Oscillation of aerated liquid on perforated plates and flow-induced vibration of column trays

Abstract

The occasional structural failure of distillation and absorption trays operating at low vapour rates has been attributed in the literature to fatigue of tray parts caused by flow-induced vibration. We suggest that synchronous oscillation of gas flow through tray perforations can occur when the perforations are evenly spaced, and the velocity through them is not too high. The controlling mechanism is that of a Helmholtz resonator, in which an oscillating mass of liquid near to each perforation cyclically exchanges kinetic energy with potential energy (head) in the bulk liquid across the tray. The rate of exchange is limited by the wave speed in the bulk liquid. A correlation scheme is suggested for the oscillation frequency based on literature data. We show that effective elastic constants can be calculated for perforated tray panels, and draw attention to the stress concentration around holes near the tray edge that exposes trays to fatigue cracking during vibration. Vibration analysis of an idealised tray structure identified the first 30 modes with resonant frequencies between 25 and 70Hz, the range where excitation by flow-induced pressure pulsations is possible. Recommendations are made to avoid vibrational damage caused by such pulsations.