Trays Downcomer Clearance

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Referring to Fig. 3.5, note that the downcomer B is flooding. The cause is loss of the downcomer seal. The height of the outlet weir is below the bottom edge of the downcomer from the tray above. This permits vapor to flow up downcomer B. The upflowing vapor displaces the downflowing liquid. That is, the vapor pushes the liquid up onto the tray above—which is a cause of flooding. On the other hand, Fig. 3.6 shows what happens if the bottom edge of the downcomer is too close to the tray below. The high pressure drop needed for the liquid to escape from downcomer B onto tray deck 1 causes the liquid level in downcomer B to back up onto tray deck 2. Tray 2 then floods. Once tray 2 floods, downcomer C (shown in Fig. 3.6) will also back up and flood. This process will continue until all the tray decks and downcomers above downcomer B are flooded.

On the other hand, all trays in a tower below downcomer B will lose liquid levels and dry out when flooding starts in downcomer B. Thus, the following rules apply:

• When flooding starts on a tray, all the trays above that point will also flood, but trays below that point will go dry.
• An early indication of flooding in a distillation column is loss of liquid level in the bottom of the column.
• If the downcomer clearance—which means the distance between the bottom edge of the downcomer and the tray below—is too great, the downcomer becomes unsealed. Vapor flows up the downcomer, and the trays above flood.
• If the downcomer clearance is too small, then liquid backs up in the downcomer, and the trays above flood. To calculate the height of liquid in the downcomer, due to liquid flowing through the downcomer clearance:

To guarantee a proper downcomer seal, the bottom edge of a downcomer should be about 0.5 in below the top edge of the outlet weir. This dimension should be carefully checked by process personnel when a tower is opened for inspection. It is quite easy for sloppy tray installation to distort this critical factor.

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