Comparative Study on European Pipe Fitting Standards and Chinese Pipe Fitting Standards

[Purpose] To meet the needs of recent European power station construction, it is urgent to understand the requirements of European pipelines and pipe fittings standards in power station engineering.
[Method] For pipe fittings, compare the corresponding clauses of European pipe fitting standards with domestic pipe fitting standards.
[Result] After comparison, the differences between European pipe fitting standards and domestic pipe fitting standards were obtained: European standard pipe fittings are more comprehensive in terms of industry usage scope and pipe usage scope; The size deviation, welding end wall thickness deviation, and shape and position deviation of European standard pipe fittings are generally stricter than the requirements of national standards; The European standard for calculating the strength of pipe fittings is more conservative and biased towards safety; European pipe fittings have their characteristics compared to national standard pipe fittings in terms of end grooves, flow area, and bending radius of elbows; In addition, pipe fittings entering the European market need to comply with the EU administrative directive PED.
[Conclusion] Through the differences in the above standards, the basic requirements of the European standard pipe fittings have been mastered, which can guide the practical application of European standard pipe fittings in power plant engineering.

European Pipe Fitting Standards Vs Chinese Pipe Fitting Standards

With the implementation of the national Belt and Road policy, the business of domestic power design institutes has gradually expanded to the European continent. European countries generally adopt European standards, different from the conventional use of American standards (mainly used in Southeast Asia, the Middle East, and South America) or national standards by domestic design institutes. Currently, the owners of upcoming projects tend to use European standards. This is a new topic for the power design institute; after all, there were few projects that did European standards in the past, and China is not very familiar with European standards. Pipeline engineering involves designing, manufacturing, and inspecting pipelines and fittings by European standards. Due to space limitations, this article selects pipe fittings for standard comparison. The standard for pipe fittings in the domestic power industry generally adopts the “Steel Butt Welding Pipe Fittings for Power Plants” (DL/T 695-2014). This time, we will learn and master the butt welding pipe fittings (EN 10253). EN 10253 is divided into four parts, namely Part 1: Forged carbon steel with general and no special inspection requirements; Part 2: Nonalloy and ferritic alloy steels with special inspection requirements; Part 3: Forged austenitic and austenitic ferrite duplex stainless steel without special inspection requirements; Part 4: Wrought austenitic and ferritic austenitic duplex stainless steels with special inspection requirements. The second part is commonly used in the power industry. Therefore, this comparative study of standards mainly focuses on the comparison between “Steel Butt Welding Pipe Fittings for Power Plants” (DL/T 695-2014) and “Butt welding pipe fittings part 2: nonalloy and ferritic alloy steels with specific inspection re requirements” (EN 10253-2-2007).

1. Comparison of the scope of use of pipe fittings standards between China and Europe

DL/T 695 specifies the basic requirements for butt welding pipe fittings of carbon steel, alloy steel, and austenitic stainless steel. This standard of pipe fitting applies to steel butt welded pipe fittings used in Fossil fuel power stations and non-nuclear grade steel butt welded pipe fittings in nuclear power plants.
Part 2 of EN 10253-2 specifies the technical requirements for seamless and butt welded pipe components (elbows, concentric and eccentric reducers, equal and reducing tees, heads) made of carbon steel and alloy steel. These metal materials are designed to transmit and distribute fluids and gases at room, low, or high temperatures. Part 4 of EN 10253-2 specifies the basic requirements for stainless steel welded pipe fittings.
By comparison, the scope of use of the two pipes is similar, but the national standard is more suitable for steel butt welded pipe fittings used in Fossil fuel power stations and nonnuclear grade steel butt welded pipe fittings in nuclear power plants, while EN 10253-2 does not specifically refer to the power industry. Therefore, in the power industry, if European owners require the use of European standard pipes, European standard pipe fittings can be used.

2. Comparison of steel and pipe diameter standards between China and Europe for pipe fittings

2.1 Comparison of Standard Steel for Pipe Fittings between China and Europe

DL/T 695-2014 specifies various commonly used steel types in China, including GB 3087, GB 5310, ASTM, and EN 10216-2; The second part of EN10253 specifies that carbon steel includes: P235TR2-P265TR2-P235GH-P265GH-P355N-P355NH-P355NL1-P215NL-P265NL and other steel names can be found in EN 10027-1. The national standard does not mention European carbon steel materials, while the European standard covers carbon steel, alloy steel, and stainless steel. In terms of the scope of use of European standard pipes, the European standard is more comprehensive.

