1 Introduction With the development of the oil and gas industry, the market has placed higher and higher demands on the exhaust pressure of natural gas compressors. In addition to high quality requirements for the main components of the compressor, higher and higher requirements are placed on the strength, toughness and weldability of the process pipeline. A branch pipe refers to a pipe fitting with holes opened on the main pipe and welded for overall reinforcement.
Reciprocating piston compressors are the most commonly used positive displacement compressors. They have the advantages of mature technology, wide pressure range, high thermal efficiency, strong adaptability, large exhaust volume adjustment range, and low material requirements. They are widely used in various gas compressor applications in the fields of petroleum and chemical industry.
Butt-welded branch pipes are often used for transitional connections between the main pipe and the safety valve branch pipe, and threaded branch pipes are often used for transitional connections between the main pipe and the temperature and pressure measuring end.
Compared with traditional connection types, branch pipe connections have outstanding advantages such as safety and reliability, reduced cost, simple construction, improved medium flow path, series standardization, and convenient design and selection.
However, the special structure and weld form of the branch pipe increase the difficulty of detection and process complexity [3]. Incomplete penetration during welding construction is difficult to scientifically detect and effectively repair. The lack of penetration in the joint has a great impact on the structure. It reduces the effective load-bearing area of the weld, reduces the strength of the welded joint and causes stress concentration, increasing the safety risk of the unit.
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2 Observation and analysis of the internal morphology of the branch pipe welding joint The appearance of the branch pipe and the main pipe weld of a unit is shown in Figure 1, and the internal section is shown in Figure 2.
As can be seen from Figure 2, there are a lot of incomplete penetration in the branch pipe connection welds during the actual production process. Possible reasons for incomplete penetration include irregular opening, too small group gap, improper welding position, too small current, too fast running speed, arc blowing, etc.
3 Cause analysis 3.1 Irregular opening The appearance of the weld using mechanical opening is shown in Figure 3. As can be seen from the figure, when other factors remain unchanged, mechanical opening is used, and most areas of the weld root are still incomplete, indicating that irregular opening is not the main reason for the incomplete penetration of the weld.
3.2 Adjusting the group gap After adjusting the group gap, the appearance of the weld using the flame opening method is shown in Figure 4, and the appearance of the weld using the mechanical opening method is shown in Figure 5. As can be seen from Figures 4 and 5, most areas have been welded through, indicating that the group gap is the main reason for incomplete penetration.
The penetration effect of vertical welding and overhead welding is not good. Compared with flat welding, these two positions cannot cause the molten droplet to detach from the end of the welding wire under gravity, which is conducive to the transition of the molten pool.
Manual tungsten inert gas arc welding is used for base laying. The accessibility of the welding gun in the flat welding position is better and the molten pool is easy to observe. The welder is more likely to use short arc welding, the heat is more concentrated, the heat affected zone is smaller, and the deformation is smaller, which is conducive to improving the welding quality.
The relationship between the gap and the penetration rate of different groups is shown in Table 1. It can be seen from Table 1 that too small and too large gaps are not conducive to penetration. If the gap is too small, the welding gun is not easy to penetrate into the root of the weld, and the arc cannot be well transferred to the back of the weld through the gap.
If the gap is too large, the molten hole is too large, it is very easy to burn through, and the back of the weld is too convex, which is not conducive to gas circulation, and it is not conducive to connecting temperature measurement, pressure measurement and other parts. When the gap is 2.5~5 mm, the penetration length ratio of the single branch pipe platform weld is greatly improved.
Under the premise of ensuring a reasonable gap, the welding penetration length of a single branch pipe using the mechanical opening method is slightly greater than that of the flame opening method, and the weld appearance quality is also better than that of the flame opening method. This is because the use of mechanical opening is conducive to ensuring uniform gaps between the groups, and the weld width difference and residual height difference are lower.
3.3 On the basis of ensuring that the welding position adopts machined grooves and ensuring the gap between the welded parts, the welding position is adjusted and the weldment is welded in the flat welding position. The results are shown in Figures 6 and 7. It can be seen from the figure that the penetration area of the same branch pipe station has reached 100%.
Radiographic flaw detection was performed on the complete sample, and all areas met the domain level requirements, indicating that all welds were fully penetrated and there were no unfused or included defects.
4 Parameter verification The weld microstructure of the qualified sample after cutting, grinding and corrosion is shown in Figure 8, and the heat-affected zone microstructure is shown in Figure 9. The structure of Figure 8 is mainly ferrite and pearlite, and the structure of Figure 9 is mainly ferrite and slat body.
There is no Wei-type structure in the weld zone and heat-affected zone, and acicular ferrite structure appears. The acicular ferrite nucleates and grows in the austenite crystal, which has the effect of refining the austenite grains and improving the metal toughness of the weld and heat-affected zone.
The needle-shaped ferrite has fine subgrain structures and high-density movable dislocations, which are easy to achieve multi-slip and beneficial to the improvement of performance [6].
Using the same parameters to weld the pipe-to-pipe butt test pieces, the weld zone and heat-affected zone were tested to have similar structures. The bending specimens produced met the standard requirements, and the tensile and impact specimens produced exceeded the standard requirements.
5 Preventive measures (1) Strictly inspect the incoming branch pipes according to their manufacturing standards to ensure that their shapes, dimensions and tolerances meet the requirements, prevent non-standard manufacturing from affecting the quality of the welds, and reasonably control the gap when assembling.
(2) Use welders who meet the requirements to weld to prevent the welder’s skills from affecting the penetration of the branch pipe. Prefabricate the branch pipe and the main pipe. The branch pipe and the main pipe are welded in the flat welding position, which is conducive to the penetration of the branch pipe.
(3) Strictly clean, process, assemble, weld, record, and inspect according to the process requirements.
6 Conclusion (1) Too small or too large welding gap is the main reason affecting whether the branch pipe is welded through. The gap between the branch pipe and the main pipe should be 2.5~5 mm.
(2) The branch pipe and the main pipe are welded in a flat welding position. Gravity pushes the molten pool to transfer, the accessibility of the welding gun is better, and the welder can observe the molten pool more easily, which is conducive to the branch pipe penetration.
(3) Under the condition of ensuring a reasonable gap, the use of mechanical opening method is conducive to improving the penetration length ratio of a single branch pipe and the appearance quality of the weld.
Post time: Jan-14-2025
