A pipe specification is four decisions: material, how it was made, the standard it answers to, and the grade. Get those down and the size range is usually the easy part.
Seamless or welded
Seamless pipe is formed from a solid billet and has no longitudinal weld. Welded pipe is rolled from plate and seam welded. Seamless is generally specified where pressure is high or the service is critical; welded covers larger diameters economically, and modern welded pipe to a good standard is not the poor relation it once was.
The practical difference is often size. Above roughly 24 inches, seamless becomes impractical and welded is what exists.
Carbon steel
The default for most service. API 5L covers line pipe, with grades B and the X series stepping up in strength. A106 and A53 cover seamless and welded pipe for general and high temperature service. A333 covers low temperature work, where ordinary carbon steel would become brittle.
Alloy steel
Where temperature is the problem. A335 chrome moly grades, P11 through P91 and beyond, hold strength at temperatures that would soften carbon steel. Power plant steam lines are the classic application.
Stainless and higher grades
Where corrosion is the problem. A312 covers the common austenitic grades, TP304 and TP316 and their low carbon variants, with TP321 and TP347 stabilised for high temperature. Duplex grades under A790 carry both strength and corrosion resistance, which is why they appear offshore and in chemical service.
Reading a specification
A line such as A312 TP316L tells you the standard, the grade and the carbon content in one string. The L matters: low carbon resists sensitisation during welding, which is exactly the failure mode a welded stainless line is exposed to.
Before ordering
Confirm the standard and the grade together, not one or the other. Confirm the schedule, since wall thickness and not just diameter determines the pressure rating. Ask what certification is required, because mill test certificates to EN 10204 are simple to supply with the order and difficult to obtain afterwards.
Materials, norms, grades and size ranges are tabulated on our pipes page.
