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Analysis and solution of welding splitting of fluid unit components
From the inspection results, the 40th forged steel meets its corresponding mechanical properties, except for the section shrinkage, which does not comply with the standard technical requirements. However, this material is insufficient to withstand the stress caused by the 85-ton weight of the rotor and the large shaft flange. This deficiency is one of the key reasons behind the de-welding and fracture of the rotor coupling bolts. Following this, issues were also identified during the construction process.
During the inspection of the coupling bolts on units No.1, No.2, and No.3, it was observed that the external force required to remove the bolts on units No.1 and No.2 was significantly greater than that for unit No.3. This discrepancy occurred despite using the same disassembly tools and the same arm length, suggesting that the installation process for the rotor coupling bolts may not have strictly followed the required procedures.
We conducted further tests on the original coupling bolts from units No.4, No.5, No.10, and No.12 of Unit 2. The results showed that the elongation values of bolts No.4, No.10, and No.12 did not meet the design tensile strength requirements. Additionally, the turbine's vibration varies under different operating conditions, and the de-welding or fracture of the coupling bolts can lead to varying degrees of damage or failure.
In the design and selection of the hydro-generator, it is essential to conduct detailed analysis and calculation of the forces acting on the unit under five different working conditions, using the maximum value as the design reference. While drainage conditions (before water filling) and water filling conditions (hydrostatic load before operation) do not directly affect the rotor coupling bolts, other operational factors must be carefully considered.
Under normal operating conditions, when the unit is under full load, it should not cause any damage to the coupling bolts. However, in practice, the flow is often uneven, leading to fluctuations and impacts. These result in increased horizontal vibrations in the water guide and vertical vibrations in the runner casing, causing strong vibrations and noise in the high-load region. This can negatively impact the integrity of the coupling bolts, contributing to de-welding and fractures.
The issue of de-welding and fracture in the coupling bolts of units No.1, No.2, and No.3 presents a major safety risk. To address this, an expert group recommended replacing the original 40 forged steel with 35CrMo round steel. With the cooperation of quality supervision and inspection stations, the mechanical properties of 35CrMo were tested. The results confirmed that 35CrMo meets the manufacturer’s design requirements and effectively prevents future failures.
In conclusion, the replacement of the rotor coupling bolts in units No.1, No.2, and No.3 at Guangxi Guiping Hydropower Plant eliminated a significant hidden safety hazard. Since the replacement, no faults or accidents have been reported in over one year, and the units are operating smoothly. Overall, the safety and reliability of the units have been greatly improved. The use of 35CrMo material has proven to be an effective solution that meets the design standards for rotor coupling bolts.