Discussion on storage and peak shaving in pipeline natural gas dispatch

Abstract: A large amount of natural gas enters the city, and the imbalance of monthly and seasonal natural gas demand is increasingly affecting the city gas company’s rational user production scheduling with the increase in natural gas consumption. Dispatchers should coordinate the upstream long-distance pipeline and downstream users to make corresponding peak shaving measures.
Keywords: pipeline, unbalanced gas consumption, storage, peak shaving
Introduction: With the successive operation of “West-East Gas Pipeline” and other pipeline projects, a large amount of natural gas has entered the use range of urban users. In view of the characteristics of natural gas safety, environmental protection, and low price, natural gas users have gradually increased, and industrial users have begun to use natural gas in large quantities. However, the gas consumption of various natural gas users is uneven, especially industrial users with large gas consumption. In the production cycle, in order to meet the seasonal, monthly, and daily variations of the gas load, and to meet the needs of industrial gas peaks and valleys, pipeline storage and peak shaving must be considered in natural gas production scheduling.
1. Classification of natural gas pipeline systems At present, the supply of natural gas is mainly developed in the direction of pipeline gas transmission, consisting of long-distance pipeline high-pressure pipeline transmission and distribution systems and urban pipeline transmission and distribution systems. The natural gas pipeline of the “West-East Gas Transmission” project adopts high-pressure gas transmission technology, and the pressure of natural gas supplied to downstream cities has been greatly increased compared with traditional pressure. With the expansion of urban natural gas supply scale and the increase of users, the medium and low pressure transmission and distribution system of the original urban pipeline network can no longer meet the daily peak shaving. Therefore, the urban natural gas transmission and distribution system has been allocated to multiple pressure levels. The development of the gas system will gradually form the first-level high-pressure natural gas pipeline at the periphery of the city, the second-level middle- and low-pressure natural gas pipeline and the high-pressure spherical tank mixed peaking system, which is conducive to meeting the pressure needs of different users, while reducing the internal transmission and distribution pipeline network in the city. The operating pressure of the pipeline network is increased, and the capacity of the pipeline network is increased. At present, the transmission and distribution systems of some large cities in China have tried to adopt multi-level systems to ensure that the uneven gas consumption of users is satisfied.
2. The unevenness of natural gas consumption In the urban gas supply system, the urban gas consumption in the urban gas supply system varies monthly, daily, and hourly with the characteristics of urban civil, industrial and other users. The peaks and valleys vary greatly. In addition, there is a shortage of gas caused by emergencies. In order to solve the contradiction of unbalanced gas consumption, cities must build gas storage facilities. 1. Uneven production cycle The gas consumption of users in the urban natural gas transmission and distribution system will fluctuate with the seasons, the cycle of the industry’s production scale and equipment, and people’s daily habits. Among them, residential gas has the characteristics of stable gas consumption, small fluctuations, and small consumption, which is easy to predict and adjust. As long as a small amount of gas storage facilities are reasonably configured, city gas companies can handle daily and hourly peak shaving by themselves. However, industrial users are different. The gas consumption is relatively large. The gas consumption in the peak production season is often several times that of the off-season, and the general production cycle is difficult to predict. If it is only solved by the city gas system, this requires a lot of investment, and the idle rate is too high. This is related to long-term peak adjustments such as months and quarters, which is the primary consideration in pipeline production scheduling. 2. Unpredictable accidents When the long-distance pipeline supplies gas to the urban natural gas transmission and distribution system, abnormal shutdowns and gas restrictions caused by pipeline, equipment damage and irresistible factors will directly affect the reliability of downstream gas supply Therefore, accidental peak shaving of the gas source needs to be considered.
