There are many kinds of pipelines used to transport gas, which must be determined according to the service conditions (design pressure, temperature, medium characteristics, area of use, etc.) of gas, welding performance of data and other factors after technical and economic comparison [1,2].

## 1. Determination of steel grades of high pressure and sub high pressure pipelines

When the diameter of high pressure gas pipeline and sub high pressure gas pipeline is greater than 150mm, welded steel pipes are generally used, and the selected welded steel pipes shall comply with the provisions of Steel Pipes for Pipeline Transportation Systems in Petroleum and Natural Gas Industries (GB/T 9711-2017); Seamless steel pipes shall be used when the diameter is less than or equal to 150mm. The steel grade and the type of welded steel pipe shall be determined through technical and economic comparison.
The method to determine the steel grade is: calculate according to the calculation wall thickness formula of steel pipe straight pipe section given in Article 6.4.6 of GB50028-2006 Code for Design of Urban Gas Engineering, obtain the calculation wall thickness of steel pipe, and determine the selected wall thickness by comparing the minimum nominal wall thickness of steel pipe (see Table 1, from Table 6.3.2 of GB50028-2006 Code for Design of Urban Gas Engineering). The steel grade shall be determined after analysis and comparison according to the selected wall thickness and price of different metal materials.
Table.1 Minimum nominal wall thickness of steel pipe

 Nominal diameter of steel pipe/mm Minimum nominal wall thickness/mm Nominal diameter of steel pipe/mm Minimum nominal wall thickness/mm 100 – 150 4 600 – 700 7.1 200 – 300 4.8 750 – 900 7.9 350 – 450 5.2 950 – 1000 8.7 500 – 550 6.4 1050 9.5

The formula for calculating the wall thickness of steel pipe straight section is as follows:

In the formula

• δ–Calculated wall thickness of steel pipe, mm;
• p – Design pressure, MPa;
• D – Outer diameter of steel pipe, mm;
• σs – MPa, the lowest yield strength of steel pipe;
• φ – Weld coefficient, 1.0;
• F – The strength design coefficient is determined according to the region grade.

For example, the design pressure of 1.6Mpa and pipe outer diameter of 219.1mm are used in a projectWelded steel pipeBased on GB / T 9711-2017 “steel pipes for pipeline transportation system of petroleum and natural gas industries”, the steel grades of grade IV areas are compared and selected. The results are shown in Table 2.

Table.2 Comparison results of different steel grades in a project

 Design pressure/MPa 1.6 1.6 1.6 Outer diameter of steel pipe/mm 219.1 219.1 219.1 Weld coefficient 1 1 1 Strength design factor 0.3 0.3 0.3 Steel grade L245 L290 L360 Minimum yield strength/MPa 245 290 360 Calculation of wall thickness/mm 2.38 2.01 1.62 Minimum nominal wall thickness/mm 4.8 4.8 4.8 Adopt wall thickness/mm 5 5 5 Steel pipe price by length/(yuan).m-1） 118.8 121.44 129.36

Based on the analysis of the data in Table 2, when the design pressure is 1.6MPa and the outer diameter of the pipe is 219.1mm, the calculated wall thickness of L245, L290 and L360 steel pipe straight sections calculated according to Formula (1) is less than the minimum nominal wall thickness of 4.8mm in Table 2, so 4.8mm is taken as the lower limit of the wall thickness selected for the pipe specification. According to Table 1 in GB/T 21835-2008 Welded Steel Pipe Dimensions and Single Specific Weight, it is determined to use pipes with a wall thickness of 5.0mm. For different steel grades D219.1 ×  5.0 The price of steel pipes calculated by length is compared. On the premise of meeting the design requirements, the L245 with the best economy is finally selected.

## 2. Analysis of welded steel pipe types of high pressure and sub high pressure pipes

On the basis of determining the steel grade, the type of welded steel pipe is further selected. Welded steel pipes are divided into two categories according to the weld shape: straight seam welded pipe and spiral seam welded pipe. According to different welding methods, longitudinal welded pipe can be divided into longitudinal high-frequency resistance welded pipe and longitudinal double-sided submerged arc welded pipe. Spiral seam welded pipe welded by submerged arc welding process is called spiral seam submerged arc welded steel pipe.

### (1) Introduction to welded steel pipe

#### a. Straight seam welded pipe

The pipe is made of hot-rolled or cold-rolled steel plates or steel strips by coil welding, and is welded in a straight seam on the welding equipment. The welded pipe shall ensure that the hydrostatic test, tensile strength and cold bending efficiency of the weld meet relevant regulations.

