Tray deck weeping

Valve sticking can be alleviated by proper selection of valve type. Several designs have the valves seated on tumed-down nibs which hold the disk about 0.1 in (138) above the tray deck to prevent sticking (Figs. 6.5a, 6.6a, b, d). This clearance, however, may enhance the weeping tendency and somewhat reduce turndown. One experience has been reported (85) where weeping from this type of valve at low liquid rates did not permit adequate liquid circulation in a pumparound loop. 

The first continuous distillation tower built was the "Patent Still" used in Britain to produce Scotch whiskey, in 1835. The patent still is to this day employed to make apple brandy in southern England. The original still, and the one 1 saw in England in 1992, had ordinary bubble-cap trays (except downpipes instead of downcomers were used). The major advantage of a bubble-cap tray is that the tray deck is leakproof. As shown in Fig. 4.5, the riser inside the cap is above the top of the outlet weir. This creates a mechanical seal on the tray deck, which prevents hquid weeping, regardless of the vapor flow. 

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