The Spiral Forming Principle
Spiral welded steel pipe (SSAW) is manufactured by feeding steel strip or coil at an angle onto a forming line so that the strip wraps into a helix. The edges meet along a continuous spiral seam, which is welded by double-sided submerged arc welding (DSAW). Because the pipe diameter is set by the strip width and the helix angle, one coil width can produce many diameters, giving SSAW exceptional flexibility for large-diameter pipe at low cost.
The Forming Line Step by Step
Uncoiling and flattening:
the steel coil is uncoiled, levelled and straightened before entering the line.
Edge milling:
both edges are milled to a precise bevel, removing defects and creating the welding groove.
Helical forming:
the strip is bent around a forming cage at the set helix angle; the angle controls the pipe diameter.
Seam tacking:
the approaching edges are positioned and tacked to hold the spiral geometry.
Internal welding:
the inside seam is welded first by submerged arc welding.
External welding:
the outside seam is welded, completing the double-sided weld.
Cutting:
the continuous pipe is cut to length at the discharge end.
Why the Helix Angle Matters
The helix angle relates strip width to pipe diameter by the geometry of the helix. Typical forming angles are 40-70°:
Smaller angle:
wider effective seam spacing, generally better for pressure service because the seam is more favourably oriented relative to hoop stress.
Larger angle:
better coil width utilization for very large diameters.
Design codes:
API 5L and ASTM limit the helix angle range for line pipe to keep seam stresses acceptable.
Welding and Quality Control
Submerged arc welding gives deep penetration, stable quality and a clean weld surface. The quality programme includes:
100% ultrasonic testing
of the full spiral seam.
Radiographic sampling or full RT
where specified.
Hydrostatic testing
of every pipe at the specified pressure.
Flattening, bend and tensile tests
on the seam and body.
Dimensional checks
on OD, wall, straightness and end bevel.
Advantages and Limitations
| Aspect | SSAW (Spiral) | LSAW (Straight Seam) |
|---|---|---|
| Diameter flexibility | Very high (one coil, many sizes) | Fixed by plate width |
| Seam length per metre | Longer (helical path) | Shorter (one straight seam) |
| Wall thickness range | Moderate | Wider, thicker walls |
| Pressure rating | Medium-high | High |
| Typical use | Water mains, piling, low/mid-pressure gas | High-pressure trunk lines |
Frequently Asked Questions
What does SSAW stand for?
Spiral Submerged Arc Welded, pipe welded along a helical seam using submerged arc welding.
How is the pipe diameter controlled in spiral forming?
By the strip width and the helix angle; changing the angle changes the diameter, so one line can produce many sizes.
Is spiral pipe as strong as straight-seam pipe?
For medium-pressure service, yes, when the seam is fully NDT-inspected; for very high pressures, straight-seam LSAW or seamless is preferred by design codes.
Why is the internal weld done first?
Welding the inside first allows the external weld to complete full penetration and avoids slag entrapment between the two passes.
What testing guarantees SSAW seam quality?
100% automated ultrasonic testing of the entire spiral seam plus hydrostatic testing of every pipe, with radiographic checks where specified.
What are SSAW pipes used for?
Large-diameter water transmission mains, cooling water lines, piling, structural columns and low-to-medium pressure gas gathering lines.
