Tray liquid drainage

Under normal service conditions, the liquid collector is a separate tray from any liquid distribution device. The basic collector is a chimney tray with a sump (Fig. 9). The chimneys provide passage for vapor rising through the tray. The sump(s) provides for liquid drainage into a draw nozzle (for product draw) or... 

Once a vapor slug passes through a deep liquid layer, the vapor space previously occupied by the slug is left empty and turns into a vapor gap under the bottom tray. Fast drainage of a deep liquid layer can also cause a vapor gap or induce excessive pressuring and depressuring rates. The consequences are described in Sec. 12.9. The description includes an experience where a vapor gap was initiated by vapor slugging. 

Ptdp) filtei. s. These filters employ one or more packs of filtermasse (cellulose fibers compressed to a compact cylinder) stacked into a pressure case. The packs are sometimes supported in individual trays which provide drainage channels and sometimes rest on one another with a loose spacer plate between each two packs and with a drainage screen buried in the center of each pack. The liquid being clarified flows under a pressure of 345 kPa (50 psig) or less through the pulp packs and into a drainage manifold. Flow rates are somewhat less than for disk filters, on the order of 20 L/(min-m ) [0.5 gal/ (min-ft")]. Pulp filters are used chiefly to polish beverages. The filtermasse may be washed in special washers and re-formed into new cakes. 

Provide drainage holes in seal pan to allow adequate drainage, flushing and cleaning, but not too large (number) to prevent liquid backup sufficient to maintain a seal on the tray. 

During shutdown, it is necessary to drain the trays and seal pans of unevaporated liquid and steam condensate. At least two drain holes are needed for each tray or seal pan, at 0.50 in. minimum diameter and preferably 0.75 in. to avoid plugging. As a rule of thumb, a 0.75 in. hole will deliver about 2gpm of liquid during tower drainage. 

It is desirable to place the draw-off nozzle in a portion of the tray floor that is lowered to form a sump. This lowers the height of liquid on the chimney tray by an amount equal to the nozzle diameter and reduces liquid leakage and the mass of liquid that the tray must support. The outlet nozzle should be flush with the sump floor to ensure adequate drainage. This practice is particularly fruitful in leak-tight applications, because it eliminates th> need for weep holes. 

Drainage can be troublesome when the bottom trays are flooded (i.e., covered with liquid), emd the rate of liquid withdrawal fiwm the column exceeds the rate of liquid downflow through the trays and downcomers (61,192,207,356). Under such conditions, a vapor gap may form below the bottom tray. The presence of this gap leaves the bottom tray with the task of supporting all the liquid above it, and may lead to failure of the bottom tray. Once the bottom tray fails and the liquid drains from it, the vapor gap moves to the space below the next higher tray. This tray may fail as well, and a cascade of collapsing trays may ascend the column (Fig. 12.8). 

In a general sense, flooding means not only the formation of pools of water on the floor of a room but also the collection of liquid in higher locations, if sufficient drainage is not assured. For example, water (arising from sprays or condensed steam) can collect in cable trays even if they are located well above the floor level. Equipment located in such a place should then be considered to be subject to flooding. In addition, water from these trays may be drained to other undesired locations. 

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