Case Study On APH Vibration

Vibration and noise problems are caused when air or flue gases flow over tube bundles, which may be arranged in line or staggered. Vortices are formed and shed beyond the wake of the tubes, resulting in harmonically varying forces on the tubes perpendicular to the flow direction. It is a self-excited vibration. If the frequency of vibration of the Von-Karman vortices, as they are called, coincides with the natural frequency of vibration of the tube bank, resonance occurs, which leads to tube vibration.

Another phenomenon that occurs with vortex shedding is acoustic vibration, leading to noise and high gas pressure drop. The duct or the bundle enclosure vibrates when the acoustic oscillation frequency coincides with the vortex shedding frequency. The acoustic oscillation is normal to both the direction of gas flow and the tube length.

Here is the case of air preheater ducts vibration with huge noise, which leads to frequent failures of the duct & casing plates.

Continuous repairing in casing plates

APH Vibration

At the time of our visit the plant was in operation, and APH vibration levels were seen. It’s about a humming noise observed from APH. Physical dimensions were collected for drawing preparation and analysis. The following are the observations based on calculations,

There are three types of frequencies: Vortex shedding frequency, Natural frequency & Acoustic frequency. In any case either of the two frequencies is close to the other, then vibration would not be avoided.

Here, in this case, all three frequencies were very close to each other, so we provided an engineering modification to shift the frequency range.

Site Observation

Whenever the steam load crossed 85 TPh. Physical conditions of APH were checked, and found that airside casings were found to be cracked due to vibration.
Whenever there is an interference of the frequency of the tube bundle with the frequency of the ducts, this will lead to vibration.
In this case, all-natural frequency, Vertex shedding frequency & Acoustic frequency are very close to each other. We proposed a modification to shift the natural frequency.
Boiler operation was reviewed and found to be with excess air operation; this shall be attended to. High excess air operation will lead to more air, and this will also cause vibration.