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 ... 

The froth height in the center downcomers in the bottom section is only slightly above 30 percent, and increasing the downcomer clearance will suffice to overcome the problem. However, this is unlikely to suffice for the side downcomers in the bottom section. In this example, idle clearance under the downcomer will be increased to 2.0 in in the center downcomers, and to 2.25 in in the side downcomers. The weir height on idle cenler-to-side flow trays in the bottom section will be lowered to 1.5 in to lower tray pressure drop. 

Although there is no need to change the downcomer clearance in the top section, it is a good idea. The froth in the downcomer bottlenecks the top section, the head loss in the downcomer apron is quite high (1.26 in) and the tray spacing relatively low. The clearance in idle top section will therefore also be increased to 2 in. 

Note that in the top trays the weir height is now equal to the clears ance under the downcomer (both are 2 in). In the bottom section, the outlet weirs are shorter than the clearances under the downcomers. This may raise concerns about having adequate ssal on the trays. However, the practice of using outlet weirs shorter than the downcomer clearances is usually edequate for high liquid loads (1), such... 

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. 

This clearance should be greater than the normal clearance imder the tray downcomers, and should be at least 2 in, because the bottom seal pan is one of the most sensitive areas for solids accumulation. 

Dimensions of small magnitude, such as clearances under downcomers, weir heights, notch dimensions, seal pan widths, distances between downcomers and inlet weirs, downcomer widths (for narrow downcomers), clearances between chimney tray or redis-... 

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. 

Valve trays are most vulnerable to vapor gap problems, particularly if their downcomers and/or downcomer clearances are small. The... 

Here, denotes the relative hquid holdup in the downcomer, the height of downcomer clearance, the pressure loss of a tray expressed in clear liquid height, and the clear liquid height on the tray (see Sect. 5.4.1.3). The orifice dis-... 

In column design, increasing tray spacing, downcomer clearance, and the number of tray flow passes can effectively avoid downcomer backup. Increasing tray spacing is... 

Specifying the downcomer clearance above the tray below is one of our more critical design criteria. If the clearance is too small, the... 

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. 

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