P&ID Conventions That Save You Rework Later

P&IDs outlive the phase of the project they were drawn in. A convention that saves five minutes today can save days of rework six months later when piping, instrumentation, and HAZOP all need to agree on the same drawing.

Tag numbers are load-bearing

Line numbers, equipment tags, and instrument tags get referenced by every downstream discipline and by procurement. Decide the numbering convention before the first P&ID is drawn, not after the third one. Changing a tag numbering scheme mid-project is one of the most expensive “small” mistakes a project can make.

Show what actually matters for safety review

A P&ID prepared for HAZOP needs to clearly show:

• Every protective device (relief valves, rupture disks) and what they protect
• Interlocks and their trip conditions, even in summary form
• Normal flow direction and any lines that can see flow reversal

If a HAZOP team has to ask “wait, which way does this normally flow?”, the drawing hasn’t done its job yet.

Keep redlines traceable to a revision

When markups come back from HAZOP or design review, log them against a specific drawing revision, and track each action item to closure with a reference back to where it was incorporated. Loose redline sheets circulating outside a controlled revision are how action items quietly get lost.

Consistency beats cleverness

A slightly less elegant convention that’s applied consistently across every P&ID in a project is worth more than an elegant one that’s inconsistently applied. Downstream disciplines are pattern-matching against your drawings at speed; surprises cost them time and introduce errors.