Research on Several Key Issues in the Engineering Design of Ultra-High Pressure Natural Gas Pipelines in Towns

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1 Introduction In Beijing’s project to introduce natural gas from Shaanxi, Gansu, and Ningxia into the city, the design pressure of the Dn700 natural gas pipeline along Highway Ring Road 1 has reached 2.5 MPa. The design of ultra-high pressure gas pipelines (>1.6 MPa) differs from that of ordinary high-pressure gas pipelines. The Code for Design of Town Gas stipulates that when installing gas pipelines with a design pressure greater than 1.6 MPa, the design shall be carried out in accordance with the current Code for Design of Gas Pipeline Engineering. The \"Code for Design of Gas Pipeline Engineering\" focuses primarily on long-distance pipelines from oil and gas fields to their destinations, and the plants and stations mentioned in it are mainly those related to the construction of oil and gas field facilities; it is not applicable to the pipelines and plants/stations used in urban gas distribution systems. Therefore, when laying ultra-high-pressure gas pipelines in towns, it becomes a practical issue as to how to implement the \"Code for Design of Town Gas\" and the \"Code for Design of Gas Pipeline Engineering\" and integrate them organically. With the development of the domestic natural gas industry, the design scale of urban natural gas distribution systems is also increasing. To ensure the economic viability and feasibility of such projects, it is necessary to raise the design pressure of the distribution systems. To date, the ultra-high-pressure natural gas pipelines designed by our institute and already installed in Beijing total nearly 200 km, with several dozen more kilometers of such pipelines currently under installation. Below, based on the design and implementation of the Dn700 natural gas pipeline project for the first highway ring in our institute, several key issues in the design of ultra-high pressure natural gas pipelines in urban areas are studied and summarized, in order to further improve the quality of such projects. 2 Project Overview: The urban natural gas project introduced in Beijing from the Shaanxi-Gansu-Ningxia region is the largest downstream project within the natural gas transmission network of the Shaanxi-Gansu-Ningxia basin. It is also the largest urban natural gas distribution project in China at present. The main components of the project include one city gate station, three storage and distribution stations, five pressure regulation stations, over 200 kilometers of transmission and distribution main lines, a transmission and distribution dispatch automation system, as well as supporting production facilities. Between 1991 and 1994, the project carried out its pre-feasibility and feasibility studies; in 1994, preliminary design work began on the ten sub-projects, and in 1995, detailed design work for each sub-project started. To raise funds, Beijing’s **included this project as a sub-project under the “Beijing Environmental Improvement Project” in 1993 and applied for a loan from the Asian Development Bank, which was approved by the bank at the beginning of 1994. The high-pressure pipeline project (third sub-item) is one of the main projects in the downstream section of the Shaanxi-Gansu-Ningxia natural gas project leading to Beijing. The ultra-high pressure natural gas pipeline runs roughly along the highway ring road, is 45 km long, has a diameter of Dn700, and a design pressure of 2.5 MPa. Along the project route, it crosses 5 railways, 1 subway line, 11 existing and planned interchanges, 15 rivers, and 1 rocky hillside. The total investment for the project (including some 1.OMPa pipelines and related works) amounts to 640.69 million RMB (of which 1.2496 million USD), of which the cost of the project itself is 218.21 million RMB. 3 Research on Issues (1) Selection of design specifications and design principles: The design pressure for the natural gas pipeline in the road ring road is 2.5 MPa, which exceeds the transmission pressure range of 1.6 MPa specified for high-pressure Class A pipelines in the \"Code for Design of Urban Gas Systems\". In accordance with the Code for Design of Urban Gas, the current Code for Design of Gas Pipeline Engineering shall be adopted. The \"Design Code for Gas Transmission Pipelines\" is based on the principle of ensuring the safety of the pipelines themselves, and this principle does not exactly coincide with that of the \"Design Code for Urban Gas.\" However, the explanatory notes in both design codes state that long-term practical experience, as well as statistical data on explosions and poisoning incidents caused by gas pipeline leaks, show that the occurrence of gas accidents is not necessarily related to the distance between the gas pipelines and buildings within a certain range. Increasing the distance between pipelines and buildings does not completely prevent accidents; on the contrary, it increases the difficulty in selecting pipeline locations during design or raises project costs. Therefore, the horizontal clear distance between underground gas pipelines and buildings or structures specified in the \"Code for Design of Urban Gas Systems\" takes into account the spacing required for construction and maintenance, as well as the influence of gas transmission pressure; revisions to the code also tend to reduce the emphasis on the impact of gas transmission pressure. The \"Code for Design of Gas Pipeline Engineering\" is primarily aimed at long-distance main pipelines used to deliver gas to cities. This standard adopts improving the strength and safety of the pipeline itself as the design principle for gas transmission pipelines. It refers to the American standard ANSI B31.8, uses regional