How to read a P&ID: a step-by-step guide

What a P&ID shows, and what it does not

A P&ID shows the process equipment, all piping with its size and specification, every valve, every instrument and control loop, and the safety devices. It shows how things are connected, not where they are: distances, elevations and routing belong to the plot plan and the isometric drawings.

It sits between the process flow diagram (PFD), which shows only the main flow and conditions, and the construction drawings. See P&ID vs PFD.

Step 1: read the title block

Bottom right of every sheet. It gives the drawing number, sheet number, revision, date and the system or unit shown. Check the revision first: a P&ID that is not the latest revision is the most common source of error in the field. Revision clouds and triangles on the drawing mark what changed.

Step 2: open the legend sheets

The first sheets of every P&ID set are the legend: every symbol, line type, abbreviation and tag format used on the project. Symbols vary between companies, so the legend always wins over memory. Our P&ID symbols library shows the common forms.

Step 3: follow the main process line

Find the thick lines with flow arrows and follow the main flow from where it enters the sheet, through each piece of equipment, to where it leaves. Off-page connectors at the sheet edges tell you which drawing the line continues on.

Each line carries a line number such as 6"-P-1001-A1A: size, service code, sequence number and piping specification. The specification tells you the pressure class and material.

• Process line
• Minor process line
• Off-page connector
• Concentric reducer

Step 4: identify the equipment

Equipment is drawn as a simplified outline with its tag nearby: P for pumps, V for vessels, E for exchangers, K or C for compressors, T for tanks. The equipment name and design data are often listed along the top or bottom of the sheet.

• Centrifugal pump
• Vertical vessel
• Shell and tube heat exchanger
• Centrifugal compressor

Step 5: read the valves

Almost every valve starts from two triangles meeting at their points. What is added tells you the type: a dot for a globe valve, a circle for a ball valve, a filled half for a check valve. Valves drawn solid black are normally closed. An actuator on the stem means the valve is moved by the control system.

• Gate valve
• Globe valve
• Check valve
• Control valve

See all valve symbols and control valve symbols.

Step 6: read the instrument bubbles

Instruments are circles (bubbles) with a tag inside. The letters follow ISA 5.1:

The bubble shape and the line through it tell you where the instrument is: no line in the field, a solid line in the control room, a circle in a square on the DCS. See instrument symbols.

Step 7: trace a control loop

A loop is a group of instruments with the same number. Find the transmitter (for example LT 101 on a vessel), follow the dashed signal line to the controller (LIC 101), then to the control valve (LV 101) that it moves. Reading loops this way tells you how the system is controlled, and what happens when a signal fails.

• Level transmitter
• DCS shared display
• Electrical signal line
• Control valve

Step 8: check the safety devices

Find every relief valve (PSV), rupture disc and shutdown valve on the sheet, with its set pressure and where it discharges. These are checked in every HAZOP and safety review.

• Pressure safety valve
• Rupture disc
• Shutdown valve

Reading P&IDs at project scale

A large project has hundreds or thousands of P&ID sheets, and every tag on them has to become a line in the tag register, assigned to a system, with inspection records to follow. Typing that register by hand takes engineering teams months. Deskely Extract reads the sheets and builds the register for engineers to check, and Delta shows what changed when a new revision arrives.