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LPI Evaluation, Root Cause Analysis, and Corrective Action of LNG Expander Wheel

LPI Evaluation, Root Cause Analysis, and Corrective Action of LNG Expander Wheel Fracture

LPI was requested to perform an engineering evaluation of a fractured expander wheel to determine the root cause of failure and corrective actions required to prevent future failures. The fractured expander wheel failed after only a short time in service. The expander wheel had a history of numerous failures in the past and went through several design changes without solution of the problem.

LPI performed metallurgical investigation, material strength testing, and analytical studies to determine the root cause of the failure. The metallurgical investigation revealed that it was a fatigue failure. Tensile testing of the expander wheel material revealed that the material met the specified material requirements. Fracture toughness of the expander wheel in accordance with ASTM Standard E1820 determined the fracture toughness of the material. Crack growth evaluation of the expander wheel calculated that the dynamic loading had the potential to cause a fatigue fracture of the expander wheel in the time period reported. Finite element stress analysis (FEA) revealed that the maximum stress in the expander wheel coincided with the location of the fracture origin.

Detailed dynamic analysis investigated the gyroscopic effects on the shift of natural frequencies and captured the critical frequencies via a Campbell diagram analysis. The results of this analysis indicated a vibration mode to be potentially damaging to the expander wheel and the likely cause of failure.

In order to operate the machine at the maximum efficiency, LPI recommended that the manufacturer of the expander wheel optimize the design, particularly in view of the repeated fatigue failures. Modifications to the expander wheel design included stiffening the outer rim, but not at the expense of additional weight which would have increased the problem of the shaft whirl. The investigation also looked into the feasibility of stiffening the shaft. Since design modifications have been made no reported expander wheels failures have been reported to date.


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