Stainless Steel Pipe vs Tube: Which to Specify?

Stainless Steel Pipe vs Tube: Which to Specify?

“Rør” og “rør” are often used interchangeably, but they are two completely different products with different dimensions, tolerances and applications. Specify the wrong one and you will either pay too much or get a part that does not fit. This guide explains the difference between stainless steel pipe and tube, how dimensions are measured, and which one to use for pressure, structural or decorative applications.

The Core Difference: Dimensioning

Pipe is specified by nominal pipe size (NPS) and schedule. A 2-inch NPS pipe has an outside diameter of 2.375 inches — not 2 tommer. The nominal size is a historical label, not a measurement. Tube is specified by exact outside diameter and wall thickness. EN 25 mm OD tube has a 25 mm outside diameter, period. This is the fundamental difference: pipe uses a nominal label that does not match the actual size, while tube uses the actual dimensions. For piping, this works because flanges and fittings are standardized to NPS. For structural or mechanical applications, you need exact dimensions, so tube is used. Se vår 304 ark og 316L plate.

Stainless Pipe: For Pressure and Fluids

Stainless pipe is used in pressure systems: water, damp, olje, gas, chemical process piping. It is manufactured to ASTM A312 (sømløs) or ASTM A358 (welded). The wall thickness is determined by the schedule (Sch 5S, Sch 10S, Sch 40S, Sch 80S), which is sized for pressure rating. Pipe is typically supplied with a mill finish surface, and it is welded or threaded into flanges and fittings. For high-purity applications (farmasøytisk, mat, semiconductor), electropolished pipe is available with an internal Ra of 0.25 µm or better. The key advantage of pipe is that it is standardized: every 2-inch Sch 40 pipe in the world fits the same flange.

Stainless Tube: For Structure and Decoration

Stainless tube is used in structural, mechanical and decorative applications: rekkverk, møbler, architectural railings, varmevekslere, instrumentation and automotive trim. It is manufactured to ASTM A249 (welded tube) or ASTM A269 (general service tube). The dimensions are exact: a 25.4 mm OD tube has a 25.4 mm outside diameter, with wall thickness specified in millimeters or gauge. Tube is available in round, kvadrat, rectangular and oval shapes. The surface can be mill finish, børstet, polished or mirror. Tube is typically welded or pressed into fittings, not threaded. The key advantage of tube is that you can specify exact dimensions and surface finish — you get what you measure.

Welded vs Seamless

Both pipe and tube come in welded and seamless forms. Seamless is made by piercing a solid billet and rolling it to size — no weld seam. Welded is made from a strip that is rolled and welded longitudinally. Seamless is stronger and handles higher pressure, but it is more expensive and has looser tolerance. Welded is cheaper, has tighter tolerance, and is available in longer lengths. For most pressure applications under 1000 psi, welded pipe is sufficient and code-approved. For structural or decorative applications, welded tube is the standard — the weld bead is removed and the surface is polished. The myth that “seamless is always better” is wrong: for most applications, welded is cheaper, straighter and more accurate.

Vanlige karakterer

304 is the standard grade for general service pipe and tube. It handles water, air and mild chemicals, and is the cheapest stainless. 316L is the marine-grade option, med 2 – 2.5 % molybdenum for chloride resistance. For high-temperature service, 321 or 304H are used, with stabilized carbon or titanium. For sanitary applications, 316L electropolished is the standard. The grade selection depends on the environment, not on whether it is pipe or tube — 304 pipe and 304 tube have the same corrosion resistance.

Buying Advice

Never say “2-inch stainless tube” if you mean pipe — suppliers will quote you a 2-inch OD tube, which is not the same as 2-inch NPS pipe. If you need to connect to flanges or fittings, specify NPS and schedule. If you need structural or decorative tubing, specify exact OD and wall. Always confirm the tolerance: pipe tolerance is ±10 % on wall, tube tolerance is ±0.05 mm. For polished tube, specify the grit (f.eks. 180 grit brushed, 240 grit polished, speil #8). And always ask for the mill certificate to confirm grade and seamless/welded condition.

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