Pipe Joint Solutions Butt Weld Socket Weld

I. Technical principle and structural design

1. Butt welding

Connection method: Align the end faces of the two pipes directly and fuse them by filling solder to form a continuous weld.

Structural features:

• The weld is consistent with the pipe wall thickness, there is no protruding structure, and the flow resistance is small.
• The groove (V-type, U-type or J-type) needs to be processed to ensure the penetration rate.
• Standard specification: ASME B16.25 specifies the dimensional tolerance and surface treatment requirements of the butt welding groove.

2. Socket welding

Connection method: Insert the pipe into the pipe fitting with a socket (such as a valve or a tee), and weld the fillet weld along the outer edge of the pipe fitting.

Structural features:

• There is a 1.6mm expansion gap between the pipe and the pipe fitting (ASME B31.3 requirement).
• The weld is located between the outer wall of the pipe and the socket of the pipe fitting to form a mechanical lock.

Standard specification: ASME B16.11 defines the pressure grade and size series of socket welding pipe fittings.

II. Comparison of process requirements and operation procedures

1. Butt welding construction process

Groove processing:

• Use a groove machine to cut a V-shaped groove (angle 60°±5°, blunt edge 1.6mm).
• Clean the oil and rust within 20mm of the inner and outer walls of the groove.

Assembly and spot welding:

• Use a clamp to ensure that the concentricity deviation of the pipe is ≤1mm, and fix it at 3~4 spot welds.

Multi-layer welding:

• The base welding adopts TIG process (argon purity ≥99.99%), and the filling welding and cover welding can be selected by SMAW or GMAW.
• The interlayer temperature is controlled at 150~250℃ (carbon steel) or ≤150℃ (stainless steel).

Post-weld treatment:

• Perform 100% radiographic testing (RT) or ultrasonic testing (UT) to eliminate unfused or pore defects.

2. Socket welding construction process

Pipeline pretreatment:

• Cut the pipe to a length tolerance of ±0.8mm to ensure that the insertion depth meets the requirements.

Assembly and positioning:

• Insert the pipe into the socket of the pipe fitting, leaving a 1.6mm axial gap (to prevent thermal stress cracking).

Fillet weld welding:

• Use SMAW or GTAW process to weld 1~2 welds around the pipe mouth.
• The height of the weld leg must be ≥1.09 times the pipe wall thickness (ASME B31.3 regulations).

Inspection and acceptance:

• Visual inspection (VT) to check the continuity of the weld, and perform penetration testing (PT) if necessary.

III. Comparison of core performance and economic efficiency

IV. Analysis of industry application scenarios

1. Typical applications of butt welding

Long-distance oil and gas pipelines:

Requirements: withstand high pressure (≥10MPa), resist hydrogen sulfide stress corrosion.

Solution: X65 steel grade pipeline + U-shaped groove, multi-pass welding with post-weld heat treatment (PWHT).

Nuclear power main steam pipeline:

Requirement: zero leakage, thermal fatigue resistance.

Solution: Austenitic stainless steel + automatic orbital welding, 100% radiographic flaw detection.

2. Typical applications of socket welding

Chemical instrument pressure pipe:

Requirement: small pipe diameter (DN15~25), easy to disassemble and calibrate.

Solution: 316L stainless steel socket welding valve + gas shielded welding of fillet weld.

Ship ballast water system:

Requirement: fast installation, vibration resistance.

Solution: galvanized steel pipe with socket welding flange, epoxy anti-corrosion coating after welding.

V. Key factors for selection decision

System pressure and temperature:

• High pressure and high temperature (P≥5MPa, T≥200℃) give priority to butt welding.
• Medium and low pressure and normal temperature systems can consider socket welding to reduce costs.

Pipe diameter and space restrictions:

• When the pipe diameter is below DN80 and the installation space is narrow, socket welding is easier to operate.
• For large pipe diameters (≥DN150), butt welding must be used to ensure structural strength.

Maintenance and inspection requirements:

• For pipelines that need to be frequently disassembled and repaired (such as sampling systems), socket welding is more economical.
• For pipelines involving highly toxic or radioactive media, the full penetration structure of butt welding is safer.

VI. Common problems and solutions

1. Typical defects of butt welding

Incomplete penetration: adjust the groove angle or increase the welding current to ensure root fusion.

Heat-affected zone cracks: control the interlayer temperature, slow cooling after welding or perform stress relief heat treatment.

2. Typical defects of socket welding

Insufficient gap: check with a feeler gauge before installation to ensure a 1.6mm expansion gap.

The weld is not filled: increase the number of welds or use a larger diameter electrode.

VII. Industry standards and compliance requirements

ASME B31.3: specifies the process assessment and inspection standards for butt welding and socket welding.

API 570: requires wall thickness inspection of butt-welded pipes every 5 years, and socket welding needs to focus on checking the corrosion of fillet welds.

ISO 15614-1: Certification requirements for welding procedure specifications (WPS).

VIII. Future development trends

Automated welding: Robotic butt welding system improves the construction efficiency of large-diameter pipes with an accuracy of ±0.2mm.

Argon-free technology: Flux-Cored Wire reduces the reliance on gas protection during butt welding.

Intelligent detection: AI-driven real-time weld quality monitoring system (such as Olympus OmniScan X3).