2.2 Comparison of Standard Pipe Sizes for Pipe Fittings between China and Europe

1) Comparison of nominal sizes
DL/T 695 provides two outer diameter size series in the DN series, namely the A and B series. The A series is an international standard known as the ISO series.
The second part of EN 10253-2 has a nominal diameter DN derived from EN ISO 6708; The outer diameter series comes from EN 10220; the outer diameter series of seamless steel tubes for pressure purposes and technical delivery conditions. Part 2: nonalloy and alloy steel tubes with specified elevated temperature properties EN 10216-2 and welded steel tubes for pressure purposes technical delivery conditions Part 2: Electric welded nonalloy and alloy steel tubes with specified elevated temperature properties EN 10217-2 To.
The nominal size and outer diameter series of the European standard are completely consistent with the A series of the national standard.
2) Size range comparison
DL/T 695 specifies butt welding fittings for the outer diameter control pipe size series DN15-DN1200 and butt welding fittings for the inner diameter control pipe size series 146-1067.
The second part of EN 10253-2 specifies the size series as DN15-DN1200.
The standard domestic DL/T 695 specifies the dimensions of outer and inner diameter control pipes, respectively. The European standard does not distinguish between the outer diameter control pipe size and the inner diameter control pipe size range, and the pipe diameter range is consistent with the domestic control outer diameter pipe size.
The difference in size range regulations is due to the actual processing capabilities of different countries. When using different standards, attention should be paid to the regulations of their size range to avoid selecting products that cannot be purchased.

3. Comparison of Standard Pipe Fitting Types and Pressure Grades between China and Europe

3.1 Comparison of Standard Pipe Fitting Types between China and Europe

The domestic standard DL/T 695 does not distinguish between Class A and Class B according to whether the wall thickness of pipe fittings are the same as or increased with the wall thickness of connected straight sections.
EN 10253-2 defines two types of pipe fittings: the welding end of A-type pipe fittings, and the wall thickness of the main body of the pipe fittings are the same as the wall thickness of the connecting straight channel. Usually, their bearing capacity for internal pressure is smaller than that of straight pipes of the same size. The wall thickness of B-type fittings increases at the main body of the fittings, aiming to withstand the same internal pressure as straight pipes of the same size. These two types of pipe fittings are used in applications covered by EU Directive 2014/68/EU. Type A: Butt welded pipe fittings with reduced pressure coefficient X; The pressure coefficient X is defined as the pressure borne by the pipe fitting/the pressure borne by the straight pipe. For example, taking over OD711 × The pressure coefficient X of the welded elbow with a 2D bending radius of 7.1 (with a wall thickness corrosion allowance of 1 mm) is only 67%. Type B: Butt welded pipe fittings used under full working pressure. According to this definition, the pressure pipeline system in the power industry generally adopts Class B pipe fittings.

