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Key takeaways
- The P&ID shows what is connected to what, with every line, valve and instrument, but not where things are in space.
- For piping, the key details are line numbers, spec breaks, valve types, reducers, vents and drains, and layout notes such as “no pockets” or “slope”.
- Anything shown on the P&ID must appear in the model, and anything in the model should trace back to the P&ID.
What a P&ID is and isn’t
A piping and instrumentation diagram shows the process system as a schematic: equipment, lines, valves, instruments and control loops. It is not to scale and it does not show routing. It tells the piping designer *what* must be connected, not *how* to route it.
The details piping must pick up
Line numbers. Every line carries a size, fluid code, sequence number and piping class. This is what you route and what becomes an isometric.
Spec breaks. The symbol between two classes tells you where material or rating changes. You need a physical break there, usually a flange or valve.
Valves. The symbol tells you the type (gate, globe, ball, check, butterfly) and often whether it is locked open (LO) or locked closed (LC), car sealed (CSO/CSC) or fail-safe.
Reducers and size changes. These often sit next to control valves and pumps. Note eccentric or concentric if the P&ID says so.
Vents and drains. High-point vents and low-point drains are shown for hydrotest and operation. Their position on the P&ID is indicative; you place them in the model at real high and low points.
Insulation and tracing. Codes on the line number or notes tell you whether the line is insulated, heat traced or jacketed, which affects pipe spacing on racks.
Layout notes. This is where process tells you about requirements piping must honour:
- “No pockets” or “free draining”: route continuously falling.
- “Slope 1:100 towards V-101”: a minimum fall in one direction.
- “Min. straight run 10D”: typical for flow meters.
- “Close coupled”, “as short as possible”, “symmetrical”: typical for relief valve inlets and two-phase lines.
Instruments
Instrument symbols follow ISA 5.1. The bubble letters tell you the function (FT is a flow transmitter, PSV a pressure safety valve, LG a level gauge). Inline instruments such as flow meters and control valves are part of the piping and need flanges, straight runs and access.
Keep model and P&ID in step
P&IDs change throughout a project. Each revision cloud is work for piping. A simple habit helps: before issuing isometrics, check each line against the latest P&ID revision, item by item.
Code reference
ISA 5.1 Instrumentation symbols and identification.
ISO 10628 Diagrams for the chemical and petrochemical industry.
What to take back to your desk
Print the P&ID, take a highlighter, and mark every layout note before you open the model. Most layout rework comes from notes nobody read.
Educational content. Always design to your project specification.
