Sieve trays with downcomers entrainment flooding

Figure 8-123 illustrates a typical sieve tray capacity chart. Entrainment by jet flooding or limitation by downcomer flooding are two of the main capacity limiting factors. The liquid backup in the downcomer must balance the pressure drop across the tray, with the process balance [209]. 

With no clearance between the bottom of tray 3 s downcomer and tray deck 4, the resid could not flow through the downcomer. Hence, it had to drain through the sieve holes. At 4,000 Ib/hr of stripping steam, a large static head of liquid was needed to overcome the force of the up-flowing steam. This static head of resid filled the space between the tray decks and caused excessive re-entrainment of resid into the flash zone— which then flooded the wash oil trays. 

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