classification to determine the strength design factors, and then carries out calculations for pipeline strength. The standard does not specify a specific horizontal clearance between the pipeline and buildings or structures. In the design, this is specifically implemented by using different strength design coefficients corresponding to various regional classifications for pipeline strength calculations, in order to ensure the safety of structures and buildings surrounding the pipelines. The regional grade is determined based on the number of households in various 2-km long sections arbitrarily divided within a range of 200 meters on each side of the pipeline’s centerline. Grade 1 area: Segments with 15 households or fewer ； Grade 2 areas: Zones with between 15 and 100 households. Grade 3 areas: Zones with 100 or more households, including suburban residential areas, commercial areas, industrial areas, development zones, as well as densely populated areas that do not meet the criteria for Grade 4 areas ； Grade 4 areas: refer to locations where four-story or higher buildings (excluding the number of basement floors) are commonly found, traffic is frequent, and there are many underground facilities. Urban gas supply areas should generally be classified as level four areas. The design intensity coefficients for various regions are shown in the table below. Regional grade intensity design coefficient (F): 0.72 for Grade 1 regions, 0.6 for Grade 2 regions, 0.5 for Grade 3 regions, and 0.4 for Grade 4 regions. Urban gas distribution systems must not only ensure the overall rationality of the system but also meet the requirements of relevant design standards. Although the \"Design Code for Gas Pipeline Engineering\" does not specify the horizontal clear distance between pipelines and buildings or structures, this does not mean that such a distance is not required; at the very least, the distance necessary for construction and maintenance should be taken into account. At the same time, pipes at a higher pressure level should never be located closer to buildings and structures than those at a lower pressure level; the impact of the gas transmission pressure must be taken into account. Therefore, after thorough deliberation and discussion with the planning authorities, fire safety departments, and relevant higher-level regulatory bodies, our institute has decided to use the principle outlined in the \"Design Code for Gas Transmission Pipelines\", which emphasizes controlling the safety of the pipelines themselves, for conducting strength calculations. To ensure consistent application of these standards, the selection of pipeline routes is carried out in accordance with Article 5.3.2 of the \"Design Code for Urban Gas\", which specifies a horizontal clearance of 6 meters between high-pressure Class A pipelines and buildings or structures. By using these two sets of specifications together, it not only makes it possible to select routes for ultra-high pressure natural gas pipelines in urban areas, but also, by adhering to the principle of \"prioritizing the safety of the pipelines themselves,\" it ensures the safety and reliability of laying such pipelines in urban zones from a design perspective. At the same time, it set a precedent for laying ultra-high-pressure natural gas pipelines in urban areas, serving as a model for similar engineering designs in the future. (2) Selection of pipe materials ① Pipeline strength calculation: When designing ultra-high pressure gas pipelines, the determination of the horizontal clear distance between the pipeline and other buildings and structures is always based on the need to ensure the safety of the pipeline itself. Therefore, when designing gas pipelines with a pressure greater than 1.6 MPa, it is necessary to calculate the wall thickness of the pipeline, elbows, and bends. According to Article 5.1.2 of the \"Design Code for Gas Pipeline Engineering,\" the formula for calculating pipeline strength is as follows:　　　　　　　PD 　　δ= ———— 　　　　　　　2σsψFt Where: δ – calculated wall thickness of the steel pipe (cm) ； 　　　　P —— Design pressure (MPa) ； 　　　　D —— Outer diameter of steel pipe (cm) ； 　　　　σs —— Minimum yield strength of the steel pipe (MPa) ； 　　　　F--—Strength design factor ； 　　　　ψ——- weld coefficient ； 　　　　t———Temperature reduction factor ； When the temperature is less than 120°C, the value of t is 1.0. In previous calculations of gas pipeline strength, due to the lack of strict requirements regarding pipe manufacturing standards and welding techniques, a welding coefficient less than 1 had to be taken into account in such calculations to ensure safe gas transmission; this actually increased the amount of steel used in pipeline construction. At present, China’s pipe manufacturing technology has seen significant improvement. New steel pipe standards, such as those for spiral-welded submerged-arc welded steel pipes used in oil and gas transmission pipelines, are formulated with reference to the American APISpec 5L standard, and their technical requirements are essentially identical. The \"Code for Design of Gas Pipeline Engineering\" also sets strict requirements for pipeline construction, welding, and inspection to ensure the safe operation of the pipelines. Therefore, when performing strength calculations for pipelines, the design stress of the steel reduced due to welding is no longer taken into account, and a welding factor of 1 is specified. Furthermore, the \"Design Code for Gas Pipeline Engineering\" stipulates that when calculating the strength of pipelines, no corrosion margin is taken into account for the pipe wall. This is because the specifications clearly state that the anti-corrosion design of gas transmission pipelines must comply with the relevant provisions of the **current standards, namely