There are three processes for metallic pipe manufacture. Centrifugal casting of hot alloyed metal is one of the most prominent process. Pipes are generally manufactured in such a fashion. Seamless pipe is formed by drawing a solid billet over a piercing rod to create the hollow shell. As the manufacturing process does not include any welding, seamless pipes are perceived to be stronger and more reliable. Historically seamless pipe was regarded as withstanding pressure better than other types, and was often more easily available than welded pipe.
Process control and non-destructive testing allow correctly specified welded pipe to replace seamless in many applications. Welded (also Electric Resistance Welded ("ERW"), and Electric Fusion Welded ("EFW")) pipe is formed by rolling plate and welding the seam. The weld flash can be removed from the outside or inside surfaces using a scarfing blade. The weld zone can also be heat treated to make the seam less visible. Welded pipe often has tighter dimensional tolerances than seamless, and can be cheaper if manufactured in the same quantities.
There are a number of processes that may be used to produce ERW pipes. Each of these processes leads to coalescence or merging of steel components into pipes. Electric current is passed through the surfaces that have to be welded together; as the components being welded together resist the electric current, heat is generated which forms the weld. Pools of molten metal are formed where the two surfaces are being connected as strong electric current is passed through the metal; these pools of molten metal form the weld that binds the two connected components.
ERW pipes are manufactured from the longitudinal welding of steel. The welding process for ERW pipes is continuous as opposed to welding of distinct sections at intervals. ERW process uses steel coil as feedstock.
The High Frequency Induction Technology (HFI) welding process is used for manufacturing ERW pipes. In this process the current to weld the pipe is applied by means of an induction coil around the tube. HFI is generally considered to be technically superior to “ordinary” ERW when manufacturing pipes for critical applications, such as for usage in the energy sector In addition to other uses in line pipe applications, as well as for casing and tubing.
Large-diameter pipe (25 centimetres (10 in) or greater) may be ERW, EFW or Submerged Arc Welded ("SAW") pipe. There are two technologies that can be used to manufacture steel pipes of sizes larger than the steel pipes that can be produced by seamless and ERW processes. The two types of pipes produced through these technologies are Longitudinal submerged arc welded (LSAW) and Spiral submerged arc welded (SSAW) pipes. LSAW are made by bending and welding wide steel plates and most commonly used in oil and gas industry applications. Due to their high cost, LSAW pipes are seldom used in lower value non-energy applications such as water pipelines. SSAW pipes are produced by spiral welding of steel coil and have a cost advantage over LSAW pipes as the process uses coils rather than steel plates. As such, in applications where spiral-weld is acceptable, SSAW pipes may be preferred over LSAW pipes. Both LSAW pipes and SSAW pipes compete against ERW pipes and seamless pipes in the diameter ranges of 16”-24”.
Tubing for flow, either metal or plastic, is generally extruded.
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