Pipe, Schedules and Materials

Process piping is specified by nominal size, wall thickness schedule, material and a piping class that ties those together with the flange rating, gasket, bolting and valve types permitted for a given service.

Size and schedule

Nominal pipe size is a designation, not a measurement — the outside diameter is fixed for a given nominal size, and the schedule number sets the wall thickness. A higher schedule means a thicker wall and a smaller bore. This matters practically: two pipes of the same nominal size but different schedules have different internal diameters, so a replacement spool of the wrong schedule changes both the pressure rating and the flow.

Wall thickness allowances

The required wall is calculated from design pressure, temperature, material allowable stress and diameter, then increased by a corrosion allowance and by manufacturing tolerance. The corrosion allowance is consumable: inspection tracks how much is left, and a line that has consumed it must be repaired, replaced or re-rated.

Materials

• Carbon steel — the default for most hydrocarbon service, with a corrosion allowance and often an internal coating or inhibitor programme.
• Low temperature carbon steel — impact tested, for lines that get cold on depressurisation.
• Stainless and duplex steels — for corrosive, sour or chloride-bearing service. Duplex requires careful welding control.
• Alloys and clad pipe — for severe corrosion, used where the cost is justified by the life required.
• Non-metallic — GRE and HDPE for produced water, firewater and low-pressure service, with their own jointing and support rules.

The piping class is a control

Substituting material, schedule, gasket or bolting outside the piping class is a modification, not a repair. It requires engineering approval through management of change — a "like for like" replacement that is not actually alike is a common route to a loss of containment.