Design Calculation of High-pressure Pipe Fittings with Large Diameters (Part One)

Abstract: In terms of domestic high-pressure pipe fittings with large diameters used in valve boxes and stations of China's natural gas long-distance pipeline engineering, quality of pipe fittings with large walls are difficult to be guaranteed by using mathematical analysis methods, while wall thickness calculated by the verification test method for imported pipe fittings is thinner. There are no problems in manufacturing equipment and process of pipe fittings, and the quality as well as safety of pipe fittings are guaranteed. Through the comparison of imported and domestic pipe fittings in engineering examples, it is explained that the verification test method should be used for design calculations of pipe fittings. It is proposed that the standards required for the verification test method should be based on foreign standards, and use of the pipe fitting verification test method to calculate wall thickness is discussed. Relevant technical issues include standardizing structure types and dimensions as well as stipulated manufacturing process specifications.
 
Preface
Since the construction of the West-east natural gas pipeline project, China's natural gas high-pressure long-distance pipelines with large diameters have developed rapidly. The design pressure of pipelines has reached 12 MPa, and the pipe diameter has been increased from DN 1,000 (1016 mm) to DN 1400 (1420 mm). The material of the main steel pipeline generally adopts X 80. Although manufacturing and construction welding technology of the main steel pipeline has been well developed, pipe fittings are used a lot for the main pipeline valve box and station, especially the tee and elbow; their problems of manufacturing process, equipment and quality have not been resolved for a long time. This is mainly because the stress distribution of pipe fittings under the action of internal pressure is more complicated and not distributed evenly, which is completely different from the film theory of pressure vessel cylindrical shells. Therefore, the calculation of pipe fittings is stipulated for two design methods in the specification. At present, China still uses relatively conservative mathematical analysis methods that do not match the high-pressure large-diameter long-distance pipeline in the design and calculation methods of pipe fittings, resulting in extremely large wall thicknesses of high-pressure pipe fittings with large diameters.
 
1. A comparison of engineering examples
In a long-distance pipeline project, high-pressure large-diameter pipe fittings designed and manufactured by a foreign pipe fitting company were once imported. The technical standard adopted was the American pipe fitting standard MSS SP-75-1998 Technical Specification for High-quality Steel Butt Welding Pipe Fittings. The pipe fittings are made from steel WPHY 70 and WPHY 60 which are specified for pipe fittings in the standard. The calculation method of pipe fitting design is the verification test method. At that time, the main imported pipe fittings were tees and elbows. The design parameters, specifications, wall thickness and materials of some imported tees and elbows are shown in the Table 1 and Table 2 respectively. In order to facilitate the comparison of the wall thickness differences of the pipe fittings caused by the different pipe fitting design calculation methods, the specifications, wall thickness and materials of some domestic pipe fittings for a domestic long-distance pipeline project are listed in the Table 3 and Table 4. The standard for the design of pipe fittings is the technical specification of the project and MSS SP-75-1998 Technical Specifications for High Quality Steel Butt Welding Pipe Fittings, and the pipe fitting materials are X70 and X60 API specified in the Spec 5 L-2000 Steel Pipe Specification (the 42nd Edition) 12. The pipe fitting design calculation method is mathematical analysis method, that is, tees adopt equal area reinforcement, and the elbow is calculated by the formula.
 
Table 1 Design parameters, specifications, wall thickness and materials of the imported tees

 
Table 2 Design parameters, specifications, wall thickness and materials of the imported elbows

 
Table 3 Specifications, wall thickness and materials of the China's tees

 
Table 4 Specifications, wall thickness and materials of the China's tees

 
Tables 1 to 4 can at least indicate the following matters:
1. The material selection of domestic pipe fittings is the same as that of foreign pipe fittings. The material is determined according to the design pressure and nominal diameter of the pipe fitting. The American standard MSS SP-75-1998 Technical Specifications for High Quality Steel Butt Welding Pipe Fittings stipulates that the minimum required tensile strength of WPHY 70 and WPHY 60 is 82,000 psi (565 MPa) and 75,000 psi (517 MPa) respectively and the minimum values are 70 000 psi (483 MPa) and 60 000 psi (414 MPa) respectively, which are completely consistent with that of the API Spec 5L-2000 Steel Pipe Specifications the 42nd Edition.
 

2. Since the same specification MSS SP-75-1998 Technical Specifications for High-quality Steel Butt-welded Pipe Fittings is adopted, the main structural dimensions of the pipe fittings are the same. In fact, the size of pipe fittings in MSS SP-75-1998 Technical Specifications for High-quality Steel Butt-welded Pipe Fittings and that of other pipe fittings in ASME B16.9-2001 Factory-manufactured Forged and Rolled Butt Welded Pipe Fittings, domestic pipe fitting standard SY/T 0609-2006 Specifications for High-quality Steel Butt Welded Pipe Fittings and SY/T 0510-2010 Specifications for Steel Butt Welding Pipe Fittings are the same. There may only be a few structure and size differences due to design or manufacturing of the mold structure and manufacturing process, but these differences will not bring about the difference in strength calculation.

 

3. It can be seen from the Table 1 and Table 3 that imported tees and domestic tees require different specifications due to different projects, and tees with the same specifications are rarely available for comparison. However, the following wall thickness differences can also be seen:

 
With the same design pressure (10 MPa), materials (WPHY 70 or X 70) and specifications, the design pressure is 10 MPa, and wall thickness of the two shoulder parts of imported tees with DN 1,000 × 600 is 30.65 mm, which is thicker than that of domestic tees by 46.8 mm and has a reduction of 34.5%.
 
With the same design pressure (10 MPa) and materials (WPHY 60 or X 60), the imported equal tee with a size of DN 700×700 which has the same main pipe but different branch pipes has wall thickness of the two shoulder parts of 30.64 mm, which is thicker than domestic reducing tees with a size of DN 700x500 whose two shoulder thickness is 36.0 mm by 5.36 mm.
 
The reason why the wall thickness of the imported tee is thinner than that of the domestic tee is that the calculation method of the tee design is different. The calculation of the imported tee uses the verification test method, while the calculation of the domestic tee adopts the equal area reinforcement method of the mathematical analysis method.
 

4. It can also be seen from wall thickness of the elbow in the Table 2 and Table 4 that because the imported elbow adopts the verification test method, while the domestic elbow adopts the mathematical analysis method, the wall thickness of the imported elbow is thinner than that of the domestic elbow. Moreover, the proportion of thinning is also larger. For example, an elbow with  design pressure of 10 MPa, DN 1,000, bending radius of 1.5 D and in WPHY 70 or X70, the wall thickness of the imported elbow is 26.46 mm, which is 6.32 mm thinner than that of the domestic elbow whose thickness is 32.8 mm by 6.32 mm. The thickness reduction ratio is 19.3%.