Steel Butt-Welding Fittings are key components in piping systems used to change direction, connect branch pipes, and alter pipe diameters. Their types and parameters are mainly specified in accordance with the national standard GB/T 12459 (equivalent to ISO 4200), chemical industry standards HG/T 20592 (European system) and HG/T 20615 (American system). The following is a detailed summary of types and parameters:

I. Types of Fittings

Based on function and shape, they are mainly classified into the following categories:

1. Elbows

Used to change the direction of the pipeline.

• 45° Elbow: for small-angle direction change.
• 90° Elbow: the most commonly used, for vertical direction change.
• 180° Elbow: for U‑bends or return pipelines.
• Special angles: such as 60°, 11.25°, etc. (custom‑made).

2. Tees

Used to branch off from the main pipe.

• Straight Tee: main pipe and branch pipe have the same diameter.
• Reducing Tee: branch pipe diameter is smaller than main pipe diameter.
• Lateral Tee: branch pipe connects to main pipe at 45° or other angles (less common, used for special fluid dynamic requirements).

3. Crosses

Used to connect pipes in four directions simultaneously.

• Straight Cross: four outlets have the same diameter.
• Reducing Cross: two opposite outlets share one diameter, and the other two share another diameter (e.g., large to large, small to small).

4. Reducers

Used to connect pipes of different diameters.

• Concentric Reducer: centers of both ends are on the same axis, commonly used in vertical pipes or pump suction nozzles.
• Eccentric Reducer: centers of both ends are not aligned, with one flat side. Widely used in horizontal pipelines to prevent air or liquid pockets (e.g., pump discharge lines).

5. Caps

Used to close the end of a pipe.

• Elliptical Cap: most commonly used, with uniform stress distribution.
• Dish Cap: shallower, easier to fabricate, but with slightly lower pressure performance.
• Conical Cap: used for special equipment connections.

6. Couplings / Nipples

Used to connect two pipes of the same diameter (usually for repair or extension).

II. Key Parameters

1. Nominal Diameter (DN)

Definition: nominal diameter of the fitting, not necessarily equal to the actual inner diameter.Series: DN15, DN20, DN25, …, DN1200, DN1400…American standard equivalent: NPS (Nominal Pipe Size), e.g., NPS 1/2, NPS 1, NPS 2.

2. Nominal Pressure (PN)

Definition: represents the maximum allowable working pressure of the fitting at the reference temperature (usually 200℃ or 250℃).Series (GB/T 12459): PN2.5, PN6, PN10, PN16, PN25, PN40, PN63, PN100.American standard equivalent: Class (Class 150, Class 300, Class 600…).

3. Radius (R) — for elbows only

The most important geometric parameter of elbows, determining flow resistance and space occupation.

• Long Radius (LR): R = 1.5DFeatures: low flow resistance, most widely used, default selection.
• Short Radius (SR): R = 1.0DFeatures: compact size, lower cost, but higher flow resistance; only used where space is limited.

4. Wall Thickness

• Schedule (Sch.): commonly used in American standards, e.g., Sch.40, Sch.80, Sch.160, XXS.
• Series: commonly used in Chinese standards, divided into Series I (corresponding to Sch.40) and Series II (corresponding to Sch.80).
• Grades: STD (Standard), XS (Extra Strong), XXS (Double Extra Strong).

5. End Preparation

Both ends of butt-welding fittings must be beveled for welding.

• Standard bevel: generally V‑shaped bevel (30°–37.5°) with a root face.
• Caps: usually plain end without bevel; however, large‑diameter caps may sometimes be beveled.