Seamless vs Welded Pipe Selection

Quick Answer

Seamless pipes have no weld seam and handle higher pressures; welded pipes (ERW, SAW) cost less, offer better availability in large diameters, and are sufficient for most moderate-pressure and structural applications.

The choice between seamless and welded pipe depends on pressure requirements, pipe size, budget, and lead time. Both types can meet the same ASTM and API specifications, but their manufacturing differences affect performance and cost.

Key Differences

Seamless pipes are formed from a solid billet with no longitudinal or spiral weld. Welded pipes are made by forming steel plate or coil into a tubular shape and welding the edges together. The weld seam is the fundamental differentiator, as it introduces a heat-affected zone (HAZ) that may have different mechanical properties from the base metal.

In practice, modern welding techniques (especially ERW/HFW) produce joints with strength equal to or greater than the base metal. Still, many engineering codes and client specifications mandate seamless for high-pressure, high-temperature, and sour service.

Feature	Seamless (SMLS)	Welded (ERW/SAW)
Weld seam	None	Longitudinal or spiral
Pressure rating	Higher (no weld factor)	Lower (weld joint factor 0.85-1.0)
Size range	NPS 1/8 to ~NPS 24	NPS 1/2 to NPS 80+
Wall uniformity	±12.5% eccentricity typical	±10% or better
Cost	20-40% higher	Lower
Lead time	Longer for large sizes	Shorter, wider stock
Surface finish	Rougher (hot-finished)	Smoother
Common specs	ASTM A106, A335, API 5L	ASTM A53 Type E, API 5L, ASTM A671/A672
NDT requirement	UT per ASTM/API	RT or UT on weld seam

When to Use Seamless

• Design pressure above ASME B31.3 limits for welded with a joint factor < 1.0
• Sour service (NACE MR0175/ISO 15156) where HAZ cracking risk must be minimized
• High-temperature service above 400°C (ASTM A335 alloy grades)
• Low-temperature service below -46°C (ASTM A333)
• Small-bore piping (NPS 2 and below) where seamless is standard practice

When Welded Is Acceptable (or Preferred)

• Large-diameter pipelines (NPS 24+) where seamless is unavailable or prohibitively expensive
• Structural applications per ASTM A500, EN 10219
• Water transmission, low-pressure utilities
• Projects where cost and delivery drive decisions

Common Mistake

Specifying seamless pipe for all sizes and services regardless of actual pressure needs. This inflates project costs without improving safety. Review your pipe class specifications before defaulting to seamless.

The weld joint efficiency factor (E) in ASME B31.3 Table A-1B determines whether welded pipe meets the design pressure requirement. For ERW pipe examined per the applicable standard, E = 0.85 to 1.0. For seamless, E = 1.0 always.

Read the full guide to pipe types