the ‘Design Code for Anti-Corrosion of Steel Pipelines and Tanks’ and the ‘Design Code for Forced Current Cathodic Protection of Buried Steel Pipelines’. These two specifications were developed based on domestic and international practical experience, and they propose effective methods to prevent external corrosion of pipelines. When transporting natural gas that meets specification requirements, corrosion generally does not occur on the inner wall of the pipes. At the same time, due to economic factors such as project costs and metal consumption, it is generally not permissible to use the method of increasing corrosion allowance to address internal corrosion in pipe walls. Therefore, after applying anti-corrosion measures to the pipeline, the corrosion margin can be disregarded when determining the wall thickness. Calculations were performed on several commonly used pipe materials, with the results shown in the table below: For steel grades Q235B, 20, SM41B, X42, 16Mn, and X52, the minimum yield strength is ≥235 MPa, ≥245 MPa, ≥245 MPa, ≥290 MPa, ≥340 MPa, and ≥358 MPa respectively. The design pressure is 2.5 MPa for all of them. The calculated wall thicknesses of the pipes are as follows: for DN700, it is 0.957 cm; for DN500, it is 0.918 cm; for DN400, it is 0.776 cm; for DN300, it is 0.662 cm. The values for DN700 are 0.628 cm. Note: The calculated wall thickness values in the table are obtained under the conditions where ψ and t are set to 1, and F is set to 0.4. While performing strength calculations on the pipeline, its wall thickness must also be no less than the minimum nominal wall thickness. The wall thickness calculated under low internal pressures may be quite thin; to meet the requirements for transportation, lifting, pipeline laying, and maintenance, it must also be verified under various loading conditions. It is generally believed that instability of the circular cross-section occurs only when D/δ > 140 under normal conditions of transportation, laying, and pipe burial. The table below lists the minimum nominal wall thicknesses for several common pipe diameters. Minimum nominal wall thickness: Nominal diameter of steel pipe (mm) – Nominal wall thickness (mm): DN300: 4.5; DN350, DN400, DN450: 5.0; DN500, DN550: 6.0; DN600, DN650, DN700: 6.5. In summary, when determining the wall thickness of pipes, it is necessary to take into account not only the calculations related to pipe strength but also the minimum nominal wall thickness. Additionally, factors such as the skill level of personnel involved in pipe manufacturing, transportation, and installation, as well as the current level of management, which may cause damage to the pipes, should be considered, resulting in an appropriate increase in the pipe wall thickness. ②Principles for selecting pipe materials ★ Technical principles: In accordance with the \"Design Code for Gas Transmission Pipelines,\" the steel pipes to be used must comply with the relevant provisions of current standards such as those for \"Spiral-welded submerged-arc welded steel pipes for oil and gas transmission pipelines.\" If pipes other than those specified in these standards are used, they must be made of killed steel and must meet the following basic requirements: — The ratio of yield strength to tensile strength should not exceed 0.85 ； 　　——The carbon content should not exceed 0.25%, and the carbon equivalent should not exceed 0.45% ； 　　——The sulfur content in the material melt analysis should not exceed 0.035% ； The phosphorus content should not exceed 0.04%. ★ Economic principles: For pipes of the same diameter, the transportation and installation costs are roughly identical; the price of welding materials has little impact on the project cost. In terms of economics, only the price of the pipes is taken into consideration. ★ Principles for construction and operation management: The selection of piping materials should facilitate aspects such as material ordering, welding during construction, wiring under pressure, emergency repairs, and operation management. ⑧The selection of pipe materials is determined based on the aforementioned principles for material choice and pipeline strength calculations, ensuring compliance with technical standards while also facilitating construction and operation management. Economic considerations are also taken into account. The following table provides a comparison of several commonly used materials for DN700 pipes:

| Pipe Material | Q235 | BSM41 | BX42 | 16Mn |
|----------------|------|-------|------|------|
| Unit Price of Pipe (RMB/t) | 40 | 70 | 413 | 413 | 4300 |
| Welding Requirements | Low | Medium | High | Very High |
| Design Initial Wall Thickness (cm) | 1.0 | 3.0 | 5.0 | 7.9 | 7.9 |
| Weight per Meter of Pipe (Kg/m) | 177.98 | 164.34 | 136.97 | 136.97 |
| Cost per Meter (RMB/m) = A/1000 * B | 724.46 | 78.75 | 65.75 | 89.0 |

Note: 1. The wall thickness values follow the APISpec 5L standard series. 2. When determining the preliminary design wall thickness, calculate the wall thickness by increasing it up to the minimum wall thickness for standard weight pipeline pipes. 