Papers and Case Studies

Hydrocracker Cooling Tower Fan

Since instillation, the motor of fan 21F030 has experienced elevated vibration amplitudes when operating speeds are in certain zones. The maximum vibration amplitude was measured at the OBH location of the motor reaching 0.98 ips 0-p at 1,252.5 cpm (1x motor running speed) in November 2021. This fan is 1 of 4 in the cooling tower and while it is not the only fan that records high vibration amplitudes dependent on running speed, it is the one that records the highest amplitudes. These fans are run off of a VFD and automatically fluctuate depending on ambient conditions. “Start-up” and “Coast-down” plots were collected for 21F030 when the ambient temperature was cold enough to be able to slow down the fans and speed them up without affecting process conditions. The plots showed 3 different critical speeds, with 1,250 rpm being the most severe, likely due to resonant conditions within the motor frame.

Figure 1: Coast-Down Plot of 21F030 Showing Different Critical Speeds

Reliability Recommendation:

In the meantime, lockout critical speeds if possible and run other fans at higher or lower load depending on what is necessary to obtain proper process conditions.

Perform an FEA to design a more robust motor mounting structure to move potential resonant conditions out of the operating range.

Due to the motor recording high vibration amplitudes since it was installed into service, investigate the design and acceptance standards for the cooling towers to understand why these critical speeds are present within the designed operating speed range of the fans.