Overview:
The manufacture of steel pipe dates from the early 1800’s. Initially, pipe was manufactured by hand – by heating, bending, lapping, and hammering the edges together. The first automated pipe manufacturing process was introduced in 1812 in England. Manufacturing processes have continually improved since that time. Some popular pipe manufacturing techniques are described below.
Lap Welding
The use of lap welding to manufacture pipe was introduced in the early 1920’s. Although the method is no longer employed, some pipe that was manufactured using the lap welding process is still in use today.
In the lap welding process, steel was heated in a furnace and then rolled into the shape of a cylinder. The edges of the steel plate were then “scarfed”. Scarfing involves overlaying the inner edge of the steel plate, and the tapered edge of the opposite side of the plate. The seam was then welded using a welding ball, and the heated pipe was passed between rollers which forced the seam together to create a bond.
The welds produced by lap welding are not as reliable as those created using more modern methods. The American Society of Mechanical Engineers (ASME) has developed an equation for calculating the allowable operating pressure of pipe, based on the type of manufacturing process. This equation includes a variable known as a “joint factor”, which is based on the type of weld used to create the seam of the pipe. Seamless pipes have a joint factor of 1.0. Lap welded pipe has a joint factor of .6.
Electric Resistance Welded pipe manufacturing
Electric resistance welded (ERW) pipe is manufactured by cold-forming a sheet of steel into a cylindrical shape. Current is then passed between the two edges of the steel to heat the steel to a point at which the edges are forced together to form a bond without the use of welding filler material. Initially this manufacturing process used low frequency A.C. current to heat the edges. This low frequency process was used from the 1920’s until 1970. In 1970, the low frequency process was superseded by a high frequency ERW process which produced a higher quality weld.
Over time, the welds of low frequency ERW pipe was found to be susceptible to selective seam corrosion, hook cracks, and inadequate bonding of the seams, so low frequency ERW is no longer used to manufacture pipe. The high frequency process is still being used to manufacture pipe for use in new pipeline construction.
Electric Flash Welded pipe manufacturing
Electric flash welded pipe was manufactured beginning in 1927. Flash welding was accomplished by forming a steel sheet into a cylindrical shape. The edges were heated until semi-molten, then forced together until molten steel was forced out of the joint and formed a bead. Like low frequency ERW pipe, the seams of flash welded pipe are susceptible to corrosion and hook cracks, but to a lesser extent than ERW pipe. This type of pipe is also susceptible to failures due to hard spots in the plate steel. Because the majority of flash welded pipe was produced by a single manufacturer, it is believed these hard spots occurred due to accidental quenching of the steel during the manufacturing process used by that particular manufacturer. Flash welding is no longer used to pipe manufacturing.
Double Submerged Arc Welded (DSAW) pipe manufacturing
Similar to other pipe manufacturing processes, the manufacture of Double Submerged Arc Welded Pipe involves first forming steel plates into cylindrical shapes. The edges of the rolled plate are formed so that V-shaped grooves are formed on the interior and exterior surfaces at the location of the seam. The pipe seam is then welded by a single pass of an arc welder on the interior and exterior surfaces (hence double submerged). The welding arc is submerged under flux.
The advantage of this process is that welds penetrate 100% of the pipe wall and produce a very strong bond of the pipe material.
Seamless pipe has been manufactured since the 1800’s. While the process has evolved, certain elements have remained the same. Seamless pipe is manufactured by piercing a hot round steel billet with a mandrel. The hollowed steel is than rolled and stretched to achieve the desired length and diameter. The main advantage of seamless pipe is the elimination of seam-related defects; however, the cost of pipe manufacturing is greater.
Early seamless pipe was susceptible to defects caused by impurities in the steel. As steel-making techniques improved, these defects were reduced, but they have not been totally eliminated. While it seems that seamless pipe would be preferable to formed, seam-welded pipe, the ability to improve characteristics desirable in pipe is limited. For this reason, seamless pipe is currently available in lower grades and wall thicknesses than welded pipe.
Conclusion
Continual advances in materials and welding techniques have resulted in dramatic improvements in the reliability of pipes. As mentioned, however, there is still pipe in use that is susceptible to corrosion and seam-related defects. These defects are identified through integrity assessments and are repaired when found.
pipe manufacturing today is subject to non-destructive tests such as ultrasonic testing and x-ray, as well as pressure-testing. Each individual section of pipe must be pressure-tested by the manufacturer, and new pipelines are also pressure-tested during the actual construction process.
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