[Objective] Excessive horizontal vibration dominated by the once-per-revolution (1X) frequency component was observed in a vertical centrifugal pump during commissioning of a coal-fired power unit. This study aimed to distinguish structural resonance from rotor, electrical, bearing, connection, and operating-condition faults and to evaluate a field reinforcement strategy. 

[Methods] Start-up vibration velocity, spectra, and phase were recorded at three heightwise locations on the motor and supporting cylinders in the horizontal, vertical, and axial directions. Motor no-load measurements were used to assess whether the response depended on hydraulic load, while shutdown external-excitation response tests were used to identify the first-order horizontal natural frequencies of the motor, intermediate cylinder, and support cylinder. Common fault sources were screened using the available online balancing check, connection inspection, operating records, electrical observations, and bearing and electromagnetic frequency features. A frequency-separation margin was calculated to quantify proximity to the 49.95 Hz 1X excitation and was treated as a continuous descriptive quantity rather than a fixed risk or acceptance threshold. The equivalent-stiffness ratio was estimated from the squared natural-frequency ratio only when the same mode was tracked, the boundary conditions were comparable, and the added mass was small. The retained historical records documented the instrument model, measurement positions and directions, operating states, and measured peaks, but did not preserve the complete acquisition configuration; this limitation is stated explicitly. 

[Results] Before reinforcement, the horizontal vibration velocities at measurement points 1, 2, and 3 were 8.37, 6.43, and 5.92 mm/s, respectively, all above the site start-up control value of 4.5 mm/s. Horizontal response increased with structural height and remained dominated by 1X. During no-load operation, the horizontal velocities at the motor non-drive end, motor drive end, intermediate-cylinder upper flange, and intermediate position were 4.09, 3.16, 3.07, and 2.41 mm/s, respectively, while the recorded spectra remained 1X-dominant. The first-order horizontal natural frequencies of the motor, intermediate cylinder, and support cylinder were 48.74, 48.67, and 47.49 Hz, giving separation margins of only 2.4%, 2.6%, and 4.9%. These mutually consistent observations supported a diagnosis of insufficient lateral support stiffness. An adjustable horizontal support was therefore added at the motor top, and axial stiffening ribs were added to the intermediate cylinder. After reinforcement, the corresponding natural frequencies increased to 79.0, 66.5, and 79.0 Hz, and the separation margins increased to 58.2%, 33.1%, and 58.2%. The estimated relative stiffness multipliers were 2.63, 1.87, and 2.77. The horizontal start-up velocities decreased to 2.36, 1.69, and 1.83 mm/s, representing reductions of 69.1% to 73.7%, and all values were below the site control value. 

[Conclusions] The agreement among direction-dependent start-up response, persistent no-load 1X response, natural-frequency proximity, frequency migration, and post-treatment vibration reduction identifies horizontal structural resonance as the dominant fault mechanism in this pump unit. The combined diagnostic path is useful for similar field cases when speed and boundary conditions are stable and competing fault sources have been examined, but the case-specific margins should not be treated as universal acceptance criteria.