3.2 Comparison of Standard Pressure Grades for Pipe Fittings between China and Europe

DL/T 695-2014 does not specify pressure level and strength.
The second part of EN 10253-2 establishes a series of 8 pressure levels for the wall thickness of Class B pipe fittings (these 8 pressure level series do not specifically define pressure values). Each series corresponds to a straight pipe with a selected wall thickness, and each pipe fitting has specific wall thickness parameters corresponding to the selected wall thickness of the straight pipe. For elbows, the minimum inner arc wall thickness Tint is defined based on different bending radii.
From this, it can be seen that neither the national nor the European standard specifies the strength (wall thickness) of specific pressure-grade pipe fittings corresponding to specific materials. The European standard only lists the wall thickness of pipe fittings under the recommended 8-pressure class series but does not specifically define the pressure value.
In addition, although the standard domestic DL/T 695-2014 does not specify the pressure grade and strength, to facilitate the selection, processing, manufacturing, and procurement of steam and water pipes and fittings in Fossil fuel power stations, China Electric Power Design and Planning Association has prepared Typical Design of Parts and Components of Steam and Water Pipes in Fossil fuel power station (GD 2016). The 2016 Code applies to the design of metal steam and water pipeline parts and components for units with ultra-supercritical parameters and below, pipeline design temperature not greater than 628 ℃ and capacity not greater than 1000 MW within the Fossil fuel power station scope. The content of the “2016 Standard Pipe” manual covers steel pipe varieties, elbows, reducers, tees, heads, flanges, and assemblies. This standard pipe has the characteristic of setting identification codes for pipes or fittings. The identification code should have at least 6 units to express: the first unit is the name code; The second unit is the level code, represented by design pressure or design temperature; The third unit is the type of material; The fourth unit is the material code; The fifth unit is the type code; The sixth unit is the specification code. After identification and coding, listing pipes and fittings is convenient for understanding the characteristics and materials of pipes and fittings, facilitating retrieval and standardized design. It is understood that due to intellectual property issues, there is no corresponding regulatory manual prepared by domestic industry associations in European standards.
It is recommended that when using the European standard, manufacturers with the ability to design and manufacture European standard pipe fittings should calculate the strength (strength design) based on the specific pressure grade, material, and pipe fitting type of the pipe fittings according to the dimensions specified in such standards to determine the specific wall thickness of the pipe fittings.

4. Comparison of standard size, wall thickness, and geometric tolerance of pipe fittings between China and Europe

4.1 Comparison of standard dimensional tolerance of pipe fittings between China and Europe

DL/T 695-2014 lists the dimensional tolerances of various pipe fittings for outer diameter control pipes in Appendix E.1, which are specific numerical values. The positive deviation in size ranges from 1.0 mm for DN 15 to 6.4 mm for DN 1200; The negative deviation ranges from 0.8 mm for DN 15 to 4.8 mm for DN1200.
The second part of EN 10253-2 defines the size deviation of pipe fittings as ± 1% or ± 0.5 mm, whichever is greater, but with a maximum of ± 5 mm.
From above, the size deviation of pipe fittings in the European standard is slightly stricter than in the national standard. This puts forward higher requirements for domestic pipe fittings manufacturers to produce European standard pipe fittings.

4.2 Comparison of Wall Thickness Tolerances for Welding Ends of China Europe Pipe Fitting Standards

DL/T 695-2014 stipulates that the maximum deviation of welding end wall thickness is from 0.9 mm (DN ≤ 65) to 5.6 mm (1050 ≤ DN ≤ 1200).
Part 2 of EN 10253-2-2007 specifies the wall thickness deviation at the welding end (for Class B pipe fittings):
When the outer diameter is ≤ 610 mm, regardless of the wall thickness, the deviation is -12.5%,+20%.
When the outer diameter is>610 mm, there are three types:

• 1) Seamless pipe fittings, wall thickness -12.5%, +20%.
• 2) Welded pipe fittings, wall thickness ≤ 10 mm, deviation -0.35 mm,+20%.
• 3) Welded pipe fittings, wall thickness>10 mm, deviation -0.5 mm, +20%.

From the above, it can be seen that for welded pipe fittings, the European standard requires a relatively small absolute value for the negative deviation of wall thickness, and there is no specific requirement in this regard in the national standard. In addition, the European standard has a positive deviation of 20% for both seamless and welded pipe fittings, but there is no specific requirement in the national standard. It can be seen that the European standard generally has a narrower range of wall thickness tolerance control.

4.3 Comparison of Standard Geometric Tolerances for Pipe Fittings between China and Europe

DL/T 695-2014 specifies that the geometric tolerance X=1 mm when the nominal size is 15-100; X=2 mm at 125-200; X=3 mm at 225-400; X=4 mm at 450-600; X=5 mm at 650-1200.
Part 2 of EN 10253-2 stipulates that the geometric tolerance X is ≤ 1% of the outer diameter of the measuring point or 1 mm, whichever is greater.
When the diameter of large-diameter pipe fittings in the European standard is greater than 500, the deviation in shape and position is lower than the requirements of the national standard; When the diameter DN is less than 500, the deviation in shape and position is consistent with the national standard, both of which are 1% of the outer diameter. The overall European standard is more stringent regarding form and position deviation than the national standard.
After comparing the size, wall thickness, and shape deviation of Chinese and European standard pipe fittings, the European standard has a narrower allowable deviation range than the national standard, and the overall requirements are stricter than the national standard, mainly due to the more refined processing and manufacturing processes in Europe than in China. If domestic pipe fittings factories need to produce and manufacture European standard pipe fittings, they must improve their processing and manufacturing levels.

