Short answer: Solid heavy lines are usually process lines. Dashed or hatched lines are signals: pneumatic, electrical or data, according to the legend's line-style table. Arrows show the intended flow direction. Continuation flags (off-page connectors) carry a line onto another sheet, with a reference you must follow. Trace each process line from its source to its destination before you trust any single symbol.
Steps
1. Read the line-style table on the legend sheet. ISA-5.1 gives common signal-line conventions, but company legends vary.
2. Find each source. In gas panels, look for gas inlets, purge supply, vent or exhaust, and vacuum connections.
3. Follow the flow arrows. Check them against directional components such as check valves, regulators, filters and MFCs. An arrow that disagrees with a check valve is a drawing error to raise.
4. Follow the continuation flags. Note the target sheet and zone, then confirm that the matching flag on that sheet points back.
5. Note line numbers and specs. These usually give size, material or service, and the line spec code. They drive tube and fitting selection.
6. Mark the branches and tees. Each tee is a physical fitting, or a weld, in the assembly.
Worked example (generic)
Sheet 1 shows a process gas inlet that runs through a manual valve to a continuation flag "to sheet 2, zone B3." On sheet 2, the flag "from sheet 1" enters a manifold with three branches, one per chamber. Each branch has a pneumatic valve with a dashed pneumatic line going to a solenoid manifold. That dashed line is a signal tube, not a process line, and it needs its own tubing in the assembly.
Common mistakes
- Treating pneumatic actuator lines as process lines, or the other way around.
- Missing a continuation, which leaves a branch "dead-ended."
- Ignoring the line spec code. It changes the tube material, size or fitting type.
- Reading a crossing as a junction. Junctions are usually shown with a dot or a tee, crossings with no dot or a gap. Check the convention.
More steps: tracing a line properly
1. Start at each source and highlight the line as you follow it, using a different color per service (process gas, purge, vent, vacuum).
2. Stop at every branch and note the branch tag or line number.
3. At each continuation flag, write the sheet and zone, jump to it, and continue. Tick off flags in pairs.
4. At each end point, record what it connects to: tool inlet, exhaust, vacuum pump, another panel.
5. Check for loose ends. Every highlighted line should start and end at a defined source or destination. A line that just stops is either a missing continuation or a drawing error.
6. Reconcile with the line list if the project has one, comparing line numbers, specs and endpoints.
Edge cases
- Bidirectional lines. Some lines flow in different directions in different modes, such as purge in and vent out through the same segment. The drawing may show two arrows or a note. Record both modes. Don't pick one.
- Conditional paths. A path that only exists when certain valves are open is still a physical connection. The 3D model must build it even if it is rarely used.
- Line spec breaks. The spec can change partway along a line, for example at a regulator where pressure drops. The break is usually marked with a symbol or a note. Tube or fitting choices change at that point.
- Signal tubing vs process tubing. Pneumatic actuator lines are real tubes in the assembly but follow a different spec and route. Don't mix them in the BOM.
- Instrument impulse lines. Short lines to gauges or transducers are process lines, even though they look like part of the instrument.
- Revision clouds. A recently changed line may be clouded. Check that the continuation on the other sheet was updated too.
A second worked example (generic)
A vacuum line leaves a gas stick, goes to a continuation flag "to sheet 4, zone A2," and sheet 4 shows it joining an exhaust header. Sheet 4's flag, however, says "from sheet 3." The designer checks sheet 3 and finds an older version of the same line ending there. The revision table shows that the line was rerouted in the last revision, but the flag on sheet 4 wasn't updated. The designer raises a query instead of choosing a route. The process owner confirms the new route, and the flag is corrected in the next revision. Building the 3D route from the outdated flag would have connected the stick to the wrong header.
FAQ
Are line crossings connected?
Only if the legend's junction convention says so, usually with a dot or a drawn tee.
What if two sheets disagree?
Raise it with the process owner. Don't choose one silently.
Does flow direction affect the mechanical design?
Yes. Directional components must be installed the right way, and some fittings are directional too.
What is a line list?
A table of every line with number, service, size, spec and endpoints. Where it exists, it should match the P&ID.
How do I show a spec break in 3D?
Change the tube or fitting type at the component or joint where the break is marked, and note it in the route.
Should I trace signal lines too?
Yes, if they are physical tubes or cables you have to route. Keep them in a separate list.