Mechanical Solutions, Inc. (MSI) was contracted to solve excessive vibration problems on Induced-Draft and Forced-Draft fans and their respective drive trains. The fans were used in a paper mill. This case history discusses the Induced Draft (ID) fan/gear problem and solution. Another case study entitled “Solving a Forced Draft Fan Problem” presents information about the motor driven Forced Draft (FD) fan issue.
MSI collected detailed natural excitation vibration data on all components of both fans and their drive trains, including their bases and piping. MSI uses a specialized operating force response vibration technique that allows the data to be used to produce very detailed animated operating deflection shapes (ODS) of the fan system. The ODS animation shows the relative motion of various system components in an exaggerated fashion to help efficiently identify the problem root cause(s) (Figure 1 and 2). MSI also performed impact modal testing while the units were operating to determine the natural frequencies and mode shapes. This is a typical vibration testing that is used on any Rotating Machinery with vibration issues.
The vibration levels on the ID fan were highest in the gearbox, considerably lower in the turbine and even lower in the fan. The frequency of the peak vibration levels on the gearbox was between 900 and 980 Hz, or about 13x the rotational speed of the drive end of the gearbox. The ODS animation showed, at this frequency, an “accordian-like motion” of the top casing of the gear, indicating a possible internal crack in the casing, oriented laterally. This gear box motion was verified as the 2nd harmonic. The modal test confirmed the natural frequency.
MSI recommended that the customer perform a non-destructive dye penetration test on the ID fan gearbox casing as soon as possible in order to avoid severe damage of the gear box. The customer, however, decided to replace the turbine and gear box with a direct drive induction motor like the one driving the FD fan.
