4.5.5.3 Process of Manufacture

Three different processes are used to manufacture pipe that is used for line pipe. The properties and capabilities of the pipe vary with the type of process used.

4.5.5.4 Seamless Line Pipe

Seamless pipe is generally the industries first choice for high-pressure flow lines and pipelines.

Seamless pipe is a wrought steel tube without a welded seam, manufactured by hot working steel and, if necessary, subsequently cold finished to produce the desired properties.

Generally speaking, seamless pipe is preferred by the oil industry for use in well flow lines and other high pressure lines, although welded pipe described below is similarly used for high pressure lines in larger sizes where seamless pipe is not available. Availability is limited to a maximum diameter of about 20
inches because of the process of forming seamless pipe.

4.5.5.5 Furnace Welded Line Pipe

About the only type of furnace welded pipe available today is manufactured by the continuous welding, butt-weld process.

In the butt-weld process, pipe is manufactured with one longitudinal seam formed by mechanical pressure to make the welded junction after the entire steel strip from which the tube is formed has been heated to proper welding temperature.

The cost of the CW, continuous weld, butt weld line pipe is 15 to 20% lower than Grade B seamless or electric weld line pipe.

4.5.5.6 Electric Welded Line Pipe

Electric welded pipe has one longitudinal seam formed by electric flash welding, electric resistance welding or electric induction welding without the addition of extraneous metal. There is probably more pipe manufactured by the electric weld process than any other method because of the low initial
investment for the equipment and the adaptability to different wall thicknesses. Most electric weld line pipe is not fully normalised after welding. Some is normalised in the weld zone only. Therefore, there is a heat runout zone on each side of the weld resulting in non-uniformity of hardness and grain structure.

Like furnace weld, electric weld is not recommended for use where internal corrosion is expected.