What LSAW and Seamless Pipe Are
LSAW pipe, or longitudinally welded pipe, is made by forming steel plate into a cylinder and closing the longitudinal seam with submerged arc welding, usually with one inside and one outside weld pass. The process suits large diameters and heavy walls, and it is common in line pipe, structural piling and water and gas transmission. Seamless pipe, often abbreviated SMLS, is produced from a pierced and rolled billet with no weld seam at all. Its dimensional range is smaller and the cost per tonne is higher, but it is free of any welded joint.
Key Differences Between LSAW and Seamless Pipe
| Feature | LSAW pipe | Seamless pipe |
|---|---|---|
| Construction | Plate formed and welded along one longitudinal seam | Pierced and rolled from a solid billet, no seam |
| Typical outside diameter | About 406 mm up to 1420 mm | About 10 mm up to 610 mm |
| Wall thickness | Heavy walls available, often 6 mm to 40 mm and above | Usually up to about 40 mm, depending on size |
| Weld seam | One longitudinal seam, inspected separately | None |
| Pressure capability | High, where the weld is fully examined | High, with uniform circumferential wall |
| Relative cost | Lower for large diameter and heavy wall | Higher, particularly in alloy grades |
| Main standards | API 5L, ASTM A671 and A672, GB/T 9711 | ASTM A106, A333, A335, API 5L seamless, EN 10216 |
When Substitution Is Acceptable
LSAW pipe can be used in place of seamless pipe only when the governing design code, the material specification and the end user all accept a welded product for that service. In practice this means checking four points before a substitution is approved:
Code allowance: pressure piping and pipeline codes such as ASME B31.3, B31.4 and B31.8 permit welded pipe for a wide range of services but impose additional examination or restrict welded construction in specific severe services and high-temperature ranges.
Design temperature: welded pipe is normally limited to lower metal temperatures than the equivalent seamless grade, so the design temperature and allowable stress must be recalculated rather than assumed.
Fatigue and cyclic duty: lines subject to significant cyclic loading are usually specified as seamless, because the weld is a potential fatigue initiation site.
Impact and corrosion requirements: low-temperature toughness testing and the corrosion allowance must be satisfied by the welded product and its weld metal.
Where these conditions are met, LSAW pipe is a legitimate and economical solution. Where they are not met, the substitution must be rejected regardless of any short-term price advantage.
How to Confirm What Was Actually Delivered
Visual appearance is not proof of construction. A welded seam that has been dressed or painted over cannot be identified reliably by eye, so acceptance must rest on documentation and inspection rather than on appearance. Practical checks include:
Mill test certificates that state the manufacturing process, the grade and the heat number, and that can be traced to the physical marking on each length.
Full-length ultrasonic or radiographic examination of the longitudinal seam, with acceptance criteria stated in the purchase order.
Dimensional checks of diameter, ovality and wall thickness, including measurement across and away from the seam.
Hydrostatic test records at the specified test pressure and duration.
Marking that identifies the manufacturer, the specification and the process, and a certificate of conformity issued by the supplier.
Specifying the Right Product
The reliable way to avoid confusion is to define the requirement in writing. State the standard, the grade, the manufacturing process, the size and wall, the testing regime and the documentation package in the purchase order, and require the supplier to declare the process on the mill certificate. If seamless construction is genuinely required by the design, say so explicitly rather than relying on the term seamless to be interpreted correctly. If welded construction is acceptable, specify the weld examination level so that both parties know what is being supplied.
Frequently Asked Questions
Q: Can LSAW pipe be used instead of seamless pipe?
Yes, in services where the applicable code and the end user accept welded pipe and where the design temperature, fatigue duty and impact requirements are satisfied by the welded product.
Q: What is the main difference between LSAW and seamless pipe?
LSAW pipe is made from plate and has one longitudinal submerged arc weld seam, while seamless pipe is rolled from a solid billet and has no weld seam at all.
Q: How can LSAW pipe be distinguished from seamless pipe?
By documentation and inspection rather than by appearance: check the mill certificate and process declaration, then confirm the seam with ultrasonic or radiographic examination and dimensional checks.
Q: Is LSAW pipe weaker than seamless pipe?
Not inherently. A properly welded and fully examined LSAW pipe meets its specified strength, but the weld is an additional feature that must be qualified by testing and may limit fatigue performance.
Q: Which pipe sizes are available in each product?
LSAW pipe is produced mainly in large diameters from about 406 mm to 1420 mm, while seamless pipe is produced mainly in smaller diameters up to about 610 mm.
Q: What should be written on the purchase order?
Specify the standard, grade, manufacturing process, size, wall thickness, test requirements and documentation so that the delivered pipe can be verified against a clear specification.
