Welded steel pipes are manufactured by curling steel plates or steel strips and then welding them together. (common processes include ERW\SAW\LSAW\SSAW).
Seamless steel pipes are formed by piercing solid round steel billets, followed by hot rolling, cold rolling or cold drawing, with no weld seams throughout the pipe (processes include hot rolling, cold rolling, extrusion, etc.).
Generally, the maximum safe operating pressure for welded pipes is within 20 kg, which is their safest application range. They are commonly used for transporting low-pressure fluids such as water, gas, and compressed air.
Seamless pipes can withstand ultra-high pressure, and their wall thickness will increase accordingly, which requires design based on specific pressure requirements. They are typically applied in high-temperature and high-pressure equipment like high-pressure oil pipes and boiler pipes.
There is a type of fluid pipe that is actually a processed ERW pipe. The treatment methods involve planing or grinding off the original weld seam with special equipment, or hot rolling the welded pipe. Traces will remain on pipes that have been planed or ground; some traces are quite obvious, and they may become even more distinct after rust removal with tools. A careful observation will reveal that the rust on 20# steel and Q235 steel differs due to their different material compositions. Manufacturers of such pipes, out of fear of assuming liabilities, only label them as "fluid pipes" on quality certificates instead of "seamless steel pipes for fluid transportation".
