Flood trays downcomer clearance

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 ... 

Tray type Weir height Weir length Downcomer clearance Tray spacing Foaming factor Fraction of flood Surface tension Liquid viscosity... 

Sieve trays will be used with 60 cm spacing, 6 cm weir height, 0.6 cm hole diameter, 0.25 cm tray thickness, 5 cm downcomer clearance, and hole area 10% of the total tray area. The foaming factor is 0.80 and the froth density in the downcomer is 0.5. The target fraction of flood velocity is 0.70. 

In a fourth example (2066), bubble caps were installed under the tray panels this column flooded at 30 to 40 percent of design. In a fifth example (2066), the downcomer clearance was about 7 to 8 in at the feed tray due to miscommunication, and premature flooding (due to lack of downcomer seal) resulted. 

In both downcomer back up and choke cases, downcomer liquid inventory increases and downcomer liquid backs up until the downcomer froth level reaches the tray above H > Hs). This phenomenon is called downcomer flood. When downcomer flood occurs to any tray, the whole tower will be flooded very quickly. A tower under downcomer flood provides virtually no distillation. In contrast, under tray flood, liquid can still leave the tower and the tower could stiU operate if the control system allows it although distillation efficiency suffers. Downcomer flood can be prevented in design by providing adequate downcomer area and clearance underneath the downcomer and minimizing tray pressure drop. Reducing reflux rate in operation could be effective in avoiding downcomer flood in operation. 

On the other hand, if the bottom of the downcomer is too close to the tray below, then the "head loss under the downcomer" will be excessive. Typically, a minimum downcomer clearance is 1.5 to 2 inch. Too small a downcomer clearance will result in restricting the liquid flow from the downcomer. This will also cause excessive downcomer backup and flooding. Check the correct downcomer clearance on the vender tray drawings prior to the tower inspection. 

Most trays have outlet weirs devoted to maintaining the downcomer seal. But some trays have inlet weirs too, or inlet weirs, but no outlet weirs. A sketch of an inlet weir is shown in Fig. 9.2. Note the horizontal distance between the downcomer and the inlet weir (dimension x). This distance ought to be equal to or greater than the downcomer clearance—that is, the vertical space between the tray floor and the bottom edge of the downcomer. Unfortunately, a small deformation of the downcomer may push the downcomer quite close to the inlet weir. The resulting reduction in the horizontal clearance between the inlet weir and the downcomer will restrict the liquid flow. This will cause downcomer backup and tray flooding of the trays above. 

Referring to Figure 8-63, the weir height, h, must always be greater than the clearance under the downcomer, i.e., between bottom of downcomer and tray floor, hdci-Always avoid too low clearance as this can cause flooding of liquid in the downcomer. There are flow conditions... 

Fouling and corrosion. If the service is a fouling one, dirt and pol3oner may accumulate under the downcomer and restrict the flow area. This may cause excessive backup, premature flooding, and maldistribution of liquid to the tray. Clearance under the downcomer should never be less than 2 in (38, 86, 123, 172, 192) in order to avoid blockage. It is best to avoid clearances smaller than 1 in, particularly if fouling may occur. 

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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