3. Peak shaving and storage of natural gas 1. Urban natural gas transmission and distribution system In urban natural gas transmission and distribution systems, hourly and daily peak shaving is generally solved by storing a certain amount of natural gas in gas storage facilities. Gas storage facilities can be divided into low-pressure gas storage and high-pressure gas storage according to different gas storage pressures. High-pressure gas storage can be divided into high-pressure spherical tank gas storage, high-pressure pipe bundle or high-pressure pipeline gas storage and underground gas storage. For natural gas, because the supply pressure of the upstream long-distance pipeline is relatively high, in order to make full use of its pressure, high-pressure gas storage is generally used, including pipelines and storage tanks, that is, peak shaving is carried out through the construction of the city’s outer first-level pipeline network . 1.1. Gas storage in high-pressure pipelines Long-distance pipelines have a certain gas storage capacity, which can compensate for peak gas consumption in cities. The gas consumption in the urban natural gas transmission and distribution system is uneven, and it changes over time. When the urban gas consumption is greater than the gas supply, the pipeline pressure drops to make up for the lack of gas supply. When the urban gas consumption is less than the gas supply, the pipeline pressure rises and the excess natural gas is stored. The local selection of the fluctuation range of the start and end pressure of the pipeline and the diameter of the pipeline can make it have a certain capacity for gas storage and peak shaving. Calculation formula for gas storage in high-pressure pipeline: V=(Vg×To)/(Po×T)×(Pm1/Z1-Pm2/Z2) Pm1=2/3×[P1max+P2max2/(P1max+ P2max)] Pm2=2/ 3×[P1min +2/(P1min+ P2min)] where: Vg and T are the geometric volume of the pipe and the average temperature of the gas in the pipe respectively; Z1-refers to the compressibility of the gas at the average pressure Pm1; Z2-the gas is at The compression coefficient at the average pressure Pm2; Pm1——the highest average pressure, that is, the average pressure in the pipeline at the end of gas storage; Pm2——the lowest average pressure, that is, the average pressure in the pipeline at the beginning of gas storage; Pm1——the highest average pressure, that is The average pressure in the pipeline at the end of gas storage; P1max—the highest pressure at the beginning of the pipeline, that is, the start pressure at the end of gas storage; P2max—the highest pressure at the end of the pipeline, that is, the end pressure at the end of gas storage; P1min—the lowest pressure at the beginning of the pipeline, that is The starting point pressure at the beginning of gas storage; P2min——the lowest pressure at the end of the pipeline, that is, the end pressure at the beginning of the gas storage; According to the calculation formula of the above pipeline gas storage capacity, analyze the influence of the pipeline diameter, the length of the pipeline, the starting pressure, and the end pressure on the storage of the high-pressure pipeline. The influence of gas: With the increase of pipeline gas volume, the gas storage volume gradually decreases. This is because the increase in gas transmission volume, the increase in resistance loss, and the decrease in storage pressure difference, so the pipeline gas storage volume gradually decreases. The higher the starting pressure and the larger the caliber, the greater the gas storage capacity. Therefore, increasing the operating pressure of the pipeline can greatly improve the gas storage capacity and gas transmission capacity of the pipeline. 1.2. Gas storage in high-pressure storage tanks The gas consumption in the urban natural gas transmission and distribution system changes over time. When the urban gas consumption is greater than the gas supply, the high-pressure storage tank is used to make up for the lack of gas supply. When the urban gas consumption is less than the gas supply, the high-pressure storage tank stores the excess natural gas. Properly determining the inlet and outlet pressure and geometric volume of the high-pressure storage tank can enable the city gas system itself to have a certain capacity for gas storage and peak shaving. Calculation formula for high-pressure storage tank gas storage: V=Vc(P-Pc)/Po Parameter description: V——effective gas storage volume of the gas storage tank (m3); Vc——geometric volume of the gas storage tank (m3); P ——Maximum working pressure (MPa); Pc——Minimum allowable pressure of gas storage tank (MPa); P0——Atmospheric pressure (MPa). 2. Comparison of high-pressure pipeline gas storage and high-pressure spherical tank gas storage Because the above-ground storage tanks need to occupy urban land, the infrastructure cost per unit reserve is relatively high compared with other gas storage methods. Therefore, in large foreign cities, especially the need for storage Cities with a large amount of gas have gradually replaced them with other methods, such as underground gas storage and pipeline gas storage. At present, the maximum volume of domestically manufactured high-pressure spherical tanks is 5,000 cubic meters. If imported steel is available, 10,000 cubic meters of spherical tanks can be processed. The operating pressure of the natural gas pipeline has an initial pressure of 2.0Mpa and an output pressure of 1.0Mpa. The DN1000 pipeline has a length of 60 kilometers and a buffer storage capacity of about 40W. The geometric volume of the high-pressure spherical tank is 1000 cubic meters, the inlet pressure is 1.6Mpa, and the outlet pressure is 0.8Mpa. The gas storage capacity is about 8,000 cubic meters. If it is used in the urban transmission and distribution pipeline network, it can be reduced to 0.2Mpa, that is, its peak shaving range is about 1.4W square.
