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Different power sources
Although they both require energy, crude oil pipelines and natural gas pipelines have different power sources. One uses a centrifugal pump and the other uses a compressor. The centrifugal pump is the heart of the crude oil pipeline. It can increase the design pressure of the oil from the oil tank or the upstream of the pipeline and transport it to the downstream. According to the needs, two boosting methods can be used: multi-pump series or multi-pump parallel. The working characteristics of multi-pump series are the same displacement and the added head, and the working characteristics of multi-pump parallel are the same head and the added displacement. The compressor is the power source of the natural gas pipeline. It can boost the upstream gas and transport it to the downstream of the pipeline. From the working principle, it can be divided into centrifugal compressors and reciprocating compressors. The former has a relatively low inlet and outlet pressure and is mostly used in pipeline compression stations, while the latter has a relatively high pressure and is mostly used in gas storage. From the driving method, it can be divided into electric drive compressors and fuel drive compressors. The former is generally used in stations with good external power conditions, and the latter is generally used in desolate stations.
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Different temperature requirements
Although they are both transported, crude oil pipelines and natural gas pipelines have different temperature requirements. One needs heating and the other needs cooling. The freezing point of crude oil is generally high. Taking Daqing Oilfield as an example, the freezing point of crude oil is about 27°C, which is much higher than the soil temperature around the pipeline (generally not higher than 6°C in winter). In order to prevent crude oil solidification during transportation and cause pipeline blockage, crude oil pipelines in Northeast China need to set up heating facilities along the pipeline to heat the crude oil to 70°C out of the station and ensure that the downstream inlet temperature is not lower than 33°C. Natural gas pipelines are different. The lower the natural gas temperature, the more conducive to transportation. According to engineering experience, the energy consumption of the compressor station outlet temperature of 50°C is nearly 10% lower than that of the 60°C operating condition. Therefore, for some compressor stations with high compressor outlet temperatures, air coolers need to be installed at the outlet to reduce the natural gas temperature to save operating costs.
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Different flow rate requirements
Although they are both flowing, crude oil pipelines and natural gas pipelines have different flow rate requirements, one is fast and the other is slow. During the transportation process, crude oil will exchange heat through the pipe wall and the surrounding soil and gradually approach the soil temperature. In order to avoid condensation pipe accidents caused by slow flow rate and excessive temperature drop, high freezing crude oil pipelines generally set a minimum output. The designed annual output of a crude oil pipeline in Northeast China is 7.5 million tons, which is equivalent to an average daily output of 21,500 tons per day. In order to prevent the solidification of crude oil in the pipeline, the pipeline requires a minimum output of no less than 14,700 tons per day in winter, and the equivalent flow rate is about 1.085 meters per second. Natural gas pipelines do not have problems related to low output, but require that the flow rate should not be too fast, generally not exceeding 10 meters per second. If the natural gas flow rate is too fast, the friction between the natural gas and the pipe wall will increase significantly, and the vibration of the pipeline within the allowable stress range will increase, and the service life and operation safety of the pipeline will be affected.
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Different pressure drop rules
Due to the existence of friction resistance, there is energy loss in the transportation process of oil and gas pipelines, and the pressure will decrease with the increase of the transportation distance, but the pressure drop rules of crude oil pipelines and natural gas pipelines are different, one is a straight line and the other is a parabola. The pressure drop of crude oil pipelines is relatively uniform, and is in a linear proportional relationship with the transmission distance. The average pressure is at the 1/2 position of the pipeline; while the pressure drop of natural gas pipelines generally shows that it is slow at first and then fast, and the pressure drop and the transportation distance are in a parabolic law. The main reason why the pressure drop of natural gas pipelines is slow at first and then fast is that: under the same flow rate, the gas pressure at the starting end is high, the natural gas is in a relatively compressed state, and the friction resistance between the pipe wall is small, so the pressure drop is slow; while the gas pressure at the end is low, the natural gas is in a relatively expanded state, the friction resistance between the natural gas and the pipe wall increases, and the pressure drop becomes faster. According to engineering experience and simulation calculation results, the average pressure is generally at 2/3 of the distance from the starting point of the pipeline.
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Different elevation response
Long-distance oil and gas pipelines are spread all over the country, and the terrain they pass through varies greatly, including deserts, forests, plains and mountains. For mountainous areas with large elevation changes, the sensitivity of oil and gas pipelines to terrain changes is different, one is sensitive and the other is slow. Due to the high density of crude oil, crude oil pipelines are more sensitive to elevation. For every 100 meters of elevation reduction, the liquid pressure in the pipe increases by about 0.8 MPa. When transporting from high to low, the pressure in the pipe decreases slowly, or rises instead of decreasing. It is even necessary to set up a pressure reducing station in the section with a large local drop to ensure that the liquid pressure in the pipe does not exceed the design pressure of the pipeline. Due to the relatively small density of natural gas, the natural gas pipeline is relatively slow to respond to elevation, and elevation changes of less than 200 meters are generally not considered.