3. The unit prices in the table represent the ex-factory prices provided by a pipe manufacturer during the preliminary design phase of this project, in accordance with SY5036—83. When pipes are supplied in compliance with API standards, an additional cost of 1,000 to 1,500 yuan per ton is applied. Handling and miscellaneous fees are increased by 10%. From the comparative analysis of the above piping materials, it can be seen that for DN700 steel pipes, choosing a material with a lower minimum yield strength results in thicker walls, which not only increases the weight and cost of the pipes but also poses various difficulties in transportation and installation. If a material with a higher minimum yield strength is selected, the wall thickness can be reduced; however, due to limitations such as the minimum nominal wall thickness and construction requirements, the designed wall thickness tends to be larger. Moreover, the unit price of high-strength pipes is higher than that of low-strength pipes, which inevitably leads to wasted strength and increased costs. In addition, high-strength pipes require higher standards in terms of welding during installation. Therefore, selecting X42 for the DN700 pipeline is the best option. Similarly, the selection of pipes with other diameters can also be determined using the aforementioned comparative analysis method. ④Summary: This project is the first in China’s urban gas distribution systems to use APISpec 5LX42 pipes. The selection of this pipe material was the result of a careful evaluation, based on the four types of pipe materials guaranteed to be available by the former Beijing Natural Gas Company. Technical analysis and economic comparisons were conducted, taking into account various factors such as the cost of purchasing the pipes, welding quality, as well as ease of construction and operation management. Using pipes with higher strength not only solves the problem of selecting materials for ultra-high pressure natural gas pipelines in cities and towns but also saves on project costs, achieving multiple benefits at once. (3) Corrosion and electrical protection systems: Ultra-high pressure natural gas pipelines operate at high pressures and have large transportation capacities; in the event of a leak, the consequences can be severe. Therefore, ensuring the safe and reliable operation of pipelines is a fundamental design principle. The \"Design Code for Gas Pipeline Engineering\" is based on strict control over the quality of pipeline materials, corrosion prevention measures, as well as high standards for the construction, welding, and inspection of pipelines, in order to ensure their safe and reliable operation. Therefore, the anti-corrosion measures and quality for pipelines are among the key factors in ensuring the quality of pipeline projects as well as the safe and reliable operation of these pipelines. The use of an external anti-corrosion coating and electrical protection for buried pipelines is an effective way to extend their service life and reduce operational failures. In the early 1970s, following the first legislation enacted in the United States, various other laws were passed requiring that buried pipelines be protected by both anti-corrosion coatings and cathodic protection. An anti-corrosion coating provides surface protection for the outer wall of buried pipelines, primarily targeting uniform corrosion, while cathodic protection mainly offers point protection, addressing the areas where the anti-corrosion coating has been damaged. A single pipeline can become completely unusable due to pitting, preventing it from functioning properly. Over the past decade or so, there has been increasing attention in China to the issue of dual protection for buried pipelines. Various specialized seminars have been held across the country to discuss corrosion and protection issues related to buried pipelines, and in-depth discussions have taken place regarding the necessity and feasibility of cathodic protection for such pipelines. The pipeline protection scheme for this high-pressure pipeline project adopts a dual protection approach, consisting of external anti-corrosion treatment via single-layer fused epoxy powder coating and cathodic protection using sacrificial anodes. ①External anti-corrosion coatings: Over the past decade, the external anti-corrosion coatings used for buried gas pipelines in Beijing have mainly included the following types: a) petroleum asphalt + glass cloth; b) epoxy coal tar pitch + glass cloth; c) plasticized asphalt anti-corrosion strips; d) inorganic zinc-rich coating + epoxy coal tar pitch + glass cloth; e) epoxy powder spraying (+ polyethylene adhesive tape). Petroleum asphalt, epoxy coal tar pitch, and plasticized asphalt anti-corrosion strips are primarily used for the anti-corrosion protection of medium-pressure main pipelines in urban areas as well as low- and medium-pressure gas pipelines in residential complexes. Epoxy coal tar pitch outperforms petroleum asphalt in preventing bacterial corrosion and plant roots, causes less environmental pollution, and is therefore more widely used. However, due to limitations in operating temperature and curing time, construction in winter presents many difficulties. Plasticized asphalt anti-corrosion strips have only begun to be used on gas pipelines in Beijing in recent years, and their price is relatively high. However, as their use becomes more widespread, their price will decrease somewhat. Moreover, their installation is less affected by environmental temperature, making them one of the commonly used anti-corrosion methods. Inorganic zinc-rich compound + epoxy coal tar pitch entered the field of gas engineering applications with the Beijing natural gas double-track project in the North China Oilfield. Mainly used for 1.0MPa high-pressure pipelines. The reason was that cathodic protection for gas pipelines was given attention at that time; however, implementing pipeline cathodic protection required the installation of a certain number of monitoring piles. It was difficult to place such monitoring piles (above or below ground) on urban traffic roads. Moreover, due to the limited theoretical knowledge of cathodic protection among the engineering technicians at that time and the tight project deadlines, an inorganic zinc-rich primer was used as a measure for dual protection of the pipelines. Furthermore, under the conditions at that time, doubts regarding whether cathodic protection methods using