P&ID

The piping and instrumentation diagram is the controlled reference for what a system physically is: every pipe, valve, instrument and control loop, with tag numbers and interconnections. Walkdowns, installation qualification and change assessments are all performed against it, which means its accuracy is not a drawing-office concern but the foundation of several GMP activities at once.

Its characteristic failure is drift. A valve added during commissioning, a sample point relocated, a dead leg created by a modification that was never redrawn — each is small, and the aggregate is a diagram that no longer describes the plant. That matters most in cleaning validation and contamination control, where the dead leg nobody drew is precisely the location that should have been sampled.

As-built accuracy is therefore a change-control obligation rather than a project deliverable. The drawing has to be updated as part of closing the change, not as a documentation task afterwards, because the interval between modification and redrawing is exactly when someone performs a walkdown against it.

• —The controlled reference for walkdowns, installation qualification and change assessment.
• —Tag numbers on it are the identities loop checks and calibration records depend on.
• —Drifts silently through small undrawn modifications.
• —An undrawn dead leg is the location cleaning validation should have sampled.
• —Updating it belongs inside change closure, not after it.

EU GMP Part I Chapter 4 (documentation) and Chapter 3; EU GMP Annex 15 (2015) §3 for installation qualification against approved drawings; ISA-5.1 for symbology.