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Since the 1980s, liquefied petroleum gas has seen rapid development as a source of energy in our country. According to statistics, the consumption of liquefied petroleum gas increased by 24% on an average annually from 1980 to 1990. By 1998, the national consumption of liquefied petroleum gas reached 10.561 million tons; of this amount, 5.805 million tons was produced domestically while 4.766 million tons was imported from abroad. The supply of liquefied gas has thus adopted a market economy model characterized by multiple sources of supply. A considerable number of cities, especially small and medium-sized coastal cities, widely use liquefied gas vaporized or mixed with air as a city gas source, which is delivered via pipelines to various users. In the 21st century, there will be significant changes in China’s energy structure. The goal is to accelerate the exploration and development of domestic oil and gas resources, actively import natural gas from neighboring countries (such as Russia and other CIS countries**), and import LNG in appropriate amounts (by establishing LNG receiving facilities along the southeastern coast). It is estimated that in 2010, China’s natural gas production will reach 50 billion cubic meters, while LNG imports will amount to 5 million tons. Stable, safe, reliable, and clean natural gas will gradually become the primary source of city gas, and the use of natural gas is set to be the trend in the development of city gas. Cities that currently rely on centralized pipeline supply using liquefied gas that has been vaporized or mixed with air will gradually switch to using natural gas. This article introduces the conversion technologies between natural gas and pipeline liquefied gas as a reference for decision-making. 1. Centralized pipeline supply of liquefied gas: The centralized pipeline supply method for liquefied gas mainly includes two approaches: the gasification of liquefied gas and its mixing with air through centralized pipelines. 1.1 Purpose and Scale The centralized pipeline supply system for liquefied gas vaporization is mainly used for gas supply in regional areas and urban residential complexes. The service radius of gas supply from a gasification station is generally 2 kilometers, serving approximately 10,000 residential customers. In areas with high population density, the number of households connected to gas supply has increased significantly; for example, the **Central Gasification Station and Shenzhen Luohu Gasification Station serve more than 20,000 households each, while the Foshan Huanhu Gasification Station serves over 40,000 households. The centralized pipeline supply system for liquefied gas mixed with air is primarily used as a gas supply source for medium and small-scale applications in cities; it serves as a substitute for artificial gas and as a flexible source for peak shaving, as well as a substitute for natural gas during transitional periods, for peak shaving, or in emergency situations. It is also used as a gas supply source in cold regions where the properties of liquefied gas make direct vaporization impractical. The gas supply capacity of mixing stations varies; large mixing plants in China can produce up to 500,000 cubic meters of gas per day. 1. The ratio of liquefied gas to air in the mixture: Depending on the intended use of the mixture, the proportion of liquefied gas to air varies, but it must comply with the requirement specified in clause 6.4.12 of GB50028—93, the \"Code for Design of Urban Gas Systems\", which states that in a mixture of liquefied petroleum gas and air, the volume percentage of liquefied gas must be more than 1.5 times its upper explosive limit. Currently, the ratio of liquefied gas to air in the mixture is as follows: (see Table 1). Table 1 Ratio of L.P.G. to Air in the mixture Application Lower calorific value of the mixture (MJ/M3) L.P.G. % Air % Substitute artificial gas (equivalent to 5R reference gas) 1585 1682 5752 9.5 Substitute natural gas (equivalent to 10T, 12T reference gas) 4060 4524 6545 1755 4556 21.3 Distribution methods and pressure levels Depending on the supply method, gas quality, scale, and region, the distribution methods and pressure levels for liquefied gas supplied through centralized vaporization pipelines generally adopt a two-stage or three-stage system ； For the centralized pipeline supply system of liquefied gas mixed with air, a two-stage system is generally used. The gas transmission pressures are as follows: high pressure (B): 0.4 < P ≤ 0.8 Mpa; medium pressure (A): 0.2 < P ≤ 0.4 Mpa; medium pressure (B): 0.005 < P ≤ 0.2 Mpa; low pressure: P ≤ 0.005 Mpa. 1.3.1 There are several types of secondary systems. 2.1.1 Underground storage: It is commonly stored in porous underground geological formations; it can also be stored in the cracks of salt-bearing rock layers, abandoned mines (mining tunnels), or other artificially created cavities. The methods used for storing natural gas include pressurization or cooling. 2.1.2 Low-temperature storage: Natural gas is frozen into a liquid state for storage, and an LNG vaporization unit is installed to regulate gas supply during peak periods. 2.1.3 Storage in gas holders and spherical tanks: High-pressure natural gas spherical tanks are commonly used, or low-pressure storage is achieved using gas holders for artificial gas. 2.1.4 Storage in high-pressure pipelines: Utilization of large-scale buried high-pressure pipe bundles, as well as long-distance and high-pressure gas transmission mainlines. 2.1.5 Production of alternative natural gas for peak shaving and emergency use mainly involves production methods such as blending liquefied gas with air, producing gas from naphtha, and LNG gasification. 2.2 Natural gas transmission and distribution system: Long-distance natural gas pipelines use ultra-high pressure. Urban natural gas distribution systems generally adopt a three-pressure grade system: high pressure (A) of 0.8 < P ≤ 1.6 Mpa, high pressure (B) of 0.4 < P ≤ 0.8 Mpa, medium pressure (A) of 0.2 < P ≤ 0.4 Mpa, medium pressure (B) of 0.005 < P ≤ 0.2 Mpa, and low pressure of P < 0.005 Mpa. For gas supply to users, medium-pressure delivery to the premises or low-pressure delivery after regional pressure regulation can be chosen. The rated pressure of the burner is 2000 Pa. 3. Conversion between natural gas and pipeline liquefied gas In summary, the conversion between natural gas and pipeline liquefied gas is a systematic task that requires taking into account various aspects such as transmission, storage, and usage; it is necessary to select optimization solutions that are forward-thinking, practical, and advanced. The following points should be considered during the conversion. 