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Natural gas is an important energy and chemical raw material. It has a high calorific value and little environmental pollution. It is called “clean energyâ€. For half a century, the world's natural gas industry has maintained a strong momentum of growth. The share of natural gas in the world's primary energy mix has risen from less than 10% in the early 1950s to 25% in 2005. Due to the special nature of natural gas, it can only be transported by pipes. At present, 1.5 million km of natural gas pipelines have been built in the world. Natural gas pipelines have been connected into international or regional pipeline networks, forming a large-scale gas supply system.
At present, China's natural gas industry is still in its infancy. In 2005, the gas production was 27.7 billion m 3 , accounting for only about 3% of China's energy composition, while the world average has reached about 21%, which is quite different, so the development potential is great. As the country's exploration and development of natural gas resources increases, China's natural gas industry will enter a period of accelerated development. It is estimated that China will build more than 10,000 kilometers of gas pipelines in the next 10 years.
On the one hand, the operation of natural gas transmission pipelines requires safety and reliability to ensure continuous and stable supply of gas to users; on the one hand, due to the large energy consumption and loss of operation of large-scale gas transmission systems, it is particularly important to reduce gas transmission costs and save energy and reduce consumption. This paper analyzes and demonstrates the energy-saving technology of gas pipeline system.
1 The gas transmission pressure selects the friction between the airflow and the pipe wall during the gas transmission, resulting in pressure loss, and the long-distance gas transmission is realized by the continuous pressure increase of the gas station along the line. Therefore, friction loss is the basic component of energy consumption. Now let's first study the relationship between the basic process of gas transmission and energy consumption. According to the common formula derivation: Q = P 1 N 0.33 D 2.5 L 0.5 (1) where: P 1DDD pipeline starting point pressure, MPa; DDDD pipe inner diameter, mm; QDDD transmission, 10 4 m 3 / a; LDDD pipeline Length, km; NDDD gas transmission power, kW.
From equation (1), we can see the law of the change of operating parameters: when the existing pipeline transmission decreases (D, L does not change), not only the transmission power is reduced, but also the starting pressure P 1 is greater. Decline, resulting in increased gas transmission energy consumption. Numerically, if the output is reduced by 10%, the power will drop by 25% under the condition that the starting pressure P 1 is constant. If the output is reduced by 20%, the power will drop by 50% under the condition that the starting pressure P 1 is constant.
In a physical sense, increasing the pressure increases the density of natural gas in the pipe, reducing the actual velocity of the natural gas in the pipe and the pressure drop. Therefore, wear as a form of main line loss is reduced. In addition, the higher the density of natural gas, the higher the efficiency of the compressor, and the higher the pressure head produced by the same power compressor.
The maximum working pressure of the system is limited by the capacity of the gas transmission unit and the mechanical properties of the pipe. At present, the strength of the most commonly used pipeline steel in the world's oil and gas pipelines is X65, X70.
In addition, according to pipeline construction and operating conditions, pipeline steel has certain toughness in addition to strength indicators. China's West-East Gas Pipeline uses X70-grade steel pipe, which is an important choice to break through the traditional restrictions and move closer to the world's advanced level. Some foreign companies, such as British BP and Sumitomo Corporation, have been actively developing X100 grade steel pipe materials since 1997 to carry out various performance tests, such as pipeline welding test, construction pull, elbow, etc. At the scene, a series of supporting processes such as cold bending, on-site welding and lower ditch have made certain progress and prepared for industrial promotion.
In short, continuous improvement of gas transmission pressure is the direction of the development of the pipeline industry in the future.
Countries are working hard in this direction. For example, the West-East Gas Pipeline of China adopts 10 MPa gas transmission pressure; the Alliance gas pipeline in the United States and Canada is a multinational project with a length of 3000 km and a diameter of 914.4 mm; a gas pressure of 12 MPa is adopted. This is illustrated by the highest operating pressure of 15.7 MPa for the Zeepipe natural gas pipeline in Norway and Belgium.
