Pipe Schedules and Wall Thickness

What is a Pipe Schedule?

Pipe schedule is the way pipe wall thickness are designated. Pipes are designed to carry fluid, therefore their internal diameter is a critical dimension in determining how much fluid they can carry. This critical dimension is often referred to as the nominal bore, commonly abbreviated as NB. Obviously, for pipes containing pressurised fluids the wall thickness, and by implication the pipe's strength, is important - too thin and the pipe could rupture. Pipe wall thickness is expressed in "schedules", referred to as pipe schedules.

What does Nominal Pipe Size or NPS mean?

Nominal Pipe Sizing (NPS) is the name given to the approximate inside diameter of a pipe i.e. nominal bore. The European designation equivalent to NPS is DN (diamètre nominal/nominal diameter/Durchmesser nach Norm). All pipes are specified using the NPS (or DN) and schedule numbers, it is the schedule number that determines the approximate inside diameter.

What Standards Govern Pipe Sizes?

In the oil and gas and related down stream industries the most common standards relating to pipe schedule and thickness are;

- ASME / ANSI B36.10 Welded and Seamless Wrought Steel Pipe, and

- ASME / ANSI B36.19 Stainless Steel Pipe

Does Pipe Schedule Change With Pipe Size?

For all steel pipes with the same NB (nominal bore) the outside diameter remains relatively constant. Therefore any variation in piping schedule i.e. wall thickness, affects only the inside diameter. As the pipe schedule number increases, the wall thickness increases, and the actual bore is reduced. For a given schedule, the outside diameter (OD) increases with nominal pipe size (NPS) while the wall thickness stays constant or increases. Using equations and rules in ASME B31.3 Process Piping, it can be shown that pressure rating decreases with increasing NPS and constant schedule.

Pipe Schedule for Stainless Steel Pipe

Stainless steel offers superior corrosion resistance to carbon steel allowing thinner stainless steel pipe wall thickness to be used. ASME introduced different schedule numbers for stainless steel pipe and fittings. Under ASME B36.19 four new pipe schedules with an "S" suffix were introduced for stainless steel pipe. These stainless steel pipe schedules are: 5S, 10S, 40S and 80S.

Wall Thickness for Carbon Steel Pipe Schedule Charts

The wall thickness associated with a particular schedule of pipe depends on the nominal pipe size as can be seen from the pipe schedule wall thickness charts below for some of the more common sizes of carbon steel pipes encountered.

Abbreviations used: NB - nominal bore, STD - Standard wall thickness, EH - Extra Heavy wall thickness, DBL EH - Double Extra Heavy wall thickness.