Axial Resonance Troubleshooting and Resolution of an Induced Draft Fan

Abstract This case study is focused on accurately identifying the root cause of the observed resonance on the induced draft fan bearing in the axial direction, by analyzing the vibration data during steady state, and transient modes recorded during frequent balancing jobs. Also, the interpretation of Modal Analysis testing for the fan rotor in the fixed and free-free conditions to identify the influence of the structure support stiffness on the system natural frequency (bearing dependent modes). During normal operation of the fan, it was observed that vibration response in the axial direction is increasing gradually and exceeding the trip limit, however the vibration levels reduced well below alarm limit after fan cleaning and balancing. Each fan required balancing almost 3 times yearly which affected the plant availability. Closer look on the vibration levels before balancing showed that axial vibration level is more than 5 times the horizontal vibration levels. 1X vibration polar plots in the axial direction during startup and shutdown showed phase lag around 70 degrees between full speed 1X vibration vector and low speed 1X vibration vector, to be understood that the fan is approaching a natural frequency close to running speed. Modal Analysis study carried out on the fan rotor components (blade, impeller fixed mode, and impeller free-free mode), which revealed that the amplification of the 1X vibration observed in the axial direction is due to resonance, as the fan most likely found to be running at speed close to a lightly damped rotor natural frequency (first rigid mode). It was concluded that, a change in the structure support stiffness is the root cause for this resonance condition, and it was recommended to inspect the fan rotor housing and supports to find out a resolution for the resonance condition in the axial direction and to shift the natural frequency with enough separation margin away from any forcing frequencies (i.e., 1X … etc.). After site inspection and multiple investigations, the root cause of the change in the structure support stiffness was identified to be because of improper tightening torque during fan installation, which was changed as per OEM recommendations. Same solution was applied on all induced draft fans onsite (10 fans in total). Later, fans showed stable vibration levels for longer operation periods without the need for frequent balancing activities.