Could everyone please help analyze the reasons for the fracture of the shaft in a double-suction circulation water pump?
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This post was last edited by Huang Zhao on 2019-9-17 08:40. Pump speed: 990 revolutions per minute; power: 800 KW; structure: double-suction. Pump vibration: 3.8 mm/s; bearing temperature is normal. The pump suddenly stopped delivering flow; upon inspection, it was found that the pump shaft was broken. The rolling bearings rotated smoothly with no signs of jamming. Please show the photos of the pump shaft fracture. Pump shaft material: 3Cr13 ； Rolling bearing: SKF/6320, with sleeves installed at both the front and rear ends; no thrust bearing ； Axis usage time: 9 years ； Impeller surface: Coated with an anti-cavitation coating ； Impeller material: carbon steel ； Impeller type: three-dimensional flow, double-suction pump ； Manufacturer: Shandong Shuanglun ； Pump cavitation condition: Cavitation is present; the liquid level at the suction inlet is at 1/4 of the impeller’s height, and the blackened areas on the impeller blades have developed holes.
The shaft of the single-stage double-suction centrifugal pump broke at the impeller location, as stress concentration and cavitation impacts acted in combination over time. This phenomenon has occurred 5 times already.
Is it related to the starting method? ? Was it caused by the pump starting suddenly, resulting in excessive torque? ?
Remove the bushing and take a look at the part underneath it
This post was last edited by CHANGBAISHI on 2019-9-15 07:03. It is suggested to upload a panoramic photo of the device to facilitate discussion and analysis. The speed of the equipment is low, and based on the ratio of the shaft diameter to a human foot, it seems to be φ80mm? Enough thickness, moderate power. The vibration value at this speed is worth paying attention to. I don’t know what the process requirements are. Judging from the damage to the end face of the shaft and the keys on the coupling side, the causes of this damage are likely factors such as vibration and shaft misalignment. It seems the paint on the impeller is from the commissioning of new equipment, right? It is recommended to check whether there is a sufficient prestressed connection between the device’s foot bolts and the ground foundation. For example, the nut and screw are threaded to the limit, with no actual tightening against the foundation. This is the hardest to find. It is highly recommended to conduct a thorough inspection to check whether the basic fastening is proper, whether the bolts can be tightened properly, whether sufficient prestress has been applied, whether the equipment is installed level, and whether the base shims are even and level. It should be an installation issue.
This situation is likely to occur when the variable related to the pump’s operating condition is at its highest, that is, during the start-up and stop-off of the pump. It is probably related to the material of the pump shaft and the stresses resulting from processing; in the case of older pumps, it may be more associated with material fatigue. Additionally, the release of gas from the medium at the lowest pressure at the pump inlet can cause uneven stress on the pump shaft, and repeated exposure to such conditions over time can lead to premature material fatigue of the pump shaft.
The torque is too high. Look, this shaft has existing damage; it is recommended to perform dye penetrant testing on the entire shaft to check it
The power is 800 kW, and the pump shaft diameter is φ80 mm – is that considered thick enough? ——At a torque of 800 kW, this 80mm is just a bit better than noodles, I guess……
There’s also such a neat fracture surface… That doesn’t seem right; traces of shear force can only be seen on the edges. Even if there’s an issue with the material of the shaft, it shouldn’t break at this point. If the break is truly smooth, it indicates that the material of the shaft wasn’t properly treated – or at least it means the shaft wasn’t forged properly!