5. Comparison of End Groove Forms of Standard Pipe Fittings between China and Europe

DL/T 695 stipulates that when the nominal wall thickness of carbon steel and ferritic alloy steel is 5mm ≤ t_n ≤ 22 mm and the nominal wall thickness of austenitic stainless steel is 3mm ≤ t_n ≤ 22 mm, V-shaped grooves are used, Nominal wall thickness ≥ 22mm, using double V-shaped grooves; No other regulations have been made. The bevel angle at the end of the V-shaped groove is 37.5 ° ± 2.5 °. The bottom surface is 1.5 mm ± 1.0 mm.
Part 2 of EN 10253-2 stipulates that, at the manufacturer’s option, for specified wall thicknesses less than 3 mm, both ends can be cut square or slightly chamfered. If the wall thickness is between 3 and 22 mm, the angle at the end should be 30 ° ± 5 °, and the bottom surface should be 1.6 mm ± 0.8 mm.
If the wall thickness exceeds 22 mm, approval from the purchaser should be obtained.
By comparison, the Chinese and European pipe fitting standards have almost the same classification of wall thickness for the end groove form of pipe fittings. The only difference is that the angle of the bevel and the size of the bottom surface may vary slightly, depending on the processing and manufacturing characteristics of different countries, regardless of their advantages or disadvantages.

6. Comparison of minimum wall thickness calculated according to China Europe pipe fitting standards

Comparative example: seamless elbow joint OD168.3 × 4.5. The material is P265GH, with a design pressure of 1.03 MPa (g), a design temperature of 379 ℃, and an allowable stress of 91.29 MPa. For ease of comparison, the bending radius is uniformly 1.5 times the outer diameter without considering wall thickness deviation and wear allowance.
The strength design of pipe fittings in the domestic standard DL/T 695 comes from the steam specification DL/T 5054-2016. The formula for calculating the minimum wall thickness of the elbow is as follows:

In the formula:

• P – Design pressure [MPa (g)];
• D₀– Outer diameter of the pipe;
• [ σ]^t – Is the allowable stress at the design temperature (MPa);
• η—— Correction factor for allowable stress;
• I – Correction coefficient for elbow wall thickness;
• Y – Correction coefficient.

The strength design of pipe fittings in Part 2 of European Standard EN 10253 comes from the standard EN 13480-3, and the minimum wall thickness formula for the inner arc of the elbow is as follows:

When D_o/D_i ≤ 1.7;

In the formula:

• E – Minimum wall thickness of straight pipe (mm);
• P_c – Design pressure [MPa (g)];
• D_o – Outer diameter of the pipe;
• F – Allowable stress at design temperature (MPa);
• Z – Weld seam coefficient;
• R – Bending radius of the elbow (mm).

After comparison, the strength calculation standards for pipe fittings in China and Europe do not consider wall thickness deviation and wear allowance, and the European standard formula for the minimum wall thickness of elbows is more conservative.

7. Comparison of other pipe fitting standards between China and Europe

7.1 Comparison of flow area between China and Europe pipe fittings

DL/T 695 stipulates that the flow area guaranteed by the minimum inner diameter of elbows and tees should be equal to that of the connecting pipe, and the flow area should not be less than 95% of its connecting pipe flow area.
The second part of EN 10253-2 stipulates that to ensure the normal flow of fluid through fittings, the inner diameter of any part of the fitting (not applicable to the head) should be greater than 80% of the inner diameter ID. The national standard requires a higher flow area than the European standard and has better control over the flow resistance of the medium.

