Causes of Tray Inefficiency : Loss of Downcomer Seal
We stated that the top edge of the outlet weir is maintained about 0.5 in above the bottom edge of the inlet downcomer to prevent vapor from flowing up the downcomer. This is called a 0.5-in positive downcomer seal. But for this seal to be effective, the liquid must overflow the weir. If all the liquid is weeping through the tray deck, there will be no flow over the weir, and the height of the weir will become irrelevant. Figure 4.4 shows the result of severe tray deck leakage:
1. The downcomer seal is lost on tray deck 1.
2. Vapor flows up the downcomer between tray decks 1 and 2.
3. Liquid flow is backed up onto the tray above, i.e., onto tray deck 2.
4. The dry tray pressure drop through tray 2 decreases due to low vapor flow through the tray deck.
5. The hydraulic tray pressure drop on tray 2 increases due to increased liquid level.
6. Tray 2 will now start to weep, with the weeping concentrated on the low area of the tray.
7. Tray 2 now has most of its vapor feed flowing up through its outlet downcomer, rather than the tray deck, and most of its liquid flow is leaking through its tray deck.
The net result of this unpleasant scenario is loss of both vapor-liquid contacting and tray efficiency. Note how the mechanical problems (i.e., levelness) of tray 1 ruins the tray efficiency of both trays 1 and 2.