What Should I Consider When Choosing Seamless or Welded Pipe?

Not only size is key, there are a large number of facts that should be taken into account when selecting seamless or welded pipes.

Production process

Depending on the diameter or the ratio of diameter to wall thickness, different procedures are used for the production of seamless pipes. The usual raw material is a pre-heated circular shaft section in a rotary furnace or, for larger diameters, a pre-formed hollow section in a piercing machine followed by pre-rolling in a rotary rolling mill.

In the manufacture of LSAW pipes, individual steel plates or coils are used as the raw material. The pipes are produced in a series of different, sequential process stages, including cold forming, welding, heat treatment, calibration and non-destructive testing.

Sizing

Manufacturers of both welded and seamless pipes offer a wide range of standard sizes for different industrial sectors. For the individual production of welded pipes, it is possible to order and use raw materials in exact thicknesses based on wall thickness calculations, which will intentionally save resources and reduce costs.

Cost-effectiveness

When cost efficiency is taken into account, the advantages of seamless pipes are obvious for small and medium sized outside diameters and higher wall thicknesses. On the other hand, for small, medium and large outside diameters with wall thicknesses between 2 and 70 mm, welded pipes are more advantageous.

In general, the more economically efficient sizes for hot rolled seamless pipes are those with an outside diameter up to NB 100 and a diameter/wall thickness ratio of 5 : 1 or less (e.g. 50 : 10, 50 : 20, etc.), and for sizes larger than NB 100, usually 8 : 1 or less (e.g. 200 : 25, 200 : 30, etc.).

However, the range of possible sizes is limited by the fact that hot rolled seamless steel pipes with wall thicknesses less than 3 mm cannot be used.

When manufacturing welded pipes from individual steel plates or coils, the minimum ratio of outer diameter to wall thickness is always around 10 : 1, which therefore offers a very wide range of opportunities. However, these pipes can usually be produced in the welded condition, which is more cost effective.

Deciding on criteria

Whether to use seamless or welded pipes depends primarily on the wall thickness/diameter ratio and the specified pipe tolerances.

Availability

For the production of seamless hot-rolled pipes, a limited number of alloys are available. In contrast, there is generally no shortage of steel plates and coils of several different material grades for the production of welded pipes. In addition, smaller quantities of raw material can be recovered from service stores and special alloys, such as alloy C4, can be cut from stock in wide widths.

Tolerances

The wall thickness tolerance for hot rolled seamless steel pipes is 12.5%〇, which is basically specified in ASTM A999 or EN ISO 1127 T2. However, wall thickness differences caused by the manufacturing process may lead to eccentricity, i.e., deviation from symmetry (Figure 2). As a rule, wall thickness tolerances for welded pipes are specified in ASTM A999 or EN ISO 1127 T3.

The uniformity of the steel plate or coil thickness ensures that the wall thickness tolerance remains constant throughout the tube body and is much better than the required ±10%〇. This means that there can be no eccentricity. Especially for track welding or subsequent welding processes on construction sites, a constant wall thickness and thus the exclusion of any eccentricity is a key criterion.

Pipe length and quantity

In the manufacture of seamless pipes, the manufactured length depends on the weight per meter or per rod. Thus, pipe lengths of up to 13 m can be produced up to NB100 and up to about 4 m up to NB200. The minimum purchase quantity is determined by the size of the production lot of approx. 1 ton. For welded pipes, pipe lengths of up to 12 m or 18 m without circumferential welds can be freely selected. For pipes made from coils, the minimum production lot depends on the length of the coil in the particular alloy or size. When produced from individual steel sheets, small lots are also available.

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