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Modern tube making is divided into two classes, welded pipes and weldless or seamless pipes. In the former cases the tube is made from plate or strip bent in the circular form and welded along the seam. In the latter the pipe is made from a solid billet forged into the hollow form.
Welded pipes can again be subdivided into three large classes: butt welded, lap welded and electrically welded.
The butt welding process consists in forming the strip into the circular form in such a manner that the edges of the strip meet squarely without overlapping.
In lap welding the strip is scarfed at the edges, so that when bent into shape the two edges overlap, thus giving a comparatively large welding surface as compared with the thickness of the strip.
There are several types of electric welding all of which consist of converting the strip into the circular form and there after welding along the seam by arc or fusion welding.
In the weldless processes the heated billets are pierced by punches, or rotary piercers, thus forming rough thick tubes considerably shorter and thicker than the size required. These rough tubes are extended in length and reduced in thickness by passing through rolling or forging processes. This is generally carried out without reheating.
Welded or seamless pipe lengths are then to be connected to each other by means of screwed connections or by welding.
Screw pipe is threaded on each end and joined by a collar. The threaded pipe is inserted in the collar and drawn as tightly as possible by using pipe tongs which are nothing more than large wrenches. The joint so made is often defected and leaky due to the difficulty in obtaining well matched threads.
Up to the time of the World War II practically all joints of pipe lines were of the screwed and coupled type. The progress made in recent years in the technique of welding, which has resulted in low costs and high efficiency has led to welded joints for pipe line being extensively used. A number of important pipe lines have been constructed by welding the joints with the aid of oxyacetylene or electric arc welding. Welded joints may be as strong as the pipe itself and since the pipe is not weakened by the cutting of threads, it may be somewhat lighter weight than when screw joints are used. Because of this since no collars or other fittings are needed welded pipes are considered to be cheaper. If the welding is properly done, welded lines are also more secure against leakage.
Most lines being laid at present are a combination of the welded and plain end pipes. Plain end pipes are called so because neither end of any section of the pipe is threaded. The joints are connected by couplings. The pipe is welded into lengths usually 130 feet (and even much more) and joined with a plain end coupling. This allows for expansion and contraction in lines. An ideal pipe line would be a continuous pipe able to withstand all common forms of corrosion and all reasonable forces exerted upon it.
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