sacrificial anodes or external current could achieve the desired protective effects in terms of operational management were also one of the reasons for adopting inorganic zinc-rich primers. In fact, the use of inorganic zinc-rich coatings as a corrosion protection measure for metals has its specific application environments, conditions, and limitations. For the long-term protection of buried pipelines, especially as part of a dual protection system, inorganic zinc-rich coatings are not a good choice. The single-layer sintered epoxy powder spraying anti-corrosion technique is one of the highly effective anti-corrosion methods recognized internationally today. This technology was first used in Beijing’s LPG three-pipeline system in the early 1990s, in both buried and overhead configurations, and its performance has been good over the years of operation. This anti-corrosion method relies on mechanical and semi-automated assembly line processes; the raw materials and manufacturing procedures are easy to control. It features a complete set of pipeline accessories, as well as procedures for repairing leaks and damages, alongside a thorough quality assurance system. Its performance metrics are far superior to those of other common methods, and its advantages become even more evident when double protection is used. Additionally, its cost is reasonable. After careful analysis, as well as comparison and discussion with conventional external anti-corrosion methods, we have decided to use this mechanized, factory-based anti-corrosion technology, which offers high quality, fast processing speeds, and moderate costs, in the design of this ultra-high pressure, large-diameter natural gas main line. To ensure the quality of the powder coating applied, epoxy powders from the American company 3M have been selected. Choosing a good anti-corrosion method does not necessarily mean choosing a high-quality anti-corrosion project. Each type of anti-corrosion coating has its advantages and disadvantages, but they all share one common feature: they protect buried pipes. And their protective effect on the pipes depends on the quality of each step, from material selection to the backfilling of the pipe trench. If all stages are carried out in accordance with the requirements of standards and specifications, then whatever type of external anti-corrosion coating or cathodic protection system is used, it will provide the necessary protection for buried pipelines. On the contrary, no matter how high-quality the coating is, no matter how advanced the anti-corrosion methods are, and no matter how much investment is made, it is not possible to ensure proper protection for buried pipelines. Only by adopting a serious, pragmatic, and scientific approach to quality control at each stage of the process and strictly adhering to quality standards can the quality of anti-corrosion projects be ensured. In the epoxy powder coating anti-corrosion method, the three key indicators of coating quality are adhesion, thickness, and spark testing. These three indicators are also the focus for our institute’s designers in the monitoring of coating quality. Given the quality assurance of the powder, the adhesion of the coating primarily depends on the quality of oil removal, rust removal, and dust removal from the surface of the steel pipe, as well as the condition of the anchor patterns. This is also directly related to the original condition of the steel pipe, as well as the choice of materials used for rust removal and their frequency of replacement. The amount of epoxy powder used directly determines the cost-effectiveness of this anti-corrosion method. Given that the designed thickness of the coating has been determined, the actual coating is too thin and fails to meet the design requirements ； The actual coating thickness results in unnecessary waste. For example, during the pipe anti-corrosion process, there was a conflict between the shipping timeline of the epoxy powder and the project schedule, which resulted in a temporary shortage of the epoxy powder supplied by the American company 3M. To ensure progress on the project, with the approval of the design firm and the client, the anti-corrosion facility switched to epoxy powder from another manufacturer on a temporary basis. During the quality inspection of the project, we intensified our checks on the anti-corrosion coating of the pipes whose epoxy powder coating had been replaced, and promptly identified issues with the adhesion of the coating. After analysis and discussion, it was determined that one of the key issues was the application of an anti-rust primer on the surface of the steel pipes. This issue did not receive sufficient attention from the anti-corrosion manufacturers initially; the degreasing process was ineffective, and the steel grit used for shot blasting to remove rust had a negative impact, resulting in contamination of the steel pipes that did not have a primer applied. This led to the rework of dozens of properly preserved steel pipes. By strengthening the oil removal process for the steel pipes and completely replacing the steel sand, the adhesion problem was fully resolved. The timely correction of this issue, although resulting in the re-coating of dozens of steel pipes, reduced the risk of project accidents and ensured the quality of the anti-corrosion coating. Furthermore, since the pipe is a spiral-welded pipe, it is difficult to meet the quality requirements at the root of the spiral weld during shot blasting for rust removal, which in turn affects the quality of the anti-corrosion coating. After our institute’s designers identified this issue at the anti-corrosion factory, they promptly informed the factory’s technical staff. By improving the selection and ratio of the steel sand used, it was possible