Downcomer seal

The downcomer seal on the tray is recommended [5] based on the liquid flow path ... 

The bottom tray of a tower must have its downcomer sealed to prevent upflow of reboiled vapors. The downcomer of this tray is usually equal to or 6 in. longer than the other downcomers to ensure against bottom vapor surges or pulses in pressure breaking the seal. The seal pan is designed to avoid liquid back pressure and minimum restrictions to liquid flow. 

The importance of the downcomer seal is to prevent vapor from the tray from bubbling into the downcomer (see Figure 8-63), whether the trays are bubble cap, valve or sieve types. If a seal weir is not included in the tray design, then operation problems to avoid flooding, weeping and unstable performance, including pressure drop, are increased, particularly during the start-up phase. 

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. 

The bottom of the downcomer must be sealed below the operating liquid level on the tray. Due to tolerance in fabrication and tray level, it is customary to set the downcomer seal referenced to the weir height on the outlet side of the tray. Recommended seals, based on no inlet weir adjacent to the downcomer, and referenced as mentioned are given in Table 8-19. 

Once foam or froth in the downcomer backs up to the tray above, it tends to be re-entrained in the overflowing liquid, making it apparently lighter, and accentuating this height of liquid-foam mixture in the downcomer. The downcomer must be adequate to separate and disengage this mixture, allowing clear liquid (fairly free of bubbles) to flow under the downcomer seal. 

This seal acts like a typical downcomer seal from the tray above, and should be dimensioned approximately the same, except ... 

Bubbling area column area minus total of downcomer and downcomer seal areas, ft or m ... 

Hydraulic load 600 N/m2 live load on the plate, plus 3000 N/m2 over the downcomer seal area. 

To guarantee a proper downcomer seal, the bottom edge of a downcomer should be about V2 inch below the top edge of the outlet weir. This dimension should be carefully checked by process personnel when a tower is opened for inspection. It is quite easy for sloppy tray installation to distort this critical factor. 

The flange leak was taped over, and the exhaust-steam pressure dropped back to 100 mm Hg. The steam required to drive the turbine fell by 18 percent. This incident is technically quite similar to losing the downcomer seal on a distillation tower tray. Again, it illustrates the sort of field observations one needs to combine with basic technical calculations. This is the optimum way to attack, and solve, process problems. 

Bubbling area AB (also called active area) The total tower cross-sectional area minus the sum of downcomer top area Apr, downcomer seal area ADB, and any other nonperforated areas on the tray. The bubbling area represents the area available for vapor flow just above the tray floor. 

FIG. 14-25 A common design practice of ensuring a positive downcomer seal. (From Henry Z. Kister, Chem. Eng., December 29, 1980 reprinted courtesy of Chemical Engineering.)... 

Bubbling area, AB the total tower cross-section area less the total of downcomer area, downcomer seal area, and any other nonper-forated regions (often referred to as active area, A,). In practice, onperforated regions less than 4 in wide are counted as perforated... 

Downcomer seal at turned down conditions (downcomer backup less downcomer clearance, in clear liquid) 3,8 3.0 3,1... 

Ab bubbling area, column cross-section area less total of downcomer areas, downcomer seal areas and areas of any other non-perforated region, ft ... 

Active or bubbling area of tray, generally (At -2Aj), fl see Figures 8-119 and 129 Ab = Bubbling area column area minus total of downcomer and downcomer seal areas, ft or m ... 

Used in low liquid rate systems to maintain positive downcomer seal... 

Inlet weirs. These are used to insure a positive downcomer seal at high vapor rates and low liquid rates as shown in Fig. 2.23a. 

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