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Solving resonance problem of cooling oil pipe

Mode shape of first natural frequency of cooling oil pipe at 16.3 Hz

Cooling oil pipe of 700 MW transformer revealed excessive vibration amplitude measured at cooling oil pump at more than 200 microns, pp but acceptable level suggested from manufacturer is below 100 microns, pp. This can lead to pipe fatigue failure and crack, hence, unexpected plant shutdown and production lost. Modal impact test results showed resonance problem where cooling oil pump and fan running speed at 950 rpm is coincident with first natural frequency of pipework at around 16.3 Hz.

Structure modification is proposed to customer using finite element analysis (FEA) to design and simulation study. This technique can prevent “trial and error” and rework since we only have limited time frame during plant overhaul to rectify this problem. Sketch drawing was extended from FEA model for site implementation. Final vibration amplitude after adding more support to this pipe was maximum 18 microns, pp. First natural frequency of new modified structure is 25.6 Hz with about 57% separation margin. We have all required works done within one week to solve this problem.

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