Piping class (pipe spec)

Piping engineering does not select components line by line; it selects a class. Each piping class packages a coherent set of choices for a service envelope — the fluid, the pressure–temperature range, the corrosion allowance — so that specifying a line reduces to assigning its class, and every component on it follows from the class tables. The class code travels on the line number, and the specification behind it governs procurement, fabrication and inspection for everything built to it.

Where classes change, the plant has a spec break — a defined point on a line where one class ends and another begins, marked on the P&ID and consequential everywhere: the components differ on each side, the MTO must count them to the correct class, and the break's exact position is a fact worth the drawing space it occupies.

The failure modes are correspondingly concentrated: a wrong class assignment propagating into every component on the line; breaks placed ambiguously and taken off differently by different readers; and class revisions mid-project rippling through the MTO for every line that carries the code — a small edit with a long shadow.