Piping and instrumentation diagram (P&ID): what it shows and how to read one

A piping and instrumentation diagram is a schematic of a process system. It shows the equipment (tanks, pumps, heat exchangers), the pipes that connect them, every valve, and every instrument that measures or controls something, together with the signal lines that tie the instruments to each other. Engineers also call it a process and instrumentation diagram, and the abbreviation P&ID is used for both names.

The word diagram matters. A P&ID is not drawn to scale and it does not show where things physically sit in the plant. A tank that is forty metres from its pump on site can be drawn two centimetres from it on the sheet. What the drawing gives you instead is the logic of the process: what is connected to what, in which order, and which instrument is allowed to move which valve.

That is why a P&ID is usually the first drawing a controls technician opens at a new plant. Before you touch a wire or a line of PLC code, the P&ID tells you what the process is meant to do and which measurements and valves your control system depends on.

New technicians often mix up three documents that all look like boxes joined by lines. They answer different questions, so it helps to know which one you are holding before you start reading.

You move between the three constantly. A P&ID tells you a pump exists and that it is stopped on low level. The schematic tells you which contactor, overload relay and fuse make that happen, and the I/O list tells you which PLC input and output are involved.

However busy a sheet looks, each symbol belongs to one of four groups. Sorting what you see into these groups is the first reading skill, and it is exactly what the viewer above lets you practise.

• Equipment: vessels, tanks, pumps, heat exchangers and similar. Each has a tag made of a letter and a number, such as T-101 or P-101, and what the letter means is set by the project.
• Piping and valves: heavy solid lines are process pipe, and valve symbols sit on them. A handle on the valve means it is hand operated. A round actuator means something else moves it, and a signal line will arrive at that actuator.
• Instruments: circles, called bubbles. A line through the circle normally marks an instrument you can see from a control room or main panel. A plain circle is mounted in the field. The letters inside say what is measured and what the device does.
• Signal lines: thin, usually dashed lines that carry information between instruments and to the actuator of a valve. No liquid ever flows in them.

An instrument tag such as FIC-101 has two parts: letters that say what the instrument is, and a loop number that groups it with the other devices in the same control loop. The first letter names the measured or controlled variable, and the letters after it describe the function.

Take FIC-101. F stands for flow. I stands for indicating, meaning it shows a value to a person. C stands for controller. Read left to right it is a flow indicating controller, and the number 101 tells you which loop it belongs to. The transmitter that measures the flow in that loop is FT-101, the T meaning transmitter, and the valve it moves is usually FV-101.

A few tags are idioms that do not decode letter by letter. PSV is the common tag for a pressure safety or relief valve, and HV for a hand valve. The tag decoder in the Drawings and Documentation track handles these, and the viewer on this page uses the same decoder whenever you click an instrument.

One caution, which the viewer shows on the flow-loop drawing. Instruments that share a number are not automatically in the same loop. Follow the signal lines to decide what is wired to what, then use the numbers to confirm.

Open the first sample drawing in the viewer, the tank, pump and level loop. The question is simple: if the level in tank T-101 rises, what happens, and which devices take part?

The same method works on any drawing. Start at an instrument, follow the dashed lines, and stop when you reach a device that moves something. The other three sample drawings in the viewer add an independent alarm loop and a two-controller cascade, which are the two patterns you meet most often after the simple loop.

A P&ID stops at the instrument bubble. It does not say which cable carries the signal, which terminals it lands on, or which PLC input card reads it. Those answers live in the loop diagram, the wiring drawings and the I/O list, and the tag is the thread that ties them together. LT-101 on the P&ID is the same LT-101 in the loop diagram and, in a well-kept project, the same name on the PLC tag.

This is why the tag matters so much in fault finding. When an operator says the tank level looks wrong, you search the P&ID for the level transmitter, take its tag to the loop diagram to find the cable and terminals, measure the loop current at the cabinet, and compare it with the value the control system shows. Each document narrows the search and no step relies on guessing.

If the signal in question is a 4-20 mA current loop, the scaling from current to engineering units is the next skill. Practise it in the instrumentation lab, which includes a live-zero fault to diagnose, and in the 4-20 mA scaling calculator.

• Treating a P&ID as a layout. Distances and positions on the sheet mean nothing about the plant.
• Reading the loop number as proof of wiring. Follow the signal lines first.
• Ignoring the legend. A user-defined letter or a project-specific line style is explained there and nowhere else.
• Forgetting the fail position of a control valve. Whether a valve opens or closes on loss of signal or air is a safety decision and is noted next to the actuator on a real drawing.
• Confusing the controller circle with a separate box. In a modern plant the controller is usually a function block inside the PLC or DCS, and the bubble is how the drawing shows that function.

If you read the viewer drawings comfortably, the next step is to practise the same skill with graded questions. The Drawings and Documentation track starts with what a P&ID is, then instrument tags, then tracing a loop on three drawings, and goes on to loop diagrams, I/O lists, cable schedules and commissioning documents. Its first units are free with a free account.

For a step-by-step reading method with more tag examples, the guide on how to read a P&ID covers the symbol groups and an I/O list built from a drawing. For the signal side of a loop, the instrumentation simulator and the 4-20 mA scaling calculator let you work with the current a transmitter actually sends.