2 Gas transmission temperature When natural gas is selected to flow along the pipeline, the pressure will gradually decrease due to the resistance of the fluid. A decrease in pressure causes a decrease in gas density and a change in line speed. In addition, due to the heat exchange between natural gas and soil, the temperature of natural gas will also decrease. The gas transmission temperature has a great relationship with the system energy consumption. In addition to the heat loss to the soil, the efficiency of the compressor unit is closely related to the gas delivery temperature. According to the derivation of the heat exchange equation between natural gas and soil, it can be concluded that: Ne=tp " e Gz 2 m R 2 T 2 mu 2 2P 2 m DE 2 L(2) where: the power required for gas transmission at NeDDD gas station, kW ;tpDDD calculates the friction coefficient; "relative conversion efficiency of the energy consumption of the eDDD compressor station (including the efficiency of the compressor and the inlet and outlet stations); mass flow of natural gas in the GDDD pipeline, kg/s; average compression coefficient of the natural gas of D; RDDD gas Constant; average temperature of T mDDD natural gas, K; mass velocity of natural gas in uDDD pipeline, kg/m 2 s (c = u / p) pDDD natural gas density, kg / m 3; linear velocity of natural gas in cDDD pipeline, m / s; average pressure of natural gas in P mDDD pipeline, MPa; DDDD pipeline conversion diameter, mm; LDDD pipe length, km; hydraulic efficiency of the straight portion of the EDDD pipeline.
It can be seen from equation (2) that in order to reduce energy consumption, in addition to increasing the pressure of natural gas in the pipeline, the temperature of the natural gas in the pipeline must be lowered.
In a physical sense, under a certain pressure, the lower the temperature of natural gas, the higher the density, and the result is a reduction in friction loss, which is equivalent to increasing the actual pipeline output and saving energy. For example, when the temperature of natural gas is reduced from 50 ° C to 55 ° C to 25 ° C ~ 30 ° C, the actual transmission of natural gas pipeline can be increased by 4% ~ 5%.
In addition, from the perspective of operational safety, in order to prevent the hydrate blockage caused by the temperature drop and the bearing deformation and stress overrun caused by the melting of the frozen soil caused by the high temperature of the gas in the north, it is also required to cool the inside of the pipeline. Natural gas requires that the natural gas's water dew point and hydrocarbon dew point must not exceed the minimum ambient temperature.
In short, a natural gas cooler with sufficient capacity should be set up at the gas station to ensure that the gas is operated at the normal temperature with a reasonable gas delivery temperature, and the gas is saved at room temperature to save energy.
3 Pipeline drag reducing coating application The gas pipe drag reducing inner coating can greatly reduce the friction coefficient, increase the gas transmission volume and reduce the transmission power. From the Xinjiang to Shanghai, the 4000km long "West-East Gas Transmission"
The project has adopted the anti-resistance coating for natural gas pipelines for the first time in China, and the Shaanxi-Beijing double line is also preparing to use internal drag reduction coatings. At present, the natural gas pipeline inner coating drag reduction technology has been widely concerned by relevant domestic decision-making departments, scientific research, design and construction units.
Coatings are the material basis for the inner coating of pipes. In general, there are many types of coatings that can be used as anti-resistive coatings in pipes, including epoxy coatings, epoxy polyurethane coatings, epoxy phenolic coatings, and coal tar epoxy. Coatings, etc., but because of the different properties of the various coatings, the most suitable type of internal drag reducing coating should be screened for the different requirements of the pipeline.
We believe that the coatings for drag reduction inner coatings for long-distance natural gas pipelines should have the following characteristics: a) The coating surface is smooth and flat. That is, the surface roughness of the coating is small, so that the coating has a good drag reduction effect. “West-East Gas Transmission†requires that the average surface roughness of the coating is less than 10 m; b) wear resistance and hardness. Because the drag reduction coating is thin, generally only 38 ~ 80m, so the better wear resistance and hardness can ensure that the coating can withstand the wear caused by the gravel and pig in the pipeline; c) adhesion. Good adhesion can ensure that the coating does not fall off during storage, transportation, bending, laying and pigging, and running; d) pressure resistance. Capable of withstanding repeated changes in air pressure and water pressure; e) Easy to apply. It can be applied under normal temperature and humidity conditions by ordinary spraying technology; f) heat resistance. Since the construction of the epoxy powder outside the pipeline requires a high temperature of about 240 °C, if the construction method of "inside and outside" is adopted, the inner coating should be able to withstand short-term high temperature.
