According to the Chinese standard: The principle of pipe fittings selection

Pipe fittings are essential to connect, divert, or close the pipe ends in a piping system. They come in various shapes, sizes, and metal materials, each designed for specific purposes and applications. The right choice of pipe fittings can significantly impact a pipeline system’s efficiency, safety, and longevity.

1. Pipe fitting selection principle

The selection principle of pipe fittings is mainly to ensure the piping system’s safe, stable, and efficient operation. The following are some common principles of selection of pipe fittings:

• 1). The selection of pipe fittings refers to the pipeline level (class), design conditions (such as design temperature design pressure), media properties, material processing technology performance, welding performance, economy, and use to reasonably determine the temperature-pressure level of the fittings, fittings connection form.
• 2). The selection of fittings should be consistent with the corresponding standards, such as “petrochemical pipeline design equipment selection guidelines” (SH 3059), “Industrial Metal Piping Design Code” (GB 50316), and so on.
• 3). Pipe fittings are connected in various forms; the corresponding structure is also different, commonly used butt-welding connection, threaded connection, socket weld connection, flange connection, and other four types of connection forms.

According to international common practice, DN50 and above pipelines mostly use butt welding pipe fittings, and DN40 and below mostly use bending, socket welding pipe fittings, or tapered pipe thread connection pipe fittings. When selecting butt welded pipe fittings, the pipe table number of the pipe fitting should be consistent with the pipe table number of the connected pipe according to the principle of equal strength.

2. The pipe fittings selection of commonly used standards and norms

• GB 12459-90 steel butt-welding seamless pipe fittings;
• GB/T 13401-92 steel butt-weld fittings;
• GB/T 14383-93 forged steel socket weld fittings;
• GB/T 14626-93 Forged steel threaded pipe fittings;
• GB/T 17185-1997 Steel flange fittings;
• SH 3408-1996 Butt-welded seamless steel pipe fittings;
• SH 3409-1996 Butt-welded pipe fittings made of steel plate;
• SH 3410-1996 Forged steel socket weld fittings;
• HG/T 21634-1990 Forged steel socket welded pipe fittings;
• HG/T 21635-1990 Seamless butt-welded pipe fittings of carbon steel and low alloy steel;
• HG/T 21631-1990 Steel seamless butt-welded pipe fittings;
• SY/T 0510-1998 Steel butt-welded pipe fittings;
• SY 5257-91 steel pipe bends;
• ISO 3419 non-alloy steel and alloy steel butt-welded pipe fittings;
• ISO 5251 stainless steel butt-welded pipe fittings;
• ASME B16.9 Factory fabricated forged steel butt-weld fittings;
• ASME B16.11 Socket weld and threaded forged steel pipe fittings;
• ASME B16.28 Forged steel butt weld small radius elbows and return elbows;
• MSS SP-43 Forged stainless steel butt weld fittings;
• MSS SP-75 Forged steel butt weld fittings;
• MSS SP-79 Socket weld reducer inserts;
• MSS SP-83 Socket Weld and Threaded Live Fittings;
• MSS SP-95 Reducing pipe and round plugs;
• MSS SP-97 Integral reinforced pipe seats with socket weld, threaded, and butt weld ends;
• JIS B2311 General purpose steel butt-weld fittings;
• JIS B2312 Steel butt-weld fittings;
• JIS B2313 Butt-welded fittings made of steel sheet;
• JIS B2316 Socket weld fittings made of steel;
• DIN 2605 Steel butt-welded pipe fittings elbows and bends;
• DIN 2615 Butt-welded pipe fittings of steel tees;
• DIN 2616 Steel butt-welded pipe fittings reducers;
• DIN 2617 Steel butt-welded pipe fittings caps.

