Seamless vs. Welded Pipe Dimensions

When selecting steel pipe, dimensional accuracy, wall thickness range, and outer diameter control are key factors in determining its performance and application. Although both seamless and welded pipes (ERW and SSAW) are steel pipes, they differ significantly in their production processes, dimensional ranges, and tolerance standards. The following systematic comparison of the two is conducted from multiple perspectives.

Seamless Pipe Size Standards and Ranges

Seamless pipe is formed by piercing a solid steel billet and then hot-rolling or cold-drawing it without welds. Specifications are indicated as "outer diameter (mm) x wall thickness (mm)." For example, Φ89×4 indicates an outer diameter of 89 mm and a wall thickness of 4 mm.

1. Common Standards

GB/T 8163-2018 (Seamless Steel Pipe for Fluid Transportation)

ASTM A106 / ASTM A53 (American Standard)

EN 10210 / EN 10216 (European Standard)

2. Size Range

Hot-rolled seamless pipes typically have an outer diameter of 32-630mm and a wall thickness of 2.5-75mm.

Cold-drawn pipes have smaller outer diameters (6-200mm), wall thicknesses of 0.25-14mm, and higher precision (±0.2mm).

3. Limitations

Due to process limitations, extremely large diameters (>630mm) or ultra-thin wall thicknesses (<0.25mm) are difficult to produce.

Welded Pipe Size Standards and Ranges

Welded pipes are made by welding steel plates or strips that have been coiled. Common processes include longitudinal seam welding (ERW) and spiral seam welding (SSAW). Specifications are similar to those for seamless pipes, but with a wider range.

1. Common Standards

GB/T 9711 (Steel Pipe for Oil and Gas Transportation)

SY/T 5037-2018 (Spiral Submerged Arc Welded Pipe)

API 5L (Transmission Pipeline Standard)

2. Size Range

Straight seam welded pipe has an outer diameter of up to 3 meters (e.g., water pipelines) and a wall thickness of 0.5-25 mm; spiral welded pipe typically has an outer diameter of 219-3500 mm and a wall thickness of 5-25 mm (SY/T 5037-2018).

3. Flexibility

Non-standard sizes, such as square and rectangular pipes (e.g., 40×60 mm), can be produced by adjusting the steel plate width.

Seamless Pipe vs. Welded Pipe: Size Comparison

1. Diameter Range

Welded steel pipe has a wide diameter range, allowing for the manufacture of larger diameters, ranging from 1/2 inch to 100 inches. Small-diameter welded pipes can be produced directly from steel strips and plates, while large-diameter steel pipes must first be formed from round steel preforms or plates, followed by welding, painting, and packaging, resulting in relatively high costs.

The diameter range of seamless pipes is relatively small, ranging from 3/8 inch to 26 inches, which is smaller than that of welded pipes. Smaller diameter seamless pipes are produced directly from ingots, while larger diameter seamless pipes require hot rolling, cold drawing, and other processes.

2. Wall Thickness

The wall thickness of welded pipes is relatively thin and uneven, ranging from 2.0 to 11.0 mm. Due to limitations in welding processes, welded steel pipes are thinner than seamless steel pipes and are more prone to welding defects, which can shorten their service life.

Seamless steel pipes offer uniform wall thickness and high strength, generally thicker than welded steel pipes, reaching up to 45 mm. They offer excellent stability and are commonly used in high-pressure, high-strength industrial applications.

3. Pipe Diameter Accuracy

There is a significant difference in the diameter accuracy of welded and seamless pipes. Welded pipes have lower accuracy than seamless pipes.

The diameter error for welded pipes under different standards ranges from 2% to 3%, while the diameter error for seamless pipes is less than 1%. Therefore, seamless pipes are preferred for applications requiring higher precision.

4. Outer Diameter Tolerance

Welded steel pipes are formed by coil welding, using processes such as longitudinal seam welding (ERW) and spiral seam welding (SSAW). Due to metal contraction during high-temperature welding, the outer diameter tolerance is typically ±1%. For example, the outer diameter of a DN100 welded pipe is permitted to be 114.3 ± 1.1 mm.

Seamless pipes, on the other hand, are produced using a pierced cold rolling/cold drawing process, eliminating the effects of weld stress and maintaining a tighter tolerance (±0.5%). The outer diameter of the same specification is 114.3 ± 0.6 mm.

Selection Recommendations

1. Preferring Seamless Pipe

Working pressure ≥ 10mpa or media with flammable or explosive gases;

Frequent bending required (e.g., Φ50×5mm cold-drawn tubes for automotive drive shafts).

2. Preferring Welded Pipe

Large diameter (>600mm) or non-standard shape (e.g., 200×100mm rectangular tubes);

Budget limitations and low pressure requirements (e.g., φ48×3.5mm ERW pipes for construction scaffolding).

Summary

Welded and seamless pipes differ in size. Welded pipes have a wide diameter range but thin wall thickness, making them suitable for general low-pressure fluid transportation. Seamless pipes have a relatively narrow diameter range but offer uniform wall thickness and high strength, making them suitable for applications requiring frequent deformation or subject to high pressure. When selecting steel pipe, it's important to comprehensively consider the specific application and select the appropriate product to ensure project quality and economic efficiency.