It was found through analysis that epoxy resin is the best film-forming binder for the production of drag reducing coatings. With the continuous development of pipeline transportation technology and the different requirements of transportation media, the performance requirements of drag coatings are constantly changing and developing, and it has a diversified development trend. Drag reducing coatings are also developed from a single-function drag reducing coating to a variety of varieties and versatility.
4 The pressure station is optimized to meet the user's requirements for natural gas consumption. It is necessary to set up multiple compressor stations on the whole line. Its reasonable distribution and optimized configuration play an important role in the investment of the entire pipeline system and in the future safe and economic operation.
4.1 Optimized layout of the station 4.1.1 Select the appropriate pressure to pressure ratio, ie the absolute pressure of the compressor station outbound divided by the absolute pressure of the station. Its selection directly affects the number of compressor stations, single station power and total compressor unit. Is an important factor in determining the full investment and operating costs. Under the premise of satisfying a certain design transmission, if a larger pressure ratio is selected, the number of full-line compressor stations is small, and the power requirement of a single station is large. On the contrary, the number of full-line compressor stations is large, the power requirements of single stations are small, and different power levels are selected. In the case of the unit, the efficiency of the unit is different under the designed output. In this way, the gas consumption of the whole line is different when the pipeline is running, and the selection of the unit must also consider the process requirements of each station under the stepped volume. The reasonable pressure ratio should be determined according to the actual conditions of the project and the technical and economic comparison results of the pipeline system. In order to ensure the rationality of the pressure ratio selection, the West-East Gas Pipeline uses multiple pressure ratios for comparison, specifically 1.25, 1.35, 1.40 to 1.5. And 1.60 to 1.70.
4.1.2 Economic comparison of different pressure ratio schemes Hydraulic calculations are carried out at different pressure ratios (using TGNET 7.2) to obtain process parameters in different situations, including station number, single station power requirements, inlet and outlet pressures, and fuel gas consumption. .
According to the hydraulic calculation results, the compressor station is configured. Because different schemes have different configurations, the one-time investment, operation and maintenance costs of the station are also different.
On the basis of the feasible technical solution, the economic benefit optimal plan is given priority. From the above economic analysis, the following conclusions are drawn: the present value of the cost ratio 1.4 to 1.5 is the lowest among the four programs, and the economy is the best, with a pressure ratio of 1.25. Secondly, the ratio of the pressure ratio of 1.6 to 1.7 is the highest, and the economy is the worst. Therefore, the delivery scheme with a pressure ratio of 1.4 to 1.5 is recommended.
4.2. Compressor station unit configuration The unit configuration is an important part of the pipeline gas transmission system. Reasonable configuration plays a major role in the economics of the project and the safe and efficient operation of the pipeline in the future. Generally speaking, the unit configuration mainly includes the following aspect.
4.2.1 Ensure that the pipeline is normally and efficiently transported. In addition to efficient and safe operation under the designed output, the unit should be able to operate safely under different annual output and work in the high efficiency area as much as possible. It is convenient to maintain and reduce the type and quantity of spare parts to be damaged. It is required that the compressor unit of the whole line of compressor stations should choose the same model.
4.2.2 Ensure the safe and stable transportation configuration of the pipeline When the normal working unit fails, the normal transportation of the pipeline system is guaranteed. There are usually two ways: a) The unit is spare. Each compressor station completes a complete set of units. When the normal working unit fails and cannot operate, the unit starts up quickly to ensure stable supply of the system; b) power backup. When designing, the power of a single unit has a certain margin. When a station fails, one or more compressor stations next to the downstream increase the power to compensate, thus ensuring the stability of the pipeline flow. Of course, the power enrichment of the single unit should pass the station failure. Hydraulic calculation analysis to determine.