to **resolve the rust removal problem at the root of the spiral welds. Moreover, the newly selected steel sand had a longer service life, which also reduced the costs associated with rust removal. Furthermore, regarding issues such as the spark detection standards for anti-corrosion coatings, in order to strictly monitor powder quality, spraying process quality, coating thickness, as well as damage caused by human factors during installation, spark detection standards of 5000V for normal pipe sections and 10000V for on-site repairs were established through practice, summarization, further practice, and additional summarization, all within the limits permitted by quality assurance regulations. These standards have been used in subsequent similar projects to this day. ②Application of strip magnesium anodes: For the first time in this project, strip magnesium anodes were incorporated into the design of the electrical protection system for urban natural gas pipelines. Given the importance of this project, meticulous attention must be paid at every stage of the engineering design. In the past, in jacking pipe crossing projects, due to the shielding effect of the casing, apart from measures such as thickening the pipe wall and improving the anti-corrosion level for the pipe sections inside the casing, there were no better solutions in terms of the design of electrical protection systems. Furthermore, in sacrificial anode cathodic protection systems, there are certain requirements regarding the burial location and depth of the anode; at the same time, a good electrical conductivity environment should also exist around the anode. In the tunneling section of this project, the pipes are laid in stone trenches created by blasting. If block-type sacrificial anode cathodic protection is still used, it will be very difficult to dig the anode pits, and due to the poor electrical conductivity of the rocky terrain, the cathodic protection effect on the pipes will not be optimal. To address the above two issues, we reviewed a large amount of technical and engineering literature, met with various experts in corrosion prevention as well as engineering technicians from both academic and industrial fields, and conducted numerous technical investigations and discussions. Ultimately, we opted for strip-shaped magnesium anodes; with the help of some experts, we determined the methods of installation, the required quantities, and carried out the related calculations and design work. Thus, the cathodic protection problem of gas pipelines inside casings and in harsh engineering geological environments is solved. Test data obtained after the completion of this project show that the use of strip-shaped magnesium anodes fully achieved the intended design objectives, providing a practical reference solution for similar projects. ③Selection and layout of directly buried insulated joints: Insulation is a common and important technique in the electrical protection systems for buried pipelines. Without insulation, there is no electrical protection. In the past, electrical insulation for cathodic protection generally used insulating flanges. Insulated flanges have many limitations in terms of insulation performance and daily maintenance, and they require the construction of specialized maintenance wells. Through analysis and comparison of the performance, price, installation, and operation management of insulation devices at home and abroad, we conclude that directly buried insulation joints offer high insulation reliability and good sealing properties; they are designed with discharge spark gaps to prevent explosions and lightning strikes, are simple to install, and outperform insulation flanges in terms of various performance aspects. Moreover, they have lower overall costs and a better cost-performance ratio. Therefore, for the first time in domestic urban gas systems, we have identified and adopted Germany’s patented large-diameter, ultra-high-pressure, fully buried insulated joints, replacing the conventional insulated flanges installed in underground chambers. This not only improves the electrical protection of gas pipelines but also reduces the space required, facilitating the operation and management of the pipeline electrical protection system. Since then, directly buried insulated joints have been widely used in Beijing’s gas system. In the design of electrical protection systems, insulation devices are generally installed at the starting and ending points of pipelines as well as at the branch points. In this high-pressure pipeline project, the design of the electrical protection system took into account investigations into the corrosivity of the soil along the pipeline route as well as the geological conditions. The soil corrosion environments along the pipeline were divided into several typical zones, and multiple sectional insulating joints were installed along the main pipeline to electrically isolate those sections that passed through mountains or ran over long distances alongside rivers from the other sections. This was done to prevent interference between different soil corrosion environments and to avoid macroscopic galvanic corrosion caused by variations in engineering geology. (4) Welding and inspection: In the past, the design pressure for urban gas distribution systems was up to 0.8 Mpa. Carbon steel was commonly used for the steel pipes, and their wall thicknesses were chosen from standard series; the strength of these pipes was much higher than the values calculated based on the pipeline strength requirements. The welding process is a conventional one, and the inspection requirements for welds are relatively low; the sampling rate should be no less than 5% (according to the Code for Construction and Acceptance of Urban Gas Transmission and