Flow-Induced Tray Vibrations

Several mechanisms were postulated to explain the origin of flow-induced vibrations. Brierly et al. (62) proposed that vapor flow fluctuations occur when wet-tray pressure drop passes through a minimum as the hole velocity varies (Fig. 6.13). The existence of such a minimum under some conditions was experimentally confirmed by Wijn (425). Near the minimum, the vapor flow rate can fluctuate between two values with the same wet-tray pressure drop. Priestman and Brown (329-331) have proposed that fluctuations are initiated when vapor passes through the liquid as a pulsating jet. At low vapor rates, liquid reaches the rims of tray perforations and starts choking the 

The mechanism of transforming the above fluctuations into vibrations capable of damaging trays is not clear. Amplification by acoustic or mechanical resonance is commonly postulated (62, 108, 329, 425). Some predictive models and correlations have been proposed (62,108, 329, 330, 425). 

Common techniques for avoiding flow-induced vibration damage are  

Reducing the fractional hole area (62, 108, 329, 425) This enhances the pressure drop and the dampening of oscillations. This technique has been powerful for overcoming flow-induced vibrations (62). 

Altering the stiffness of support beams This technique has been successful on a number of occasions (62). 

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Heavy flow-induced vibrations occurred in two towere in a train of four while operating at low ratea The vibrations caused cracking of beam to support welds and of some trays. Increasing gas flow rates overcame the problem. 

Flow-induced vibrations at gas rates dose to the weep point caused damage to trays, support beams, and beam to column siqiparts, at times within hours of operaticm at the damaging vapor rates. 

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