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Different pipe storage capacity
The capacity of oil and gas pipelines is fixed. Due to the different compression properties of liquids and gases, the corresponding pipe storage of crude oil pipelines and natural gas pipelines (i.e. the volume of the fluid in the pipe converted to standard atmospheric pressure) is different, one is fixed and the other changes. Crude oil has poor compression performance, and the pipe storage of crude oil pipelines does not change with pressure. As a gas, natural gas has better compression performance. The higher the average pressure of the pipeline, the greater the pipe storage. For the same pipe section, the average pressure of 2 MPa pipe storage is about twice the average pressure of 1 MPa pipe storage. In order to make full use of the compressibility of natural gas, branch natural gas pipelines that supply gas to cities generally appropriately increase the pipeline design pressure and select larger pipe diameters to increase their gas storage capacity, which to a certain extent solves the peak load problem for downstream users. Due to the good gas compression performance, the storage capacity of natural gas pipelines is enhanced. It is also because of the gas compression performance that the ability of natural gas to transmit pressure in the pipeline is weakened. Once a pipeline accident occurs, if special measures are not taken, it will often cause dozens of meters, hundreds of meters or even kilometers of pipelines to be torn. On the contrary, due to the poor compressibility of liquids, crude oil has a strong ability to transmit pressure in the pipeline. When a pipeline leaks, the pressure of the front and rear sections decreases rapidly, and the pipeline will not rupture over a long distance.
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Different compatibility
After the construction of crude oil pipelines and natural gas pipelines, the compatibility with other resources is different, one is picky and the other is tolerant. Crude oil pipelines are tailored according to the physical properties of oil products. Different oil sources have their own physical properties of oil products, generally with large differences in viscosity, freezing point, density, etc. Different oil physical properties correspond to their own optimal process solutions. Therefore, after the construction of crude oil pipelines, the compatibility with other resources is poor. Once the oil source changes, the entire pipeline system needs to be modified or the process plan needs to be adjusted. A heating furnace needs to be added to the low-freezing oil pipeline to transport high-freezing oil, and a chemical drag reducer needs to be added to the conventional oil pipeline to transport heavy oil. The natural gas pipeline is more compatible with different resources. Whether it is domestic gas or imported gas, Tarim gas or Changqing gas, coalbed methane or coal-to-gas, as long as it meets the relevant quality standards, it can enter the pipeline and be mixed with other gas sources for transportation.
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Different forms of distribution
The pipeline is the link between the upstream resource area and the downstream users. The resource areas corresponding to the crude oil pipeline and the natural gas pipeline are basically similar, but the downstream users are very different. One is wholesale and the other is retail. The crude oil pipeline transports crude oil from the oil field to the refinery. The refinery has special processing requirements for different oil products. The pipeline also has corresponding conditions for different transmission volumes and different oil properties. The pipeline has determined the corresponding refinery and annual transmission scale in the design stage. It basically remains unchanged after it is put into production. It belongs to the relatively stable wholesale distribution for downstream users. The distribution location of the natural gas pipeline can be any city along the pipeline. The distribution users can be from all walks of life. The distribution time can be opened at any time according to the market development situation. The distribution scale can be adjusted at will according to the needs. It belongs to the retail distribution with scattered users, many industries and large changes.
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Different qualitative characteristics
The transportation stability of crude oil pipelines and natural gas pipelines is different, one is stable and the other is fluctuating. The upstream of crude oil pipelines is the oil field, and the downstream is the refinery, which is not affected by the season. Even during the maintenance of pipelines and refineries, they will be adjusted through large upstream and downstream oil tanks to ensure the stable transportation of pipelines and the normal production of refineries, and the transmission volume is relatively stable. The corresponding gas source of natural gas pipelines is relatively stable, while the downstream market demand varies greatly. Generally, more gas is used in winter and less in summer, more gas is used during the day and less at night. Due to the uneven gas volume download along the pipeline, the downstream pipeline transmission volume will be more volatile. In order to solve the high and low transmission volume of natural gas pipelines, gas storage facilities such as gas storage and LNG receiving stations are generally equipped around the main downstream markets of the pipeline to ensure a stable supply of gas to downstream contract users.
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Different environmental impacts
Long-distance oil and gas pipelines are linear projects. During the construction process, it is inevitable to pass through various environmentally sensitive areas such as nature reserves and water sources. However, crude oil pipelines and natural gas pipelines have different environmental impacts, one is serious and the other is minor. Once a crude oil pipeline leaks, it will cause environmental pollution at the least and fire at the worst, with very serious consequences. However, even if a natural gas pipeline leaks, the valve chambers at both ends of the leak point will generally be automatically shut off, and the gas stored in that section of the pipeline will be quickly vented. The small amount of natural gas emitted from the leak point will spread quickly, and the impact on the surrounding environment will be relatively small.
