How a Piping System Fits Together

More than pipe

A process piping system is pipe plus everything that joins it, directs it, holds it and controls it: fittings, flanges, gaskets, bolts, valves, supports, branch connections, instruments and insulation. A single line from a pump to a vessel can contain dozens of components, each one a possible leak point or failure point. Integrity depends on all of them being right and staying right.

Piping is designed and built in lines — a run of pipe with one service and one set of design conditions — and fabricated in spools: shop-made sections of pipe and fittings, welded together and joined in the field by flanges or field welds.

Worked example: following one line

A maintenance team must replace a corroded section of a cooling water return line. From the P&ID they find the line number and see that it connects an exchanger to the cooling tower return header. The line list gives the class and design conditions. The isometric gives the exact route, the spool breaks, the weld numbers and the support locations. The class sheet tells them which gasket and bolts to order. Without all four documents, the team would be guessing.

Worked example: when the documents disagree

On a walkdown, a technician finds a drain branch on the line that is not on the isometric. It may have been added years ago without the drawing being updated. Before any work, the difference is recorded and the drawing is marked up. An undocumented branch means an undocumented weld, an undocumented valve and possibly an unsupported small bore connection — exactly the items that fail.