Technology Monograph on LNG Long-distance Pipeline Transportation Technology Discussion Shaanxi Natural Gas Co., Ltd. Li Lihua and Zeng Xianzhe's consumption are increasing year by year. The LNG transmission range is constantly expanding and the transmission distance is also continuously extending. Feasibility, the establishment of LNG long-distance pipeline is really necessary. This article mainly analyzes the equipment of LNG long-distance pipelines, including the selection of conveying pipelines, conveying pumps and refrigerators; analyzes the technology of LNG long-distance pipeline transportation technologies in terms of pipeline technology, adiabatic selection, conveying technology and pipeline pre-cooling and cold shrinkage Feasibility, with a view to benefiting the development of the LNG industry.
1. Introduction In today's era of increasing energy shortages, worldwide production and trade of liquefied natural gas (LNG) are also increasingly active, and are becoming a new growth point for the world's oil and gas industry. The liquefied natural gas pipeline solves the problem that gaseous natural gas cannot be transported for long distances. Natural gas is liquefied and transported to all parts of the world, which realizes the diversification of energy supply and guarantees energy security. With the realization of energy development and the advancement of transportation technology, China's liquefied natural gas industry has gradually matured. However, the technology of long-distance pipeline transportation needs to be improved.
2LNG long-distance pipeline transportation equipment LNG long-distance pipeline generally uses 9% nickel steel pipe or 304L / 316 material austenitic stainless steel pipe. Although austenite stainless steel has good low temperature performance, its linear expansion coefficient is also very large. The shrinkage when cooled to -160 ° C at normal temperature is only 2.7mm / m. Based on this, in the pipeline Consider using expansion joints, bellows, or copper elbows to compensate for thermal expansion and contraction caused by temperature changes. At the same time, when planning the LNG long-distance pipeline, the insulation requirements of the cryogenic liquid should be taken into account and corresponding insulation protection measures should be taken.
The transportation pipeline of low temperature liquid is generally divided into vacuum insulated pipe, ordinary insulated pipe and non-insulated pipe (bare pipe). The structure of the non-insulated pipe is relatively simple, the cost is low, and the heat capacity is small, but the cooling loss during running is large, and it is mainly used in short-distance, intermittent transmission pipelines. Vacuum insulation pipes can be divided into vacuum multi-layer insulation pipes, vacuum powder insulation pipes and vacuum insulation pipes according to different insulation methods. They are mainly composed of outer pipes, inner pipes and related support parts, and the cost and construction management costs are relatively low.
The delivery pump is the power center of the LNG long-distance pipeline. At present, the delivery pumps of the pipeline include non-submersible pumps and submersible pumps. The non-submersible pump without shaft seal electric pump connects the pump body and the motor housing through a static seal structure, and there will be no shaft seal leakage; the motor wiring generally uses an airtight power connection device, and from The outlet of the device is drained to cool the motor. A large fin should be placed between the motor and the pump to prevent the motor temperature from being too low. The submersible pump installs the motor and pump in a closed metal container without shaft seal, and there will be no problems such as leakage of the shaft seal. The inlet and outlet of the delivery pump should be installed in a sealed metal container with a flange structure and the motor as a whole There is no need for shaft seal, and there is no problem of shaft seal leakage. The inlet and outlet of the pump are generally connected to the delivery pipe using a flange structure. The power cable system must use reliable materials for special design to ensure that the cable can be immersed in low temperature liquefied gas and still maintain elasticity. The electrical terminal shall adopt double-headed sealing structure and ceramic gas sealed terminal, which can withstand the impact of voltage and pressure.
Due to the long pipeline length of LNG long-distance pipelines, natural gas will inevitably rise in temperature during transportation. In order to prevent the two-phase flow caused by the temperature rise, the acceleration of the liquid gasification speed and the increase in the friction resistance cause the overpressure of the transportation pipeline. It is necessary to install low-temperature refrigeration equipment within a certain distance to effectively cool the rising temperature LNG. There are many types of cryogenic refrigeration equipment, including Stirling refrigerators, Gifford-McMahon refrigerators and pulse tube refrigerators. The principle of refrigeration equipment is to use a certain method of pressurization and heat exchange, then throttle or adiabatic expansion to achieve the purpose of cooling. From the current situation, Stirling refrigeration equipment is a small-scale refrigeration equipment that is widely used and has the most mature technology. Stirling refrigeration equipment has developed from the traditional integral type to today's split type, with a variety of driving methods, the cooling capacity can also reach milliwatt level to kilowatt level, and the refrigeration temperature can be quickly achieved from ordinary cold to deep cold.
3LNG long-distance pipeline transportation process LNG long-distance pipeline generally uses single-phase flow transportation method for stable transportation. China is currently mainly designing the LNG long-distance pipeline by the US LNG production, storage and shipment standard NF-PA59A-1994. Select 9% nickel steel as the basic material of the pipeline. The welding of the pipeline mainly uses inert gas shielded welding. After the welding, a comprehensive inspection of the weld seam is required to ensure that the pipeline does not leak.
Since the density of liquefied natural gas will change with the change of pressure and temperature, the average value of the pressure and temperature of the starting point and end point of the pipeline should be selected as the physical parameter of LNG, and it can also be calculated in sections. According to the annual equivalent cost of the pipeline and the relationship curve of the pipe diameter, the minimum value of the intersection point of the curve is the economic pipe diameter under the delivery volume. In this case, the flow velocity in the pipe is the economic flow velocity under a certain pipe transportation volume. When calculating the process of the LNG pipeline, the influence of the temperature and internal pressure of the pipeline on the physical parameters of the LNG should be fully considered, and the operation of the pipeline should be calibrated in time.
The transportation technology of the LNG long-distance pipeline is similar to the crude oil heating transmission technology. The transportation pipeline needs to be equipped with a corresponding cold pump station to cool and pressurize the pipeline. Because the pipeline of the pipeline is too long, the saturated liquid will vaporize to a certain extent after heating, causing the pipeline to have two-phase flow, building the flow of the pipeline, and increasing the resistance of the pipeline. phenomenon. For long-distance pipelines of cryogenic liquids, the pressure of the liquid should be greater than the critical pressure and the liquid temperature of the cooling riser should be ensured during the pressure transmission process to ensure that the temperature of the liquid is lower than the critical temperature to reduce the amount of LNG gasification.
Assuming that the distance between the refrigeration equipment installed in the pipeline is L, L can be calculated according to the transportation temperature and transportation pressure of the liquefied natural gas, omitting the change of potential energy and kinetic energy, then the energy equation of the two refrigeration equipment is: Among them, G represents the flow rate of cryogenic liquid (kg / s); c represents the specific heat of cryogenic liquid (AkgK); Q represents the cold loss of the pipeline between the two stations (W); v represents the specific volume of cryogenic liquid ms123); T2-Tl) and (P1-P2) represent the predetermined temperature difference (Pa) and temperature rise (K) between the two stations, respectively.
From the formula, it can be known that the sum of the cooling loss between the two refrigeration devices and the heat generated by the scrub is equal to the heat absorbed by the liquefied natural gas within this distance. At the same time, since Q and (P1-P2) can represent a function of the length of the pipeline, the reasonable distance L between the two refrigeration devices can be calculated according to the formula. When calculating L, the maximum flow and the minimum flow must be fully considered.
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