Seamless vs ERW pipes: the differences, where each is used, and how to specify them

Steel pipe enquiries often go wrong before the first quote arrives. A buyer asks for "MS pipe, 4 inch" and gets back prices that differ by 40%, because one supplier quoted seamless pipe to an ASTM standard and another quoted welded pipe for water lines. Both are "4 inch steel pipe". They are not the same product.

The first decision is the manufacturing route: seamless or ERW (electric resistance welded). This guide explains the difference, where each is used, and how to write a specification that gets like-for-like quotes.

How seamless pipe is made

Seamless pipe starts as a solid round steel billet. The billet is heated and pierced through the centre with a mandrel to form a hollow shell, then rolled and stretched to the final diameter and wall thickness. Because there is no joint anywhere, the wall is uniform in structure all the way around.

Smaller sizes may be finished by cold drawing, which gives tighter dimensional tolerances and a better surface finish.

How ERW pipe is made

ERW pipe starts as flat steel strip (coil). The strip is formed into a round tube by rollers, and the two edges are heated with electric current and pressed together to make a longitudinal weld seam. The outside weld bead is trimmed off, and on better-quality pipe the weld zone is heat-treated so its structure matches the parent metal.

Because it is made from coil, ERW pipe is quick to produce in large quantities, its wall thickness is very consistent, and it is usually cheaper than seamless pipe of the same size.

The practical differences

• Pressure and temperature. Seamless pipe has no weld line, which makes it the traditional choice for high-pressure, high-temperature and critical service: boilers, steam lines, hydraulic systems, refineries.
• Cost. ERW pipe is generally cheaper and more readily available in common sizes.
• Wall thickness consistency. ERW is made from rolled strip, so wall thickness is very even. Seamless pipe can vary more around the circumference.
• Size range. ERW is common from small bores up to large diameters. Seamless is produced across a wide range too, but large-diameter, thin-wall seamless pipe is less common and more expensive.
• Surface and length. ERW pipe usually has a cleaner surface and longer consistent lengths.

Modern ERW pipe with good seam heat treatment and non-destructive testing performs very well in many pressure applications, which is why it is widely used for oil and gas line pipe. The right choice depends on the service conditions and the standard the project specifies.

Common standards

Pipes are always specified against a standard. The ones buyers meet most often:

• ASTM A106: seamless carbon steel pipe for high-temperature service.
• ASTM A53: carbon steel pipe in Type S (seamless) and Type E (ERW) for general pressure and structural use.
• API 5L: line pipe for oil and gas, available as seamless or welded in several grades (Grade B, X42, X52 and higher).
• IS 1239 (Part 1): Indian standard for mild steel tubes, often ERW, for water, gas and general use, in light, medium and heavy classes.
• IS 3589: steel pipes for water and sewage in larger diameters.
• IS 1161: steel tubes for structural purposes.

Each standard defines the chemistry, mechanical properties, dimensions, tolerances and tests. Naming the standard and grade is the single biggest improvement you can make to an enquiry.

Sizes: NB, OD and schedule

Pipe sizes confuse many first-time buyers:

• NB (nominal bore) or NPS (nominal pipe size) is a name, not a measurement. A "4 inch" or 100 NB pipe has an outside diameter of about 114.3 mm.
• OD (outside diameter) is the measured outside diameter. It stays fixed for a given nominal size.
• Schedule (SCH 10, 40, 80, 160) or a class (light, medium, heavy) defines the wall thickness. A higher schedule means a thicker wall and a smaller inside diameter.

Always give nominal size plus schedule or wall thickness, or OD plus wall thickness. "4 inch pipe" alone cannot be quoted accurately.

Tests and inspection to ask for

• Hydrostatic test: each pipe is pressure-tested with water to a level set by the standard.
• Non-destructive testing (NDT): ultrasonic or eddy-current testing, especially of the weld seam on ERW pipe.
• Mechanical tests: tensile, flattening and bend tests per heat.
• Mill test certificate (MTC) to EN 10204 type 3.1, which certifies the actual results for your lot.
• For critical projects, third-party inspection before dispatch.

End finishes and coatings

State what you need at the pipe ends and on the surface:

• plain ends, bevelled ends (for butt welding) or threaded and coupled ends
• black (uncoated), varnished, galvanised, or with an external coating such as 3LPE or FBE for buried pipelines
• end caps for protection in transit

How to write a pipe enquiry that gets comparable quotes

Include all of these:

1. Manufacturing route: seamless or ERW.
2. Standard and grade, for example ASTM A106 Grade B, or IS 1239 Part 1 Medium.
3. Size: NB and schedule, or OD and wall thickness.
4. Length: random lengths (for example 5 to 7 m) or fixed lengths (6 m or 12 m).
5. End finish and coating.
6. Quantity in metres or tonnes.
7. Testing and certification: hydro test, NDT, EN 10204 3.1 MTC, third-party inspection.
8. Delivery location, Incoterm and required date.

Frequently asked questions

Can ERW pipe be used for high-pressure lines?

Yes, in many applications. Modern ERW line pipe with heat-treated seams and full NDT is widely used in oil and gas pipelines. Whether it is acceptable depends on the design code and the project specification, which may require seamless pipe for some services.

What does "SCH 40" mean?

Schedule 40 is a wall thickness designation. For each nominal size, SCH 40 has a set wall thickness; SCH 80 is thicker. The outside diameter stays the same, so the inside diameter gets smaller as the schedule increases.

Is seamless pipe always more expensive?

Usually, for the same size and grade, because the process is slower and uses more energy. The gap varies with size, wall thickness and market conditions, so get quotes for both if your standard allows either.

What is an EN 10204 3.1 certificate?

It is a mill test certificate in which the manufacturer's independent quality department certifies the actual chemical and mechanical test results for the specific lot supplied. It is the usual minimum for pressure piping.

Sourcing pipes on GlobalMart

Compare manufacturers and stockists in steel pipes and tubes, or go straight to seamless pipes, ERW pipes or stainless steel pipes. Need fittings too? See valves and fittings.

If your requirement is specific, post it once using the checklist above and let suppliers quote against the same specification. For overseas suppliers, our guide to Incoterms explains who pays for freight and insurance.