Common Mistakes — Piping Passion

Mistake #01

Cost: High

Working on a superseded P&ID revision

A designer works for two weeks on a P&ID that was superseded three days ago. Nobody told them. The document control system had a notification process that nobody had set up. Complete rework, schedule impact, damaged reputation.

Verify the P&ID revision every morning before starting work. Set up notifications with document control. This is the most preventable and most common expensive mistake in piping design.

Mistake #02

Cost: High

Ignoring as-built drawing inaccuracies on brownfield

A designer routes a new 6-inch line based on as-built drawings showing a clear corridor. On site: an 8-inch nitrogen line running exactly through the planned routing. Never documented. Added fifteen years ago. Discovered during construction, during the shutdown window.

On brownfield projects, treat every as-built drawing as a hypothesis. Walk the corridor. Physically verify clearances. Ten minutes of field verification costs nothing compared to redesigning during a live shutdown.

Mistake #03

Cost: Medium

Placing a valve the operator cannot reach

An Emergency Isolation Valve is correctly specified on the P&ID and modelled at the correct elevation. It is located behind a bank of large-bore pipes. Access requires climbing over a 350°C line. The valve is relocated during a subsequent shutdown at enormous cost.

Before finalising any valve placement, mentally operate it. Can the operator reach it in normal conditions? In an emergency, in full breathing apparatus, at 3am? If any answer is no, redesign. This takes five minutes in the model and avoids six-figure rework.

Mistake #04

Cost: Medium

Wrong P&ID symbol interpretation

A designer models an entire system with globe valves where the P&ID specified gate valves. The symbols were slightly different from their previous project. Nobody checked. Three weeks of rework, plus replacement of already-ordered globe valves.

Read the P&ID legend sheet on day one of every project. Never assume a symbol means the same thing as your last project. Every symbol has a precise meaning defined in that project's legend.

Mistake #05

Cost: Medium

Making design changes without formal MOC

A process engineer verbally asks a junior designer to change a carbon steel line to stainless steel. The designer changes the model. No MOC. Three months later the stress engineer discovers it: different expansion coefficient, inadequate supports, two weeks of schedule delay.

Never make a design change based on verbal instruction alone. Every change — no matter how small — requires written confirmation and, for significant changes, formal Management of Change. If someone cannot put it in writing, that is a red flag.

The pattern behind every mistake

Every one of these mistakes shares the same root cause: something was assumed instead of verified. The P&ID revision, the as-built dimensions, the welding clearance, the operator access. Experienced designers are not smarter — they verify more, and they have learned which assumptions are dangerous.