Another nuclear power plant experienced a sudden increase in gearbox vibration levels when pump flow was increased to meet new test requirements. The vibration levels increased primarily in the horizontal direction on the gearbox and at 4x diesel running speed. This increase occurred on their number one safety-related component and required the plant to correct the problem within 36 hours or shut down until it was fixed.

As a result of the increased flow rate, gearbox vibration levels increased from 0.15 to 0.90 IPS at the highest point. Plant Engineering personnel subsequently determined the maximum allowable levels on the gearbox could only be 0.56 IPS-Pk (25% higher than that allowed for the pump).
A review of work history revealed that the as-left alignment was out-of-spec by 30 mils, but due to the short time frame there was not enough time to move the diesel (or pump) due to all the associated connections. Furthermore, if the gearbox was moved to correct the alignment on one side, the alignment on the other side of the gearbox would get worse. And to further complicate the problem, the gearbox had a newly found resonance at 4x diesel speed.
Upon reviewing the data and collecting additional data, VAI recommended that the plant optimize the alignment between the gearbox and pump because the high gearbox levels coincided with 2x pump speed (4x diesel). This meant the gearbox was more sensitive to the misalignment between the gearbox and the pump, as compared to the gearbox and diesel.
VAI also recommended detuning the resonance by adding mass to the top of the gearbox. A stiffener was quickly ruled out because of the time required to engineer and approve a new stiffener and because of the possibility the new resonant frequency might be close to one of the diesel forcing frequencies.
By optimizing the alignment and detuning the resonance, vibration levels dropped from 0.90 to 0.44 IPS which was well below the required 0.56 IPS limit.
The photo below shows the steel plates (370 lbs.) that were added to the gearbox.
