Tray and Downcomer Layout

Unlike the internals discussed in previous chapters, poor tray layout of one- or two-pass trays rarely causes spectacular column failures such as flooding at 50 percent of design rates or extremely poor separation. Ill effects resulting from poor tray layout seldom extend beyond suboptimum design or performance a moderate reduction in capacity, efficiency, or turndown and some increases in capital or 

Tray layout discussions in this chapter emphasize sieve and valve trays, as these trays are most frequently encountered in industrial practice. Several of the considerations also apply to other tray types (e.g., bubble-cap trays). Considerations unique to bubble-cap trays were excluded from this chapter. The infrequent application of this type of tray in modem distillation practice argues against a detailed discussion here. A large amount of information on bubble-cap tray layout is available and is well documented in several texts (48, 257, 371, 409). 

Unlike tray layout, poor downcomer layout is as likely to cause a spectacular column failure as several internals discussed in previous chapters. Adequate hydraulics is the primary consideration, and if not achieved, premature flooding may occur. 

For each of these internals, the chapter outlines the preferred practices, highlights the consequences of poor practices, and supplies guidelines for troubleshooting and reviewing their designs. 

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Tray layout. The preliminary tray and downcomer layout is prepared in the column-sizing phase and refined during the hydraulic design phase. In addition to the parameters previously set, such parameters as hole diameter or the type of valve unit are determined. 

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. 

Primary design problems, foaming, installation mishaps, relief problems, and tray and downcomer layout problems make up the rest of the column malfunctions. Familiarity with these problems is useful to troubleshooters and operation personnel, but only one incident out of every four is likely to be caused by one of these factors. All these topics, except the primary design, are relatively narrow and are treated accordingly in this book. 

The following general factors influence tray performance and, therefore, the desired tray layout. They need to receive attention at an early stage and guide the specification of tray and downcomer layout. 

Capacity restriction mechanism(s). Column throughput is restricted by one of several different mechanisms. These include spray entrainment flooding, froth entrainment flooding, downcomer backup flooding, downcomer choke flooding, excessive entrainment and excessive pressure drop. Optimum tray and downcomer layouts vary with the mech-... 

Corrosion. The likelihood of corrosion and its potential effect on column internals must be reviewed. The consequences of operating with corroded internals, and the cost of their repair, including the cost of lost production, must be evaluated. The extent to which corrosion can be tolerated and/or inhibited must be defined. This definition affects several decisions to be made about tray and downcomer layout. 

Simplicity. The simpler the tray and downcomer layouts, the less expensive they are, and the smaller the chance of fabrication, assembly, and installation errors. Tray design and fabrication are often highly computerized any oddities [e.g., downcomers following the contours of the weir (128)] may require considerable hand work. 

Tray and downcomer layout should be as uniform as possible and their components as interchangeable as possible throughout the column. Machinery employed by manufacturers is most efficient on long production runs (128), which minimizes costs. Further, a uniform layout can eliminate costly delays when replacing defective or worn-out parts, and it minimizes the chances of interchanging dissimilar parts during column assembly. 

Others. Other considerations affecting tray and downcomer layout include efficiency, access for maintenance, and cost. These tend to be specific rather than general, and are therefore discussed in the relevant sections. 

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