Pipe is the easiest product to order incorrectly, because two of the three numbers involved are not what they appear to be.
Three numbers, two traps
- NPS (nominal pipe size) is a label, not a measurement. The outside diameter is fixed for each NPS, and for small sizes it does not match the nominal figure.
- Schedule sets the wall thickness — but the same schedule number gives a different wall at every diameter. Schedule 40 at 2 inch and Schedule 40 at 12 inch are not comparable walls.
- Wall thickness is the only unambiguous number of the three.
The stainless schedules add a further wrinkle: 10S, 40S and 80S exist alongside 10, 40 and 80, and they are not always identical.
State it like this
Grade and standard, NPS, schedule and the wall in millimetres, manufacturing route (seamless or welded), end finish, and the certification required. For example: ASTM A312 TP316L, 4 inch NPS, Schedule 40S, 6.02 mm wall, seamless, plain end, EN 10204 3.1.
Seamless or welded
Seamless suits higher pressure, cyclic and critical duty, and smaller diameters. Welded pipe is economical in larger diameters and perfectly sound for most utility and structural service, provided the weld is tested to the standard you are buying against.
Before you order
Check the fittings and flanges match the same schedule, and that your bore matters more than your outside diameter if the line is being sized hydraulically.
Send us the line list and we will quote against it, flag anything inconsistent, and tell you what is in Dubai stock now.
