Downspouts backup

Pressure Drop and Downspout Backup. Pressure drop through the caps can be a little larger than the hydraulic gradient computed by the Davies equation [Eq. (16-4)] for tray stability. At the same time, pressure drop causes liquid to back up into the downcomer. Pressure drops, by the study of T. C. Dauphine for three sizes of caps, based on the Doctoral dissertation, Massachusetts Institute of Technology, 1939. 

A vertical section and a plan view are shown schematically in Fig. 6.14. The principal part of the tray is a horizontal sheet of perforated metal, across which the liquid flows, with the gas passing upward through the perforations. The gas, dispersed by the perforations, expands the liquid into a turbulent froth, characterized by a very large interfacial surface for mass transfer. The trays are subject to flooding because of backup of liquid in the downspouts or excessive entrainment (priming), as described earlier. 

Backup in the downspout Refer to Fig. 6.IS. The distance hj, the difference in liquid level inside and immediately outside the downspout, will be the sum of the pressure losses resulting from liquid and gas flow for the tray above... 

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