4. Necessity and feasibility of peak shaving of pipelines The relationship between the urban natural gas transmission and distribution system and the supply of gas sources, long-distance pipelines and urban pipeline networks is mainly manifested in the gas supply conditions and peak shaving of urban gate stations. The high-pressure pipelines used in long-distance pipelines can participate in urban daily and hourly peak shaving, and reasonable allocation and scheduling can implement peak shaving for quarterly and monthly gas consumption. 1. The peak-shaving capability of long-distance pipelines In the gas transmission and distribution system, because users’ gas consumption fluctuates all the time, it is necessary to reasonably configure gas storage and peak-shaving facilities to ensure uninterrupted and stable gas supply to users and ensure the company and users’ Normal production and operation. If there are certain storage facilities for the daily and hourly peak shaving in the urban gas transmission and distribution system, the city gas company can solve it by itself, and long-distance pipelines do not participate in solving the short-term peak shaving problem of urban gas supply. Generally, a city with a daily consumption of no more than 100,000 Nm3 can be solved by building one or two 100m3 high-pressure storage tanks, but for cities with a consumption of more than one million, larger storage facilities need to be established. For example, Tianjin Dagang Oilfield uses the oil and gas layer structure of the oilfield to build underground gas storage. Carry out natural gas pressurized reverse injection, oil and gas storage, used to solve the seasonal peak shaving of urban gas supply. At present, cities that realize natural gas supply in China can only solve the daily and hourly peak regulation of urban gas consumption by building a large number of high-pressure spherical tanks. Seasonal adjustment can only be adjusted by long-distance pipelines and the production cycle of large industrial users. The seasonal peak shaving problem is limited by conditions (funds, environment, etc.) in most cities and cannot solve it on their own. Therefore, in the market economy situation, long-distance pipelines should be relied on, and the connection between upstream and downstream supply and extraction should be properly handled and fully utilized. The ability of long-distance high-pressure pipelines realizes the reasonable allocation of upstream, middle and downstream resources, and jointly solves the urban gas supply pressure and peak shaving. 2. Long-distance pipelines participate in the daily and hourly peak shaving of cities. Natural gas supply needs to form a pipeline network for reasonable dispatch and form a continuous, stable and safe gas supply guarantee. The forecast of natural gas supply and demand shows that, in addition to the development and utilization of domestic natural gas resources, my country needs to import 40 billion cubic meters of pipeline natural gas around 2010. Imports will increase substantially by 2020. To this end, it is necessary to build gas transmission trunk lines and build a large number of branch-line gas supply pipeline networks. According to the planning and construction plan of the national natural gas transmission pipeline network, the country’s natural gas supply will eventually become regionalized and networked. Long-distance transmission trunk lines can not only solve downstream cities The problem of unbalanced seasonal gas consumption, and it provides the possibility for long-distance pipelines to participate in urban daily and hourly peak shaving. 3. With the expansion of urban gas supply scale and the increase of various users, transmission and distribution systems often need to adopt three-stage systems (high pressure, medium pressure, low pressure) and multi-stage systems (ultra high pressure, high pressure, medium pressure, low pressure). fulfil requirements. The adoption of a three-level pressure rating system is conducive to meeting the pressure needs of different users, reducing the operating pressure of the city’s internal transmission and distribution pipeline network, increasing the capacity of the pipeline network to dispatch gas, and improving the economy of gas storage. The incoming air pressure is generally not less than 1.0 MPa. For multi-stage systems, the incoming air pressure is generally not less than 2.5 MPa, and can even be as high as 5.0 MPa or more. At present, the transmission and distribution systems of some large cities in foreign countries generally adopt multi-level systems, and the ultra-high pressure urban outer ring pipeline network is constructed as a means to meet the needs of flow transmission and partial gas storage. Moreover, the pressure level difference between the ultra-high pressure outer ring and the pressure level of the high-pressure pipe network is generally large.