Why can't non-metallic pipes or stainless steel pipes be used as long-distance pipelines

Oil and gas transportation through pipelines requires a certain pressure to achieve. 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 a dozen MPa, so the pipeline must have Sufficient ability to withstand pressure. In addition, the length of long-distance pipelines is often hundreds or even thousands of kilometers, and the diameter is generally hundreds of millimeters, which can reach more than 1 meter. The number of pipes used in pipeline construction is astonishing, and the price of pipeline materials cannot 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 inexpensive, but their strength and stiffness are lower than those of metal materials, and their pressure-bearing capacity is much lower, generally, only a few tenths of an 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 pipes, 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 because of 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 is that its content of alloying elements is relatively high. Due to a large amount of alloying elements, 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 problem is that stainless steel has only improved corrosion resistance and high-temperature oxidation resistance because of the passive film formed on the surface, not corrosion in all environments. When the passivation film is damaged or cannot be maintained, corrosion will occur. For example, chloride ions in seawater will destroy the integrity of the passivation film and cause pitting corrosion; Corrosion micro batteries are formed under the action of the electrolyte. Therefore, the corrosion resistance of stainless steel is not. 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, wide source and cheap price, and the use of coatings and cathodic protection to control corrosion. . Generally, small-diameter low-pressure pipes mostly use ordinary carbon steel pipes, while high-pressure large-diameter pipes use micro-alloyed 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.