Why non-metallic pipes or stainless steel pipes cannot be used as long-distance pipelines

The transportation of oil and gas through pipelines requires a certain pressure. The longer the pipeline, the higher the pressure required for oil and gas transportation. The pressure of long-distance pipelines is generally between a few MPa and more than ten MPa. Therefore, the pipeline must have Sufficient ability to withstand pressure. In addition, the length of long-distance transmission pipelines is often hundreds or even thousands of kilometers, and the diameter is generally several hundred millimeters, which can reach more than 1 meter. The amount of pipe materials used in pipeline construction is amazing, and the price of pipeline materials should not be too high. Therefore, the ability to withstand high pressure and economical price are the main issues to be considered in the selection of pipeline materials.

There are two types of non-metallic materials used as pipes: various engineering plastic pipes and cement pipes.

Plastic pipes are corrosion-resistant and cheap, but their strength and stiffness are lower than metal materials, and their pressure-bearing capacity is much lower, generally only a few tenths of MPa, even if the working pressure of aluminum-plastic and steel-plastic composite pipes is only 1~ 2 MPa, another problem with engineering plastic pipes is the smaller diameter. Therefore, engineering plastic pipes are often used as medium and low-pressure pipelines, such as water supply and drainage, garden gas transmission, etc., and are not suitable for high-pressure, large-diameter long-distance pipelines.

Cement pipes are not suitable for high-pressure long-distance pipelines due to their low pressure-bearing capacity, the connection between pipes is prone to leakage, and there are also corrosion problems (internal steel bars).

The characteristic of stainless steel pipe is that its alloy element content is relatively high. Due to the large number of alloy elements contained, its corrosion resistance is much better than that of carbon steel and low alloy steel, but its price is also much higher than that of carbon steel and low alloy steel. Generally, the price of stainless steel is 4 to 5 times that of carbon steel, and the high price limits the application of stainless steel in the pipeline industry.

Another key issue is that stainless steel has improved corrosion resistance and high-temperature oxidation resistance only because of the formation of a passivation film on the surface, and it does not corrode in all environments. Corrosion will occur when the passive film is damaged or cannot be maintained. For example, chloride ions in seawater will destroy the integrity of the passive film and cause pitting corrosion; when there are small differences on the surface of the stainless steel structure, such as foreign matter attached, it will Corrosion micro batteries are formed under the action of electrolyte. Therefore, the corrosion resistance of stainless steel is not absolute.

Although steel pipe has poor corrosion resistance, it is widely used in the oil and gas transportation industry due to its simple production process, high-pressure resistance, a wide range of sources, and low price, and corrosion can be controlled by coating and cathodic protection. Generally, small-diameter low-pressure pipelines use ordinary carbon steel pipes, while high-pressure large-diameter pipelines use micro-alloy high-strength steel pipes according to mechanical performance requirements.

Of course, with the development of science and technology, if the price of stainless steel pipes can be greatly reduced or the strength of non-metallic pipes can be improved, it is also possible to use them as long-distance oil and gas pipelines.

Post time: May-25-2023

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