3. The choice of branch pipe connectors

1). Due to the different degrees of standardization of fittings in different countries, branch pipe connections and the choice of fittings are different. Generally, the branch pipe connection more molded branch pipe connectors, welded lead-out connectors, and branch pipe directly welded to the main pipe and other connection forms.
2). The choice of branch pipe connectors is mainly based on the pipeline grade determined in the flange pressure level or nominal pressure (PN) to select the branch pipe connectors.
In general, when the nominal pressure of the flange PN ≤ 2.5 MPa (pounds series PN ≤ 150LB), the branch pipe is welded directly to the main pipe; when the PN24.0 MPa (pounds series PN ≥ 300LB), according to the main pipe and branch pipe nominal diameter of the different butt-welding tee, butt-welding to strengthen the fittings, socket weld or threaded connection tee, socket weld or screw to strengthen the fittings in the order of Selection.
3). Pipe table number of branch pipe connectors
It is worth noting that determining the pipe table number of the branch pipe connectors should be based on the principle of equal strength of the pipe and fittings and the fittings and pipe connection method. For example, when the pipe and fittings with butt-welding connection, the pipe, and fittings of the pipe table number should be the same; and when the pipe and fittings with socket weld or threaded connection, the standard socket weld and threaded fittings of the pipe table number is only Sch80, Sch60, XXS three grades, the lowest pipe table number is Sch80, that is to say, as long as when the socket weld or threaded fittings connected to the pipe table number is not greater than the Sch80, socket weld and threaded fittings, the pipe table number is not greater than the Sch80, the socket weld or threaded fittings. That is to say, as long as the table number of the pipe to which the socket welded or threaded fittings are connected is not greater than Sch80, the table number of the pipe for socket welded or threaded fittings is Sch80. In comparison, when the table number of the pipe to which the socket welded or threaded fittings are connected is greater than Sch80, the table number of the pipe for socket welded or threaded fittings is Sch60 or XXS.
In addition, when butt-welding tee main, branch pipe connected to the pipe of the pipe table number is different at the same time, it should be noted that the tee main, branch pipe end of the pipe table number should be respectively connected to the pipe of the pipe table number is the same, so it is very convenient for the pipeline construction, fittings do not have to be beveled on-site.

4). Reinforcement of welded branch pipes

It is worth noting that the choice of branch pipe welded directly to the main pipe in this connection accounts for the need to strengthen the pipe. Branch pipe reinforcement methods in ASME B31.3, “petrochemical pipeline design equipment selection guidelines” (SH 3059), “Industrial Metal Piping Design Code” (GB 50316), and other standards and specifications are described in detail.

4. Connection form of branch pipes and main pipes

• 1) In general, the design pressure of 22.0 MPa, the design temperature of more than 250 ° C, and the ratio of the nominal diameter of the branch pipe are greater than 0.8 or subject to mechanical vibration, pressure pulsation, and rapid changes in temperature of the pipeline branch, should be used tee, 45 ° diagonal tee, and four-way connection;
• 2) Nominal diameter ≤ 40 mm pipe should be socket welded (or threaded) forging tee;
• 3) Nominal diameter ≥ 50 mm pipe, should be butt-welded tee.

5. Reducer selection

• 1). Relative to the branch pipe connectors, the choice of reducer is simple. Also, according to the principle of equal strength, the reducer should be used with the same pipe connected to the pipe table number.
• 2). Whether to choose concentric or eccentric reducers should be based on the requirements of the process flow diagram or the requirements of the piping arrangement. For example, the horizontally placed pipe reducer on the pipe gallery is usually an eccentric reducer with a flat bottom, and the inlet pipe of the pump is usually an eccentric reducer; whether it is a flat top or a flat bottom needs to be determined on a case-by-case basis.
• 3). Usually, for DN250 mm pipe on the reducer, more butt-welding reducer, while for DN ≤ 40 mm pipe, the socket weld reducer hoop. However, galvanized pipes on the reducer are to be used as threaded connections.

6. Common pipe fittings series

Domestic standards for various pipe fittings, seamless butt-welding pipe fittings, and seamed butt-welding fittings are equivalent to ASME B16.9 and ASME B16.28. Forged steel socket weld and threaded fittings are equivalent to ASME B16.11. However, the structure of the standard fittings of the same size type differs.
The commonly used instrument nozzles include nozzle, straight thermometer nozzle, straight bimetallic thermometer nozzle, inclined thermometer nozzle, and inclined bimetallic thermometer nozzle, which are generally selected according to SYJT3000.
Pipe fittings of the nominal diameter, outside diameter, wall thickness series, and the corresponding pipe size series are consistent.
1) Outer diameter series
National pipe fittings standards GB 12459, GB/T 13401, and GB/T 14383 outside diameter are divided into A, B two series, of which the A series and the “welded and seamless steel pipe” ASME B36.10 of the pipe outside diameter series is consistent, that is, with the ASME B16.9. ASME B16.28 is the same, and the B series is along the past of China’s refining and other industries to use the series B series of DN ≤ 150 mm (except DN80) in the outside diameter series is often referred to as the “small outside diameter” series.
2) Wall thickness series
National standards GB 12459, GB/T 14383 and GB/T 14626 wall thickness is expressed in the tube table number, of which GB 12459 has twelve series, GB/T 14383 only Sch80 and Sch160 two series, GB/T 14626 has Sch80, Sch160 and XXS three series; GB/T 13401 wall thickness respectively, according to the weight and tube table number of the two methods of expression, there are XXS, STD, XS and Sch5s, Sch10s, Sch20s, Sch40, Sch80, and so on 8 series.

7. Conclusion

The selection of pipe fittings should be based on comprehensive consideration of their metal materials, specifications, performance, cost, and application requirements to ensure the piping system’s stable, safe, and efficient operation.