Vibration Stress Relief Technology HB3000+ Prevents Leakage of Transformer Oil Tanks

Abstract Leakage of transformer oil tanks is caused by welding defects, residual welding stress, and working stress during production, transportation, installation, and use. The main parts of the leakage are at the lap joints, T-joints, and other welding parts. Using vibration stress relief technology, post-welding stress relief treatment of large transformer welded oil tanks is carried out in conjunction with leakage inspection, which timely discovers potential welding defects in the welding joints, reduces the level of residual welding stress, and thus avoids the leakage phenomenon of the oil tank during use.

Keywords Vibration Stress Relief, Transformer Oil Tank, Residual Stress, Welding Defects

Chinese Library Classification Number TM403.9

Document Identification Code B

The oil tank of a large transformer is a closed container in the form of a welded structure, with complex weld shapes, and the total length of the welds is at least tens of meters and up to hundreds of meters. Since the internal tank contains the transformer body and transformer oil, even a very small hole in the weld will cause leakage, which will cause certain harm to the transformer: ① It will pollute the transformer shell and the surrounding environment; ② The reduction of oil in the oil tank will cause the temperature of the transformer to rise; ③ Leaking oil encountering an open flame can easily cause a fire. Although manufacturers have taken many measures to prevent oil tank leakage accidents [1.2], leakage oil incidents still occur. Therefore, we have adopted vibration stress relief technology for post-welding stress relief treatment of the oil tank, effectively reducing the phenomenon of leaking oil and achieving good social and economic benefits.

1 Leakage Oil Parts and Reasons of the Oil Tank

1.1 Leakage Parts and Methods

Generally speaking, the chances of leakage are very small for surface welds that are easy to weld, while the chances of leakage are relatively large for parts that are not easy to weld.

The main parts of the oil tank leakage are two: one is the part where the oil tank is connected with the pipeline; the other is the welding joint of the oil tank box. The former belongs to the on-site installation problem, as long as the correct and reasonable construction is ensured, it is generally easier to prevent and solve; while the latter has more influencing factors and is more difficult to solve. From the perspective of on-site leakage, the leakage of the welding joint is mainly around the inner and outer corner welds of the flange, the butt weld between non-magnetic steel and low carbon steel, the oil tank outer wall splicing weld, especially the T-weld, cross weld, lap weld, and edge corner joint weld, and other stress concentration areas and defect-prone areas.

The path of the leaking oil is not in the same part of the inner and outer sides of the steel plate, that is to say, the oil leakage point on the outer wall of the oil tank is at a certain part, while the oil leakage point on the inner wall is at another part. The welds of the oil tank are generally double-sided, and the part that is not welded through in the middle forms a hidden middle channel. As long as there is a defect that penetrates the weld on the inner side, the oil will flow into the middle channel. If there is also a defect that penetrates the weld on the outer side, the oil in the middle channel will leak out. The most serious situation is that if the reinforcement rib or the reinforcement groove plate just covers the weld, this kind of leakage is difficult to detect........................

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