Clearance under downcomer

Csbf Capacity parameter corrected for m/s ft/s Fd Clearance under downcomer mm in... 

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-... 

Detailed measurement of internals can be a long, tedious process in tall trayed columns. One recommended shortcut is to use templates for measuring repetitious internals (e.g., clearances under downcomers). Another shortcut which is often satisfactory is to carry out detailed measurements on the first 10 to 20 trays, and then carefully eyeball the others with periodic spot checks. However, it is important to ensure that the dimensions of all internals on all trays are at least carefully looked at. There have been cases (e.g., 102) where irregularities on a single tray were sufficient to bottleneck an entire column. 

Small clearance under downcomer Small downcomer top X X... 

Zc liquid height or liquid hold up in. of liquid hc clearance under downcomer in. 

The major factors governing the proper design far clearance under the downcomer (see Figure 8-63), and the distance between the bottom of the downcomer and the tray it is emptying onto are [190] (a) downcomer sealing, (b) downcomer pressure drop, and (c) fouling and/or corrosive nature of the fluids. TTie smaller the clearance, the more stable will be the tray start-up due to the greater restriction to vapor flow into and up the empty liquid downcomer. 

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... 

An optimal tray design, one that balances tray and downcomer area so that neither prematurely restricts capacity, and set weir height, weir geometry, clearance under the downcomer, and fractional hole area so as to maximize efficiency and capacity. 

The hardware design proceeds In two phases primary (basic) and secondary (detailed layout). The primary phase sets column diameter, type of tray, and split of tray area Into bubbling and downcomer areas. This phase also provides a preliminary (and usually close) estimate of tray spacing, number of passes, and other features of tray and downcomer layout such as weir height, fractional hole area, hole diameter, and clearance under the downcomer. These estimates are later firmed up in the secondary phase. 

The other two parameters, small clearance under the downcomer and small downcomer top area, have little effect on entrainment flooding, as they are associated with the downcomer only. Downcomer clearance affects downcomer backup, but not downcomer liquid velocity, while downcomer area affects the velocity, but has little effect on downcomer backup. 

Now retain the number of passes, tray spacing, hole diameter, weir height, and clearance under the downcomer as per Sec. 6.5.5. 

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. 

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... 

Seal check. In Sec. 6.5.7, it was decided to go to clearances under the downcomers that exceed the outlet weir heights. For such designs, it has been recommended (1) that the clear liquid height in the downcomer under turndown conditions exceeds the downcomer clearance by at least 2 in. This will be checked here. 

Comment. This check indicates that at turned-down conditions, downcomer backup exceeds the clearance under the downcomer by more than 2 in. Therefore, no seal loss problem is expected. 

Use one-half of weir height or 3/4 in (0.0191 m), whichever is greater. Clearance between downcomer and tray deck should never be less than y2 in (0.0127 m), and the velocity through the clearance should be under 1 ft/s (0.30 m/s). 

This so-called "clearance under the downcomer," represented by the area A (see Fig. 5.7-4), besrs special mention, since improperly titled tray sections can lead to inadequate clearance at one or more points in the column. Thus, the clearance is a dimension that should be checked very carefolly during tray installation. 

Seal the downcomers (i.e., the downcomer backup must exceed the clearance under the downcomer). 

Both curves in Fig. 6.17 (especially the upper curve) are very sensitive to some of the downcomer layout design parameters, particularly to the clearance under the downcomer, and the design of inlet weirs and seal pans. Figure 6.18 illustrates the effect of reducing the clearance imder the downcomer, from 1.5 to 1.0 in on the startup-stability diagram shown in Fig. 6.17. 

Three major factors govern the specification of clearance under the downcomer downcomer pressure drop, the fouling and corrosive nature of the system, and downcomer sealing. 

Downcomer pressure drop. If the clearance under the downcomer is too low, it may add substantially to the downcomer backup and consequently reduce downcomer capacity. Cases have been reported (61) where column capacity was increased by simply cutting 1 in off the bottom of the downcomer. Methods of estimating the backup caused by hydraulic losses through the opening under the downcomer are available in most distillation texts (48, 319, 371, 409). 

To avoid excessive downcomer backup, the clearance under the downcomer is usually set so that clear liquid pressure drop at the downcomer outlet does not exceed 1 in of hot liquid (61,172,192, 249). Alternatively, some designers recommend outlet pressure drops not exceeding IV2 in of hot liquid (211), or liquid velocity at downcomer outlet not exceeding 1 to 1.5 ft/s (123,243), or area between the bottom of the downcomer and the tray floor one-third to one-half the area at... 

Figure 6.18 Effect of changing clearance under the downcomer on the startup stability diagram. (Henry Z.

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. 

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