Excessive weeping

FIG. 14-43 Efficiency reduction when fractional hole area is increased, also showing little effect of vapor and liquid loads on efficiency in the normal operating range (between excessive weeping and excessive entrainment). Also shown is the small increase in efficiency with pressure. FRI data, total reflux, Dt = 1.2 m, S = 610 mm, hw = 50.8 mm, d% = 12.7 mm. (Reprinted with permission from T. Yanagi and M. Sakata, Ind. Eng. Chem. Proc. Des. Dev. 21, 712 copyright 1982, American Chemical Society.)... 

Liquid inlets. Liquid enters the top tray via a hole in the column shell, often discharging against a vertical baffle or weir, or via a short, down-bending pipe (Fig. 17), or via a distributor. Restriction, excessive liquid velocities, and interference with tray action must be avoided, as these may lead to excessive entrainment, premature flooding, and even structural damage. Disperser units (e.g., perforations, values) must be absent in the liquid entrance area (Fig. 17) or excessive weeping may result. 

As vapor rate is lowered, either at constant liquid rate or at a constant L/V"ratio, the limit of excessive weeping is reached. This limit is not identical with the weep point, as some weeping can usually be tolerated. 

Weep point prediction. Until recently, there was no reliable means of predicting weep rates. Trays were designed (4,26,67) to operate above the weep point, which could be predicted with confidence. This practice forfeited the portion of area on the stability diagram (Fig. 6.6) between the weep point and excessive weeping curves. The recent appearance of reliable weep rate correlations (below) is likely to strip the weep point of most of its practical significance for tray design. 

Sieve trays have a poor turndown (about 2 1). The minimum operating throughput in sieve trays is almost always restricted by excessive weeping, Turndown of sieve trays can be improved by... 

Valve tray turndown is normally about 4 to 5 1. The minimum operating rate in valve trays is usually restricted by excessive weeping, but it may also be restricted by the onset of vapor channeling (Sec. 6.2,13). 

Comment. Figure 6.20 predicts that at turndown, neither top nor bottom section will dump. However, both sections will operate close to the dump point, and excessive weeping is likely. This is consistent with the prediction from Fair s weep correlation (above), which indicates that at turndown both sections operate well below the weep point. 

Under some conditions, vapor velocity maldistribution induced by hydraulic gradient nr tray tilt cam lead to excessive nonuniform weeping (183a Also, see Secs. 6.2.12, 6.2.13). Such excessive weeping can be detrimental to tray efficiency. 

FIGURE 8.12 Estimate of excessive weeping. Note Operating points above the respective lines represent salf designs. A point below the line may indicate uncertainty, but not necessarily a dumping station. (From Chemical Engineering, McGraw-Hill, 1977.)... 

Dry pressure drop for sieve trays should fall between 0.75 and 3 in of liquid (19 and 76 mm) to prevent excessive weeping on the one hand or jetting on the other. Since, in this case, the dry pressure drop is barely within the recommended range, it is desirable that the tray be checked for weeping using Fig. 8.12. 

Because of their simplicity and low cost, sieve (perforated) trays are now the most important of tray devices. In the design of sieve trays, the diameter of the tower must be chosen to accommodate the flow rates, the details of the tray layout must be selected, estimates must be made of the gas-pressure drop and approach to flooding, and assurance against excessive weeping and entrainment must be established. 

Orifice Froude number = 1.11 > 0.5 no excessive weeping Fractional liquid entrainment = 0.024 Point efficiency = 0.760 Murphree tray efficiency = 0.795... 

Check tray design for excessive weeping calculate orifice Froude number, Fro If Fro > 0.5 there is no weeping problem. 

Inlet weir height should equal the cleju ance under the downcomer but be less than that of the overflow weir. Excessive inlet weir height will lead to excessive downcomer backup and excessive weep through the inlet row of perforations and should therefore be avoided. If a positive downcomer seal is required, the inlet weir needs to be higher than the clearance under the downcomer, but this may cause a reduction in downcomer capacity. 

Cracks and crevices should not take up more than 2 percent of the tray area (49). If they do, excessive weeping and/or channeling may result. 

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