ERW wins on dimensional control and cost for standard services; seamless keeps an edge at very high temperature/pressure and in severe cyclic duties. The rest is getting ovality, testing and end prep right so hydro and site fit-up go smoothly.
Processes at a glance
- ERW (HF-welded): excellent OD/ID control and straightness; fully monitored weld.
- Seamless: no weld seam; wall variation can be higher; favored for refinery/boiler extremes.
Picking the spec
- General/structural: ASTM A53, A500 (HSS), EN 10219.
- Pressure/high temp: ASTM A106 Seamless or API 5L PSL1/PSL2; grade (B/X42/X52…); sour service if required.
- Size: NPS + schedule (Sch 40/80…) or OD × WT in mm.
Ovality & straightness
Ovality % = (Dmax − Dmin)/Dnom × 100. Tighter ovality simplifies fit-up and gasket compression. ERW typically holds closer than seamless in the same size.
- Guide limits: ≤1.0–1.5% for small/medium ERW; ≤2.0% for seamless unless spec says otherwise.
- Straightness: e.g., ≤1.5 mm/m for long spools—state it in the PO.
NDT/hydro
- ERW: 100% online NDT of weld (UT/ET) + hydro or alternative per spec; add body UT for high grades/sour.
- Seamless: UT/ET on body + hydro; define acceptance and calibration notches.
Ends & coating
- Plain end or beveled 30° with 1.6 mm land; random 5–7 m / 10–12 m or fixed length.
- Bare/varnished/external coating; galvanizing for light-duty service only.
Spec line (copy/paste)
- Pipe: API 5L PSL2, X52, 168.3 × 7.11 mm (NPS 6 Sch 40)
- Ends: BE 30° with 1.6 mm land
- NDT: UT/ET per spec; hydro 100% at required pressure
- Docs: MTC EN 10204 3.1; heat traceability; stencil per order
FAQs & links
Q: When is ERW not OK?
A: Very high temp/pressure, severe fatigue, or strict NACE sour—prefer seamless.
Baoli Engineering Team · Reviewed Oct 31, 2025