Difference between seamless and welded pipe

What is the difference between seamless and welded pipe?

Pipes form an integral part of systems that require transportation of various media across refineries or industrial process plants. The capacity of the component and the media they transport will depend on the material they are made from. Some of the common materials manufacturers use include PVC, stainless steel, alloy steel, carbon steel, nickel alloys, titanium alloys, copper nickel, monel, Inconel, Hastelloy, etc. The use of said materials is judicious, which means the cost factor is also taken into account while producing them. Basically, there are two kinds of pipe construction. First is the seamless kind, which contains no seam, and the circumference of the pipe will have no joints. So the surface of these pipes will be smoother and more even. The second type of construction is a welded type. Unlike the seamless kind, the welded type will have a seam that joins the pipe, which results in the formation of a seamline on its surface.

In short, the main difference between the construction of these two pipes is the inclusion of a seamline in a welded construction. Yet, there are some other parameters that separate both constructions including the pressure capacity and the price point. In terms of the pressure bearing property, the use of a seamless pipe enables higher pressure bearing ability. On average, the pressure borne by a seamless variety pipe is almost 20% higher than a conventionally welded one. This property makes it useful for a vendor to use a seamless type for a high pressure application. Although the use of a welded pipe is suited to high stress applications, its weld seam makes it vulnerable to failure, especially if the heat affected weld zone is prone to intergranular corrosion. If the pipe has been corroded by the sensitized grain boundary, the chances of a pipe bursting at the seamline are very high. This is an undesired effect, particularly in industries that transport across toxic material or bio-hazard media or even flammable compounds. In such instances, the vendor may choose to use a seamless pipe because they aren’t predisposed to intergranular corrosion and so, the chances of pipe burst occurring are drastically reduced.

Price difference between seamless and welded pipe

Another area that separates these pipe constructions is their price difference. The reason for the occurrence of the price difference is their manufacturing process. Since a welded pipe would make use of a metal plate, it is relatively economical to produce them. Moreover, the rate of production is also faster, which is also a factor that results in a product that is priced low. On the other hand, a seamless pipe is drawn through billets, and the manufacturing process is very laborious and consumes time. These factors bring the cost of the product higher as well. Furthermore, the pressure capacity of seamless pipes is higher than welded ones, and the surface is much smoother, with the chances of corrosion being reduced. A combination of all these factors makes the product an effective, yet costlier option in comparison to the welded pipe.

The seamless and welded pipe manufacturing process

As mentioned previously, welded pipes are manufactured using metallic sheets, with the wall thickness mentioned by the buyer. The length of the sheet prior to being welded is to be matched keeping both the inner and outer diameter of the pipe. These dimensions are to be given by the buyer during order placement. The welding method used, must be conducted in a manner that is set by the various standards. Usually, manufacturers, suppliers, and buyers abide by several of the ASTM standards that act as guidelines during production.

The production of seamless pipes makes use of heat treatments. The alloy is first heated, followed by molding with the use of a solid billet. The solid billet is what guides the alloy into a component that is shaped in the form of a  cylinder. Once the alloy forms cylinders, they are rolled such that they get stretched and form hollows. During this process, the hollowed center of the cylinder is irregular shaped. To get an even, hollow center, a bullet-shaped piercer point is thrust through the core of the billet, simultaneously, as the cylinder is rolled. Following these processes, the pipes are put through additional machines that straighten them, while also eliminating deformities to create, better, smoother, and durable seamless pipes.

Seamless and welded pipe application

Since the main function of pipes is to ferry across materials, the application or the end-use of the product will be based on several factors including cost, strength, type of media being used in the application, the resistance of the alloy to corrosion in addition to the temperature and pressure requirements. Industries such as pharmaceutical, building & construction, food processing, beverage industry, nuclear & power generation, etc, make use of pipes to transport across their products. However, the industry that makes the most of these components is the petrochemical and natural gas industry.

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