Distribution Projects CJJ33—89, when there are no specified requirements in the design documents, the sampling rate should be at least 15% of the total number of welds). The quality of the welds needs to be at least grade III, unless the design documents specify otherwise. As the design pressures for urban gas distribution systems increase, there have been significant changes in the selection of pipe materials; high-strength low-alloy steels are now being used. The choice of welding materials also changes as a result of these changes in pipe materials. The safety requirements for gas distribution systems have become even more stringent, and the old methods used in the past are no longer suitable. Regarding the welding requirements for ultra-high pressure natural gas pipelines, the \"Code for Design of Gas Pipeline Engineering\" specifies that in addition to the design documents having to indicate the specifications of the base materials of the pipelines and pipeline fittings as well as the welding materials, as well as the types of welds and welded joints, and providing clear requirements regarding welding methods, pre-welding heating, post-welding heat treatment, and welding inspections, specific requirements also need to be set for the construction units. Before starting the project, the construction contractor shall conduct welding procedure tests in accordance with the requirements of the design documents, and prepare a welding procedure specification based on the results of those tests. For the pipes in urban gas distribution systems, classification by region level is required; the quantity and quality standards for weld non-destructive testing are as follows: when using radiographic inspection, at least 75% of all welds completed by a given welder on that day must be inspected along their entire perimeter. For welded parts with a pipe wall thickness of 8 mm or more, it is also possible to first use an ultrasonic flaw detector to conduct a 100% inspection of the entire circumference of all welds, and then use radiographic testing to reinspect a portion of the welds along their entire circumference; the number of welds to be reinspected should be at least 20% of all the welds completed by each welder on that day. 100% radiographic inspection shall be carried out on the pipe welds where pipes pass through water bodies, highways, or railways, as well as on the buttwelds of pipes that have not undergone pressure testing. For welds inspected by ultrasonic testing, the acceptance criteria for their quality shall be in accordance with the current **standard \"Methods for Manual Ultrasonic Testing of Steel Welds and Classification of Test Results\"; grade I represents compliance. For welds inspected by radiography, the acceptance criteria for their quality shall be in accordance with the current **standard ‘Radiography and Quality Grading of Fusion Welded Butt Joints in Steel’, with grade II considered acceptable. Whether to use radiographic inspection or ultrasonic testing combined with radiographic re-inspection in the design documents depends on the pipe wall thickness, project timeline, as well as the quality and speed of these two inspection methods. Due to the tight deadline for the construction period of projects in Shanjing City, and the long time required for X-ray inspection, we adopted 100% ultrasonic testing, along with re-inspection of 20% of all welds selected at random. For the pipe welds that pass through special areas such as railways, rivers, large sand pits, and important traffic routes, as well as for the butt welds of pipes that have not been pressure-tested, 100% radiographic inspection is required; furthermore, the quality grade of these butt welds must be deemed acceptable. Over the past year, with the introduction of Y-ray source imaging devices, the time required for radiographic inspection has been reduced, and the reliability of the inspection results is higher than that of ultrasonic testing methods used for on-site grading; as a result, pure radiographic inspection methods are once again being adopted in engineering projects. (5) Pressure test: The pressure test of pipes includes strength tests and tightness tests. The \"Code for Construction and Acceptance of Urban Gas Transmission and Distribution Projects\" CJJ33—89 (applicable to pressures not exceeding 0.8 MPa) stipulates that compressed air should be used as the medium for pressure testing of gas pipelines. The test pressure for the strength of gas pipelines should be 1.5 times the design pressure, while the test pressure for leak testing should be 1.15 times the design pressure. The \"Code for Design of Gas Pipeline Engineering\" GB50251—94 specifies, based on regional classification, that for pipelines in Class 4 areas, water shall be used as the testing medium for strength tests, and the test pressure shall not be less than 1.5 times the design pressure ； When gas used for tightness testing serves as the test medium, the test pressure shall be the design pressure. The \"Code for Construction and Acceptance of Industrial Metal Piping Projects\" GB50235-97 specifies that the pressure test should be carried out using a liquid as the testing medium. When the designed pressure of the pipeline is less than or equal to 0.6 MPa, a gas can also be used as the testing medium, but effective safety measures must be taken. When the design pressure of the pipeline is greater than 0.6 MPa, a pressure test using gas is permitted only if specified in the design documents or with the approval of the project owner. The test pressure for the pneumatic test shall be 1.15 times the design pressure. Pipelines carrying flammable fluids must undergo leak testing for leaks. Air should be used as the testing medium for integrity tests, and the test pressure for leak detection