What Are the Manufacturing Process of Pipe?

The production of steel pipe has its roots in the early 1800s. Initially, pipes were crafted manually through a labor-intensive process involving heating, bending, lapping, and hammering the edges together. The advent of the first automated pipe manufacturing process occurred in 1812 in England. Since then, manufacturing techniques have undergone continuous improvement. Below, we delve into some popular pipe manufacturing methods.

The use of lap welding in pipe manufacturing was introduced in the early 1920s. Although this method is no longer in practice, some pipes manufactured using lap welding are still in use today.

Lap Welding

In the lap welding process, steel was heated in a furnace and rolled into a cylindrical shape. The edges of the steel plate were then "scarfed." Scarfing involved overlaying the inner edge of the steel plate with the tapered edge of the opposite side. The seam was welded using a welding ball, and the heated pipe was passed between rollers, forcing 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 to calculate the allowable operating pressure of a pipe, based on the manufacturing process. This equation includes a variable known as a "joint factor," determined by the type of weld used to create the pipe seam. Seamless pipes have a joint factor of 1.0, while lap-welded pipes have a joint factor of 0.6.

Electric Resistance Welded Pipe

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 employed low-frequency A.C. current to heat the edges, a method utilized from the 1920s until 1970. In 1970, the low-frequency process was replaced by a high-frequency ERW (Electric Resistance Welding) process, which produced higher-quality welds.

Over time, it was discovered that the welds of low-frequency ERW pipes were prone to selective seam corrosion, hook cracks, and inadequate bonding of the seams. Consequently, low-frequency ERW is no longer employed in pipe manufacturing. The high-frequency process, however, continues to be utilized for manufacturing pipes intended for use in new pipeline construction.

Electric Flash Welded Pipe

The production of electric flash welded pipe commenced in 1927. In this process, a steel sheet was shaped into a cylindrical form. The edges were heated until semi-molten, then forcefully brought together until molten steel extruded from the joint, forming a bead. Similar to low-frequency ERW pipe, the seams of flash-welded pipes are susceptible to corrosion and hook cracks, though to a lesser extent than ERW pipes. This type of pipe is also prone to failures resulting from hard spots in the plate steel. Since the majority of flash-welded pipes were produced by a single manufacturer, it is believed that these hard spots occurred due to accidental quenching of the steel during the manufacturing process employed by that particular manufacturer. Flash welding is no longer employed in pipe manufacturing.

Double Submerged Arc Welded (DSAW) Pipe

In the manufacturing of Double Submerged Arc Welded Pipe, a process similar to other pipe manufacturing methods is employed. Initially, steel plates are shaped into cylindrical forms. The edges of the rolled plate are configured to create V-shaped grooves on both the interior and exterior surfaces at the seam location. Subsequently, the pipe seam undergoes welding through a single pass of an arc welder on both the interior and exterior surfaces, hence the term "double submerged." The welding arc is submerged under flux.

This process offers the advantage of welds penetrating 100% of the pipe wall, resulting in a highly robust bond of the pipe material.

Seamless Pipe

The manufacturing of seamless pipe dates back to the 1800s, and while the process has undergone evolution, certain fundamental aspects have endured. Seamless pipe is produced by piercing a hot round steel billet with a mandrel. The hollowed steel is then rolled and stretched to achieve the desired length and diameter. The primary advantage of seamless pipe lies in the elimination of seam-related defects; however, the manufacturing cost is higher.

In the early stages, seamless pipe was susceptible to defects caused by impurities in the steel. With advancements in steel-making techniques, these defects were reduced but not entirely eliminated. Despite the perception that seamless pipe would be preferable to formed, seam-welded pipe, the ability to enhance desirable characteristics in pipe is limited. Consequently, seamless pipe is currently available in lower grades and wall thicknesses compared to welded pipe.

Conclusion

Continuous advancements in materials and welding techniques have led to significant improvements in the reliability of pipes. However, it's important to note that there are still pipes in operation that may be vulnerable to corrosion and seam-related defects. These potential issues are identified through integrity assessments and addressed through necessary repairs.

Modern pipe manufacturing adheres to rigorous quality control measures, including non-destructive tests such as ultrasonic testing and x-ray examinations, as well as pressure testing. Manufacturers conduct pressure tests on each individual section of pipe, and new pipelines undergo thorough pressure testing throughout the construction process.