The P&ID is the only document on a project where every discipline’s assumptions meet on one sheet. That is exactly why errors on it are so expensive — and why a review by engineers with no stake in the schedule is worth its fee.
The author cannot mark their own work
Internal squad checks matter, but they share the author’s context: the same basis of design, the same precedents, the same deadline pressure. An unaffiliated reviewer reads the drawing the way the plant will — literally, and without goodwill.
What independent review actually catches
Across our reviews, the recurring finds are rarely exotic: isolation philosophy that cannot be maintained live, relief paths that assume a valve position no procedure guarantees, instrument ranges that cannot see the upset they exist for, and utility connections that quietly cross-contaminate. Each is cheap on paper and brutal in steel.
HAZOP is not a design review
A HAZOP examines deviations; it assumes the drawing’s basic intent is right. When the intent itself is flawed — a wrongly-placed battery limit, an unbuildable control narrative — HAZOP tends to formalise the error rather than find it. The design review belongs before the HAZOP, not inside it.
Make it contractual
The cleanest arrangement is structural: an owner’s engineer with review authority written into the EPC contract, comment closure tracked to zero, and no design responsibility of their own to defend. Independence is not a virtue here; it is a mechanism.