Are you familiar with the role of line pipes?
These carbon steel tubes are mainly used for transporting natural gas, water, petroleum products, and crude oil. Their diameters range between two and forty-eight inches.
Line pipes are subjected to various tests to check their strength, toughness, and ability to withstand high pressure. Their ends can be flanged, threaded, plain, grooved, or beveled. They must endure both internal and external loads to provide a long lifespan.
These are the factors to look for in a line pipe.
There are two main types of line pipes, welded and seamless tubes made from carbon steel. Welded models are equipped with a steel pipe joint, used for transporting gas, water, air, steam, sewage, and low-pressure fluids. Read more about the history, design, and material of steel pipes.
In contrast, seamless models are less heavy but more resistant to corrosion and capable of containing high pressure. Since there are no joints, seamless line pipes can be used for mechanical applications as well. Generally, they are chosen for the transport of liquids and gas at high speeds and volumes. Both types come with specific advantages and drawbacks, but seamless models are more widely used.
For example, welded line tubes are popular for their lower cost and the possibility for internal surfaces to be inspected before manufacturing. These models can be produced in longer lengths and larger diameters. Conversely, seamless models offer better corrosion resistance and a more homogenous grain structure. They eliminate the need for pressure de-rating and additional integrity testing.
Wall thickness is an important dimensional property of line pipes, which is directly proportional to their tonnage. The choice of wall thickness depends on the amount of internal and external load the tube will be exposed to throughout its lifetime. The internal load comes with hoop stress, referring to the mechanical stress applied to the tube when put under pressure.
External load, however, might involve a variety of loads depending on the location of the pipeline. For instance, it might include earth load in cases where the pipeline is buried or external water pressure if the pipeline is in a water body. Additionally, railway load is present when the pipeline is placed across rail lines, whereas wave and current load occur when it’s offshore.
Furthermore, temperature differences usually result in expansion loads. Also, external loads involve storage and transportation stresses. There is a wide range of international design codes and local regulations providing wall thickness specifications. For further guidance on the effects of thermal expansion, go here https://en.wikipedia.org/wiki/Thermal_expansion.
Another dimensional feature of great importance in line pipes is out-of-roundness. It refers to the difference between the maximum and minimum diameter as long as it’s measured at the same cross-section. This feature has influence over two essential aspects of line pipes, resistance to collapse and girth welding.
Out-of-roundness is proportional to the power of a line pipe to withstand external loads. The more round a line tube is, the greater its resistance to collapse. In the event of offshore applications, the out-of-roundness of these pipes must be minimal at the expense of offering greater wall thickness. The only way for offshore pipelines to withstand external hydrostatic pressure is by optimizing wall thickness.
Girth welding is a pipeline construction, requiring tubes to be welded at the ends. Any misalignment during the process of welding in large-diameter tubes might result in ruptures and leaks when exposed to pressure.
Chemical composition is believed to have a great influence over the properties, corrosion resistance, microstructure, and weldability of steels used in line pipes. Carbon has the capacity to increase pearlite volume fraction, which results in an increase in strength. Conversely, Manganese ensures strength and toughness in tubes as long as it’s kept at approximately 1.4 %. When combined with Sulphur, Manganese eliminates low ductility.
Silicone is another element increasing tube strength only moderately. When choosing a line pipe manufacturer, select one that meets environmental requirements and stringent codes. The role of silicone is that of a deoxidizer. When combined with aluminum, silicone has the potential to prevent carbon monoxide formation and porosity by eliminating the oxygen coming in contact with steel.
Copper is another desirable element because of its power to induce better resistance to atmospheric corrosion. It should be limited to 0.35 % to avoid hot shortness, eventually leading to crack development. Conversely, Chromium is popular for increasing carbon equivalent and hardness, provided its amount is limited to 0.02 %. By increasing the amount of Chromium, the weld metal will no longer keep its optimal properties.
The bottom line
These tubes are used in numerous industries due to their transporting abilities.
They undergo strict quality inspections to ensure toughness and meet the requirements of customers!