3.1 Interchangeability of gases: When selecting a solution, it is necessary to first study and analyze whether the gas currently in use can be replaced with natural gas. Typically, the interchangeability of gases is determined using the combustion characteristic known as the calorific value (heat load index) W. When switching between the two types of gas, the change in W should not exceed ±5–10%. When the combustion characteristics of the two differ significantly, flame characteristics such as flame separation, backfire, yellow flame, and incomplete combustion should also be taken into account when calculating the combustion potential Cp of the gas. To determine the interchange range and feasibility of swapping gases with different W and Cp values on burners, the combustion characteristic values (W and Cp) for various types of gases are kept within certain controlled ranges of variation. At the same time, it is possible to decide whether to retain or modify the existing gas supply system. For systems that use liquefied gas vaporization pipelines for centralized supply, since the W and CF values of liquefied gas and natural gas differ significantly (see Table 2), the two cannot be used interchangeably; the existing gas source must be discarded and the cooking appliances need to be replaced. In systems that use liquefied gas mixed with air and supplied through pipelines, if the technical parameters of the gas supply are similar to those of natural gas or adjusted accordingly, and if W and Cp remain within the specified ranges of variation (see Table 2), then the two types of gas can be used together, and the cooking appliances can adapt to them. 3.2 Transformation of the distribution system: When formulating and implementing natural gas supply plans, it is necessary to analyze the existing distribution system to determine whether it is suitable or requires transformation. Liquefied gas vaporization stations, as well as medium and small-scale mixing stations, should not be retained; the existing distribution networks can be used for natural gas supply in residential areas and neighborhoods. Those with smaller diameters need to be modified. Large mixing stations, including interconnected distribution networks, should be made full use of. Old pipelines (especially those made of cast iron or with socket joints) can be upgraded using methods such as installing PE pipes or U-shaped PE pipes inside them, as well as using steel pipes, in order to increase the operating pressure of the gas distribution network and reduce costs. The existing pressure regulation, valve, and distribution auxiliary facilities should be replaced and modified in accordance with the design and operational requirements of natural gas. For the existing pipes made of galvanized steel pipes with ducts or threads, the sealing material of the threads should be checked. For those where natural gas dry gas is not applicable, technical measures to prevent leaks must be taken. The replacement of natural gas users and the renovation of existing distribution systems require a seamless, coordinated, and synchronized integration. In cities with multiple gasification or mixing stations, transitioning to natural gas cannot be achieved simply by connecting individual neighborhoods together. Therefore, before carrying out the natural gas conversion, the gas supply area should be divided into several renovation zones, with the conversion being carried out in phases to ensure the safety and continuity of gas supply, and appropriate emergency measures should be established. For civilian natural gas metering, G2.5 diaphragm gas meters are generally used; those in use that have a capacity that is too large or that utilize sheepskin diaphragms should be scheduled for replacement. The same should apply to business associations and industrial users. 3.3 Storage and Peak Shaving: A pipeline network for LPG supply is utilized; storage of gas and peak shaving are achieved mainly by adjusting the number of LPG vaporizers in operation as well as the gas delivery pressure, with no dedicated storage facilities being used. When switching to natural gas, in addition to the upstream natural gas peak-shaving facilities required, the peak-shaving at the monthly, weekly, daily, and hourly levels must be taken into account by the gas sales system. Depending on the scale of gas supply, high-pressure pipelines, tube bundles, spherical tanks for gas storage, or the use of blending stations to produce substitute natural gas can generally be employed. Selecting among these options requires comprehensive economic analysis and comparison, in addition to ensuring compliance with design and operational requirements. 3.4 Establishing reasonable natural gas sales prices: The costs of liquefying or blending liquefied gas vary significantly from place to place, as do their sales prices. When implementing the transition to natural gas, it is necessary to establish price guidelines and reasonable price ratios among different types of energy sources, taking into account factors such as energy complementarity, substitutability, and quality differences. Economic levers are also utilized to determine the natural gas sales price, taking into account corporate profitability, users’ affordability, as well as investment returns and profit margins. If the original gas sales price is low and the adjustment range is large, it is difficult to make the adjustment in one go; at the very least, a slight profit to cover costs should be achieved. 4. Conclusion The conversion of natural gas to pipeline liquefied gas (by gasification or blending) is a relatively complex systems engineering task that requires the formulation of long-term and short-term development plans for natural gas, to be implemented in phases. Optimize various options such as the development of the natural gas market and demand, as well as the utilization and renovation of existing gas production and distribution facilities, formulate corresponding countermeasures, and properly address various technical challenges encountered during the transition. To meet the requirements of full utilization, cost savings, safe operation, and rational use, thereby achieving social, economic, and environmental benefits.