing decides for you
Several controls decisions are already made on the P&ID and it is worth reading them deliberately. The failure position of every valve is a safety decision made by the process engineer, and the controls have to honor it. The location of a function, field or panel or control system, determines what hardware is needed and what happens on a communication failure. The presence of a switch alongside a transmitter, LSH beside LT, says the designer wanted hardwired protection independent of the software.
Related documents
The P&ID is the parent of the instrument list, the I/O list, the loop sheets, and the control narrative. When it changes, all of them change. A drawing set where those documents do not agree with the P&ID is a set where somebody stopped maintaining one of them, and the P&ID is usually the one to trust.
Frequently asked questions
- What does the number in an instrument tag mean?
- It is the loop number. Every device that belongs to the same control loop carries the same number, which is how you trace a measurement to its controller and final element across a drawing set. Some plants prefix it with a unit or area number.
- What is the difference between a P&ID and a PFD?
- A process flow diagram (PFD) shows major equipment and main process streams with flows and conditions. A P&ID adds every line, valve, and instrument and the control scheme. The PFD explains the process; the P&ID specifies it.
- What do FC and FO mean next to a valve?
- Fail closed and fail open: the position the valve takes on loss of air or power. FL means fail last, meaning it stays where it was. The choice is a safety decision and the control system must respect it.
- Which standard defines P&ID symbols?
- ISA-5.1, Instrumentation Symbols and Identification, defines the tag letters, bubble shapes, and line types used on most P&IDs in North America. Many owners publish a legend sheet with their own additions, and the legend on the drawing set governs where it differs.
Related topics
- Control NarrativesWhat a control narrative is, what it must contain, how to write statements that can be tested, and how it drives programming, FAT, and the owner’s acceptance.
- 4-20 mA Signals from the Instrument SideHow a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
- ISA Standards for ControlsThe International Society of Automation standards a controls practitioner meets: ISA-5.1 symbols and tags, ISA-18.2 alarm management, ISA-101 HMI design, ISA-88 and ISA-95 for batch and enterprise integration, ISA-99 which became IEC 62443, ISA-20 specification forms, and how each is used on a water or wastewater project.
- Control SchematicsThe ladder-style drawing of every control circuit in a panel: how a schematic is organized by line number, the conventions for contacts, coils, and cross-references, what belongs on a schematic and what belongs on a wiring diagram, and reading one under pressure.
- Wiring DiagramsThe drawing that says where every wire goes: panel wiring and interconnection diagrams, and the conventions of wire numbers, conductor colors, terminal numbers, and cable numbers that make them readable. What belongs here versus on the schematic.
- Network DrawingsThe two drawings every control network needs: a logical drawing of zones, subnets, VLANs, firewalls, and conduits, and a physical drawing of switches, ports, cables, fiber, radios, and sites. What each shows and why they are sensitive.
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