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Becht has been involved in a number of flow induced vibration problems. The problems typically occur in heat exchangers, boilers, flue gas coolers and piping downstream of bypass valves used for pressure letdown. Vendors typically design their equipment to operate outside the range of fluid flows to avoid the problems. However, problems can occur at off design conditions, e.g., process upsets, and outages where more flow is diverted through a piece of equipment than  the design rates   or unit capacity "creep" where  the flow has increased flow beyond original design throughput. The driving forces are fluid phenomena such as vortex shedding, acoustic resonance, and fluid elastic instability. These phenomena can cause vibration and failures in tubes, tube banks, large diameter, thin-wall piping and small branch connection attached to larger diameter piping. Vibration amplitude is magnified if the exciting force is +/- 20% of a component's natural frequency. There are a number of industry criteria to evaluate the potential for vibration. One such criterion we have used to evaluate instability in tube banks in flue gas coolers is shown on the chart. Published industry data on the regions of Stable (low probability) and Unstable (high probability) regions of tube bank instability are shown. Overlaid on the data are the data for a superheater (SH) and high pressure section (HP1) of a flue gas cooler on the back end of a Fluid Catalytic Cracking Unit. The 1250F flue is used to generate steam. The data is shown for two mass flow rates, normal operating and   a higher flow rate when one of the units is offline for repair. At the higher flow rates the SH and HP1 sections move into the lower end of the Unstable region.