Clearance Under the Downcomer

Therefore, it is important at the downcomer layout design stage to determine whether a potential sealing problem exists. If it does, means of overcoming the problem should be sought. This is discussed in the following sections. 

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 

Kister, Hydrocarbon Processing, February 1979. Reprinted courtesy of Hydrocarbon Processing.) 

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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. 

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. 

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. 

Figure 6.18 shows the effect of the clearance under the downcomer on the startup-stability diagram. The smaller the clearance, the higher the upper curve in the diagraun, and the easier the startup. Smaller clearances restrict vapor flow up the initially empty downcomer during startup. Consequently, the impedance to downflow is reduced. 

The third criterion for satisfactory startup (Sec. 6.18) requires that downcomer backup exceed the clearance under the downcomer. A positive way of satisfying this criterion in froth regime operation is by setting the clearance under the downcomer lower than the weir height (Fig. 6.20). 

Tolerances. The clearance under the downcomer should have an installed tolerance of Vs in (38, 179, 211, 257, 399). 

Inlet weirs (Fig. 6.21a) and recessed seal pans (Fig. 6.216) are primarily used for achieving a downcomer seal in cases where a potential sealing problem exists and clearance under the downcomer is limited by one of the design criteria previously cited (Sec. 6.19). These devices provide a positive seal on the tray imder all conditions and ensure that the second and third sealing criteria (Sec. 6.18) are always satisfied. Sometimes it is argued that these devices improve liquid distribution to the tray, but this function is usually performed satisfactorily by the downcomer outlet (48, 172, 257, 404) and can rarely justify using either device. One exception is when the downcomer is circular... 

Inlet weir height should equal the cleju ance under the downcomer but be less than that of the overflow weir. Excessive inlet weir height will lead to excessive downcomer backup and excessive weep through the inlet row of perforations and should therefore be avoided. If a positive downcomer seal is required, the inlet weir needs to be higher than the clearance under the downcomer, but this may cause a reduction in downcomer capacity. 

The horizontal distance between the downcomer and inlet weir should not be less than the clearance under the downcomer. 

Trays may be clamped (Fig. 7.2a) or bolted (Fig. 7.26) to their supports. The overlap of the tray plate on the support is usually V4 to 1 in (86). Similarly, downcomer panels are usually clamped (Fig. 7.2c) or bolted (Fig. 7.2c ) to the vertical downcomer support bars. The downcomer support bars are welded to the shell. The fastening of downcomer panels to their supports should permit some on-site adjustment for ensuring correct setting of clearances under the downcomer. 

The length of downcomer panels should be carefully adjusted to set the required clearance under the downcomer. One useful technique for achieving this (274) is having wooden blocks cut to the required dimension to act as spacers. 

Narrow openings, such as clearances under the downcomer, seal pan widths, and distances between the downcomer and inlet weir, have not been blocked on any tray. Often, a fabrication error will cause this. In one case (2066), an inlet weir intended for the top tray was located five trays below, effectively closing off the downcomer. 

Special care must be exercised with columns containing valve trays. In other types of columns, vapor can freely flow upward or downward through fixed apertures as well as through the downcomers (Fig. 11.2a). In valve trays, the valves seriously restrict downflow (i.e., they act as nonreturn valves that close for vapor downflow. Fig. 11.26). Excessive pressure drops, which may cause serious tray damage, can therefore occur at relatively low downward flow rates in valve trays. This problem is most severe if the downcomers and/or clearances under the downcomers are small. 

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