Seamless vs Welded Steel Pipe: ERW, SAW and LSAW Compared

Every carbon steel pipe enquiry eventually comes down to one question: seamless or welded, and if welded, by which process. The price difference is real, but so are the differences in available sizes, wall tolerances and the design allowances the piping code applies to each seam. This article explains how seamless, ERW, LSAW and spiral SAW pipe are produced, where each fits dimensionally, and how to decide without paying for seamless where ERW will do, or specifying ERW where the code penalises it.

How each type is made

Seamless (SMLS). A solid round billet is heated and pierced, then rolled over a mandrel or plug to stretch the hollow to size. The two main routes are the mandrel mill and the Mannesmann plug mill; extrusion is used for some small sizes. There is no weld, so there is no seam to examine, but wall thickness depends on how concentrically the billet was pierced, and that eccentricity shows up as a tolerance band.

ERW / HFW. Hot-rolled coil is slit to width, cold-formed into a tube and the strip edges are fused by resistance heating under pressure, with no filler metal. Modern mills use high-frequency induction (HFI or HFW) at frequencies from hundreds of kilohertz to several megahertz, which keeps the heat-affected zone narrow. The weld flash is trimmed and the seam is normally heat treated. Wall thickness comes directly from the coil, so it is very consistent.

LSAW / DSAW. A discrete plate is formed into a cylinder by the UOE, JCOE or press-bending process and the longitudinal seam is submerged-arc welded from inside and outside, hence DSAW, double submerged arc welded. UOE includes a mechanical expansion step that tightens ovality and relieves forming stress. Plate width fixes the diameter, so each size needs its own plate.

SSAW / HSAW. Coil is fed at an angle into a forming cage and submerged-arc welded along a helical seam, again from both sides. The diameter is changed by altering the spiral angle, so one coil width serves many sizes. SSAW and HSAW are the same product under two names.

Size and wall-thickness envelopes

The overlap zone is NPS 16 to 24, where seamless, ERW and LSAW all compete. Above NPS 24, seamless is not produced commercially and LSAW becomes the only straight-seam option; SSAW covers the very large diameters where wall thickness demands are modest. Below NPS 16 the choice is almost always seamless or ERW. Size limits vary between mills, so treat the table as a market picture rather than a specification limit.

Pressure design: the weld joint quality factor

ASME B31.3, the process piping code, applies a longitudinal weld joint quality factor Ej to the allowable stress when it calculates minimum wall thickness. The factor reflects how much confidence the code places in the seam as manufactured.

Two consequences matter commercially. First, a double-submerged-arc-welded pipe with 100 % seam examination carries the same factor as seamless, which is why LSAW is accepted on high-pressure trunk lines. Second, ERW is fixed at 0.85 for B31.3 wall calculations, so for the same design pressure an ERW line needs roughly 15 % more wall than seamless. On a Schedule 40 building services system this usually changes nothing, because the schedule wall is well above the calculated minimum; on a thin-wall large-bore header it can push you up a schedule and erase the price advantage. Whether supplementary examination of an ERW seam can be credited depends on the code edition; check Table 302.3.4 first.

The figures above are for B31.3 process piping; pipeline, power piping and pressure vessel codes have their own tables and should be checked separately.

Quality differences that show up on site

Wall tolerance. Seamless pipe may be up to 12.5 % under nominal wall at any point, and the variation around the circumference is real because of piercing eccentricity. ERW and SAW walls follow the coil or plate, so they are far more uniform. For a fabricator this matters at fit-up: seamless pipe often needs more counter-boring or more careful hi-lo control at butt welds than ERW of the same schedule.

Ovality and straightness. UOE-expanded LSAW has the tightest ovality of the welded products, which is why it is favoured for pipelines using automatic girth welding. Seamless and ERW are both acceptable for ordinary fabrication.

Residual stress and sour service. LSAW carries higher residual stress from forming and welding than ERW, and SSAW has a longer weld with a more complex stress state. Many operators restrict or prohibit SSAW in sour (H2S) service; LSAW to API 5L PSL2 with NACE MR0175 / ISO 15156 compliance is the usual answer there.

Testing. Every length of A53 and A106 pipe is hydrostatically tested, or in some cases given a non-destructive electric test instead, regardless of seam type. Seam-specific NDE is additional, and it is what justifies the higher joint factor.

Cost and lead time

As a market picture, seamless pipe costs in the order of 20 to 30 % more per tonne than ERW in the sizes where both exist, and LSAW carries a premium of 30 to 50 % over SSAW at the same diameter. Lead times follow the same order: ERW and SSAW mills run continuously from coil, LSAW is batch-formed from discrete plates, and seamless depends on billet availability and mill rolling programmes. Availability in common sizes is generally better for ERW than for heavy-wall seamless, which is often an import item against order.

Which to specify: a decision guide

Where a line class says "seamless", that is not a preference to be negotiated at procurement; it is usually tied to the stress analysis or the fluid category. Where the specification is silent, the table above is a defensible starting point.

Key takeaways

• Seamless pipe has no seam and an Ej of 1.00, but its wall tolerance of minus 12.5 % and its price premium of roughly 20 to 30 % over ERW are real costs.
• ERW is fixed at Ej 0.85 under ASME B31.3; on schedule-wall building services piping that rarely matters, on thin-wall large-bore headers it can.
• Double-submerged-arc-welded LSAW with 100 % seam examination earns Ej 1.00 and is the only straight-seam pipe above NPS 24.
• SSAW is the economical choice for large-diameter, low to moderate pressure water and structural duties, and is often excluded from sour service.
• Specify by service and code, not by habit: write the process (SMLS, ERW, LSAW, SSAW) on the requisition alongside the material grade.

Frequently asked questions

Is seamless pipe always stronger than welded pipe?

No. Strength comes from the steel grade, not the seam. A seamless and an ERW pipe to A53 Grade B have the same minimum yield and tensile strength. The code applies a lower joint factor to ERW because the seam is not fully examined, not because the parent metal is weaker.

Can ERW pipe be used for steam?

For low-pressure steam within a building, yes, where the line class permits A53 Type E. For high-temperature process steam and condensate, most specifications require seamless A106, and ASME B31.3 requires seamless, heavier-wall pipe for threaded steam connections above certain pressures. Follow the line class.

What is the difference between LSAW and DSAW?

DSAW describes the welding: double submerged arc, one pass inside and one outside. LSAW describes the seam direction: longitudinal. Most LSAW pipe is DSAW. SSAW pipe is also double submerged-arc welded, but along a helix.

How Al Safsaaf can help

Al Safsaaf Trading & Contracting Company supplies carbon steel seamless pipes and ERW, SAW and LSAW pipes for MEP, industrial and infrastructure projects across Saudi Arabia. Tell us the service, size, schedule and material grade through our contact page and we will advise on the most economical process that meets your specification.