7.2 Comparison of bending radii between China and Europe elbows

DL/T 695 divides it into a long radius elbow (equivalent to 1.5 times the outer diameter) and a short radius elbow (equivalent to 1 times the outer diameter). The second part of EN 10253 stipulates that it is divided into 2D (equivalent to 1 times the outer diameter), 3D bend (equivalent to 1.5 times the outer diameter), and 5D bend (equivalent to 2.5 times the outer diameter). The expression of the European standard bending radius series is different from that in China, which is twice the bending radius in China. At the same time, the bending radius series is more diverse.

7.3 Comparison of Internal Pressure Instructions for Pipe Fittings between China and Europe

There is no administrative directive for pressure equipment in domestic standards. Part 2 of EN 10253-2 specifies that the Pressure Equipment Directive (2014/68/EU) stipulates that designs with sufficient strength should be based on computational methods or, with limitations, experimental methods. It is the ultimate responsibility of the pressure equipment manufacturer to select suitable fittings (materials, thickness) by EN 13480-3 for the resistance of fittings to internal pressure that complies with this European Standard. Only products entering the European market are required to comply with EU directives, and pressure equipment must comply with the PED directive (2014/68/EU).
In addition, the final attachment of Part 2 of EN 10253-2 also contains an explanatory table on the compatibility of this standard with the provisions of the European Union Directive PED (2014/68/EU). This is a significant difference between European standards and domestic standards.

8. Conclusion

The country’s Belt and Road policy has pushed domestic power design business out of the country to a broader overseas market and compete with large foreign engineering consulting companies for international projects. Previously, overseas engineering was mainly conducted in Southeast Asian or Middle Eastern countries. Now, our business has expanded to the European region. In addition to mastering the basic skills of national or American standards, we must also be familiar with and master the local design standards and habits in Europe. Similarly, domestic pipeline and pipe fitting manufacturers must simultaneously study European market regulations and jointly venture out to win the vast continental European market. This article introduces the domestic and European pipe fitting standards compared to them and compares the Chinese and European pipe fitting standards from multiple perspectives. The following conclusion is drawn:

• 1) The use of European standards has a broader industry scope.
• 2) The European standard has a more comprehensive range of pipe use. The nominal size and outer diameter series of the European standard are completely consistent with the A series of the national standard. The diameter range of the European standard pipe is consistent with the size of the domestically controlled outer diameter pipe.
• 3) The European standard EN 10253-2 distinguishes between Class A and Class B according to whether the wall thickness of pipe fittings is the same as or increased with the wall thickness of connected straight sections. The pressure pipeline system in the power industry adopts Class B pipe fittings without Class A classification. No corresponding regulatory manual is prepared by domestic industry associations in European standards. It is recommended that when using the European standard, manufacturers with the ability to design and manufacture European standard pipe fittings should design according to the specific pressure grade, material, and pipe fitting type of the pipe fittings according to such standards and draw the manufacturing drawings of European standard pipe fittings.
• 4) After comparing the dimensions, welding end wall thickness, and shape deviation of the European and European standard pipe fittings, the European standard is generally stricter than the national standard requirements, mainly due to the finer processing and manufacturing processes in Europe than those in China. If domestic pipe fittings factories need to produce and manufacture European standard pipe fittings, they must improve their processing and manufacturing levels.
• 5) By comparison, there are slight differences in the bevel angle and bottom side of the end groove of the pipe fittings between China and Europe, depending on the processing and manufacturing characteristics of different countries and regions.
• 6) Through example calculations and comparisons, the European standard for calculating the strength of pipe fittings is more conservative and biased towards safety.
• 7) European standard pipe fittings have unique features in terms of flow area and bend radius of elbows. In addition, all pipe fittings entering the European market must comply with the EU administrative directive PED (2014/68/EU). Part 2 of EN10253-2 provides a comparative explanation of the provisions regarding the compatibility of this standard with the European Union Directive PED (2014/68/EU). This is a particular area that domestic pipe fitting factories planning to enter the European market need to pay attention to. In addition to meeting relevant European design, manufacturing, and inspection standards, they must comply with PED administrative directives.

The above conclusion points out the similarities and differences between the European and Chinese pipe fitting standards, which has certain reference significance for domestic power design institutes and domestic pipe fitting manufacturers to understand and use European pipe fitting standards in overseas projects.
Authors: Bai Jianji, Wang Fang, Deng Chenggang