should be the design pressure. It can be seen from the provisions of the several national and industry standards currently in effect that, due to differences in the time of formulation and application scopes, the requirements for pressure testing of gas pipelines vary as well. Regarding the pressure testing of urban gas pipeline systems, especially those for ultra-high-pressure natural gas, it is necessary to follow **standards and industry standards; at the same time, careful consideration must be given to factors such as the safety of the pressure testing, the source and disposal of the testing medium, and the impact of this medium on the gas distribution system. The urban natural gas supply consists predominantly of dry natural gas, and the gas distribution systems are also designed for dry natural gas. Due to the large number of municipal pipelines beneath urban roads, the integration of these pipelines follows the principle of allowing pressurized pipelines to take precedence over non-pressurized ones, which makes the longitudinal layout of gas pipelines quite complex. If water is used as the medium for the pressure test, how to completely remove the water after the test is completed inevitably becomes a new issue. If the water is not removed thoroughly, it will inevitably cause problems in the operation of the gas distribution system in the future. Therefore, in the project to introduce natural gas to Beijing using materials from Shaanxi, Gansu, and Ningxia, and after obtaining the consent of the project owner and conducting relevant operational tests by that party, we specified in the design documents that air or inert gas should be used as the medium for strength tests; however, effective safety measures must be taken, and approval from the competent authorities is required. The test pressure for the pneumatic test shall be 1.15 times the design pressure. Air is used as the test medium for tightness testing, and the test pressure should be the design pressure. 4 Conclusion Regarding the design of ultra-high pressure natural gas pipelines in urban areas, due to the novelty of such engineering designs, many technical solutions and measures still need to be tested in practice. Some technical issues require exploration, summarization, and improvement through actual implementation. It is necessary to uphold the standards regarding design quality, meet the requirements of construction, and at the same time consider ways to reduce project costs. To this end, the designers at our institute have extended the engineering design process to cover the entire scope, from the anti-corrosion treatment of steel pipes in the anti-corrosion factory to the backfilling of the pipe trenches at the construction site, providing 24/7 support and services, with quality and service as top priorities. In addition to the examples mentioned earlier, there are numerous other instances. For example, when monitoring the anti-corrosion quality of steel pipes, we identified surface quality issues on some of the pipes; when checking the anti-corrosion quality of bent pipes, we found problems related to the processing of those pipes. At the construction site, we encountered cases where the thickness of the anti-corrosion coating on certain pipes was uneven. During the quality monitoring of electrical protection projects, it was found that the damage to the pipes’ anti-corrosion coatings caused by cathodic protection was not properly repaired. Additionally, during transportation and the backfilling of pipe trenches, construction workers failed to pay adequate attention to protecting the pipes’ anti-corrosion coatings. All of these issues require designers to follow up on them, provide feedback, and resolve them with a high sense of responsibility, quality awareness, and ownership. Only in this way can the principle of \"taking the safety of the pipeline itself as a priority\" be put into practice, thereby ensuring the design quality and overall quality of the project. The high-pressure pipeline projects for the natural gas distribution within Beijing, brought in from Shaanxi, Gansu, and Ningxia, were completed and put into operation gradually in 1997. All parties involved in the acceptance process expressed satisfaction and confidence regarding the quality of these projects. This was certainly made possible by the hard work of the construction teams, as well as by the unity and cooperation among various organizations and departments. We believe that it also included the careful design, thorough support, commitment to quality, drive for excellence, and spirit of conservation and innovation on the part of the designers from our institute. Through several years of design experience in urban ultra-high pressure natural gas pipeline projects, we have accumulated certain design expertise and also developed a team of designers who are brave, determined, and highly skilled. However, the design work for urban ultra-high pressure natural gas projects is still in its infancy, and there are many issues that need to be further studied and resolved. For example, issues such as the hydraulic calculation of ultra-high pressure natural gas pipelines in towns, stress calculation problems, reinforcement of pipelines during pressure testing, and measures to stabilize pipelines when they pass through areas with poor engineering geology. During project coordination and quality monitoring, some new issues will also arise. However, we believe that by adhering to and implementing the principle of \"prioritizing the safety of the pipelines themselves\" when addressing the quality and safety issues associated with ultra-high pressure natural gas projects, many challenging practical problems in such projects can be easily resolved. Free Postgraduate Entrance Exam Website: www.freekaoyan.com