#### b. Spiral seam welded pipe

The low carbon structural steel or low alloy structural steel strip is rolled into tube blank according to a certain spiral seam angle (called forming angle), and then the tube seam is welded. This method can produce large diameter steel pipe with narrow steel strip. Spiral welded pipe is mainly used for oil and gas transmission pipeline. The welded pipe shall ensure that the hydrostatic test, tensile strength and cold bending efficiency of the weld meet relevant regulations.

### (2) Performance comparison of welded steel pipes

This paper only compares the efficiency of longitudinal high-frequency resistance welded steel pipe, longitudinal double-sided submerged arc welded steel pipe and spiral submerged arc welded steel pipe.
The longitudinal high-frequency resistance welding is made by melting the butt joint of tube blank with resistance heat generated by high-frequency current and then extruding and fusing. It is characterized by heat concentration, small heat affected zone, welding quality mainly depends on the quality of base metal, low production cost and high efficiency.
The advantages of straight seam double-sided submerged arc welded steel pipe are high forming quality of welded pipe, small unfitness, good weld quality, small heat affected zone, small residual stress after welding, accurate pipe size, easy online inspection and 100% nondestructive testing of raw materials. It is especially suitable for application in gas transmission pipeline. The straight seam double-sided submerged arc welded steel pipe has a longitudinal weld, and the inner and outer welds are welded by one submerged arc welding. After the overall mechanical expansion (mainly to improve the size accuracy of the steel pipe), the internal stress of the steel pipe is small and evenly distributed, which can effectively prevent stress corrosion cracking. With high size accuracy, it is convenient for on-site welding construction. The process of pre welding followed by fine welding is adopted, with stable welding process and high weld quality. The range of product specifications is large, which can produce steel pipes with small diameter and large wall thickness as well as steel pipes with large diameter and large wall thickness. The disadvantage is that the pipe diameter is restricted by the width of the steel plate. A steel plate with one width can only produce a steel pipe with one diameter.
Spiral submerged arc welded steel pipe does not have 100% ultrasonic flaw detection, diameter expansion and other processes for base metal. The production is low, and automatic seam tracking is difficult. Forming and welding are completed under the dynamic condition of continuous operation. Continuous production can be realized. Steel pipes with larger pipe diameters can be produced with narrow billets, and steel pipes with different pipe diameters can be produced with billets of the same width. However, compared with longitudinal welded pipe of the same length, the weld length increases by 30%~100%, and the production speed is low. In this case, straight seam welded pipes are mostly used for steel pipes with smaller diameters, while spiral seam welded pipes are mostly used for large diameters.
The weld seam of spiral submerged arc welded steel pipe forms a spiral seam angle with the axis of the steel pipe, so that the heat affected zone of the weld seam is not in the direction of the main stress. In addition, the stress of this weld seam is good, and steel strip can be used to produce large-diameter pipes. However, due to the long weld seam, the possibility of welding defects is increased. In addition, since the steel strip rolling direction of spiral seam welded pipe is not perpendicular to the axis direction of the steel pipe, and the steel plate rolling direction of straight seam steel pipe is perpendicular to the axis direction of the steel pipe, the crack resistance of spiral seam welded pipe is superior to that of straight seam steel pipe.
The price of straight seam double-sided submerged arc welded steel pipe is higher than that of spiral seam submerged arc welded steel pipe, and the price of straight seam high-frequency resistance welded steel pipe is the lowest.
According to the production process, most of the welded steel pipes with smaller diameter (DN400mm and below) in China use straight seam high-frequency resistance welding, while most of the welded steel pipes with large diameter (DN400mm and above) use spiral seam submerged arc welding. At present, China has the ability to produce large diameter straight seam double-sided submerged arc welded steel pipes, and has been used in oil and gas transmission projects.

## 3. Conclusion

The selection of high pressure and sub high pressure gas pipelines should be based on the type, characteristics, gas consumption structure, pipeline operation conditions and environment, construction conditions, specific conditions of the construction site and other aspects of urban gas sources. It should be predictable, combined with reality, and scientific and reasonable selection should be made to meet the requirements of long-term safe operation and medium-term and long-term planning and development.
Author: Wang Zhenling
Source: Pipeline Solution Supplier
References:

• [1] Populus euphratica, Liu Qingtang Selection of gas pipeline materials [J]. Gas and Heat, 2003, 23 (1): 46 – 47
• [2] Tan Xiaoping Discussion on material selection of indoor natural gas pipeline [J]. Gas and Heat, 2009, 29 (7): B15 – B19