The design should take into account the above two types of unit configuration, and also consider the impact of the environment on the safety performance of the compressor unit, such as Gobi, desert or no man's land, etc. The final plan also needs to carry out the possible solutions from operation, unit maintenance and investment. Comprehensive comparison can be made.
4.2.3 Selection of unit drive mode There are two main ways to drive the long-distance gas pipeline compressor unit: gas turbine drive and variable frequency motor drive, both of which have their own advantages in practical applications. From a technical point of view, a compressor station with a reliable dual external power supply can meet the needs of the gas transmission process regardless of the driving mode. From the aspect of operation management, motor drive has more advantages than gas turbine drive: a) reliable operation, MTBF (sum of running time/downtime) of motor unit is 4200h, and the gas turbine unit has no fault on average. The time MTBF is 2100h; b) the management is simple, the maintenance workload is small, the maintenance cost is low, the electrical equipment is not subject to mechanical stress and thermal stress like gas generator and power turbine, although the fan, frequency converter, VFD (inverter drive The control devices, transformers and switches need to be maintained. However, after more than three years of operation, the maintenance of these components is very small, and unlike the gas turbines, it needs to be overhauled. c) China's western power resources are abundant, and motor-driven compressors are used. It can save natural gas and save valuable clean natural gas energy to the eastern region where energy is relatively scarce. It is of great significance for the rational use of energy.
Therefore, in the case where the external power is reliable and the electric drive scheme is more economical or similar than the fuel-drive scheme, the electric drive scheme should be preferred.
5 Gas consumption in all aspects of the system Gas turbine units have a large amount of natural gas venting during start-up and shutdown. In the event of an external power failure and a backup power failure, an all-station natural gas venting accident may also occur. The amount of natural gas vented by the unit during start-up and shutdown, including the amount of gas used in the turboexpander and the part of the compressor, and the pulse signal gas used in the process valve, are 40 to 200 m 3 for different types of units. The number of starts and stops of the compressor unit is related to the process requirements and the technical status of the unit. According to Russian experience, the average number of starts and stops of the unit can be 2.
In addition, the purge gas volume of the compressor station is also a kind of loss. Usually, a purge of about 30 to 40 s is required for each shift, and about 240 m 3 of natural gas is lost. The consumption of natural gas in the system is random, which is related to the quality of the manufacturing and installation of the system facilities, the operating years, the current efficiency of the unit and the level of diagnostic maintenance.
6 Conclusions a) The energy consumption per unit of gas transmission is firstly related to the gas transmission pressure. The gas transmission pressure is increased to more than 10 MPa. The use of high-strength steel pipes of X80 or higher is the development trend to improve the economic benefits of natural gas transmission systems. Should be in the material development, pipe manufacturing process, welding technology, pipe section transport, on-site pipe bending, lower trench backfilling and other technologies, safety and environmental protection, supporting the completion of gas pressure pressure upgrade tasks; b) the inner wall of the pipeline adopts drag reduction inner coating technology It is beneficial to the inspection of metallurgical defects on the inner wall of the pipe, which can shorten the drying time of the gas pipe, reduce the deposition of material on the pipe wall, improve the purity of the transport medium, improve the fluid flow performance, reduce pollution, reduce maintenance costs, and slow down corrosion of the inner wall; In the design of the gas station, the main equipment such as high-efficiency gas transmission unit should be selected. The combination of characteristics and quantity must be able to adapt to the change of gas supply volume and cope with the failure of the intermediate station to ensure the operation of the whole system in the best working condition area; d) In the design of natural gas transmission system, efforts should be made to reduce the consumption of fuel gas, starting gas, dust collector and recoil gas to reduce the amount of air leakage and accident venting to achieve energy saving and efficiency enhancement; e) in natural gas transmission system The system fuel gas and other consumption accounted for 12% of the total gas. Energy-saving and efficiency-enhancing measures in design operations are an important means to improve economic efficiency.
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