Tray Stability Diagram

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

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Figure 6.6 is a typical tray stability diagram. The area of satisfactory operation (shaded) is bound by the tray stability limits. These limits are discussed in the following sections. The upper capacity limit is the onset of flooding. At moderate and high liquid flow rates, the entrainment (jet) flooding limit is normally reached when vapor flow is raised, while the downcomer flooding limit is normally reached when liquid flow is raised. When flows are raised while the column operates at constant LIV (i.e., constant reflux ratio), either limit can be reached. At very low liquid rates, as vapor rate is raised, the limit of excessive entrainment is often reached. 

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. 

Below the dump point (100 percent weep), tray efficiency is a small fraction of its normal value, and mass transfer is extremely poor. Since no liquid enters the downcomers, they lose the liquid seal that prevents vapor from rising through them. Operation below the dump point can be accompanied by severe hydraulic instability due to unsealing of downcomers, as was demonstrated by field experience (76). The startup stability diagram (1), which defines the range of vapor and liquid rates needed for satisfactory startup, has the dump point as the lower limit- The tendency of dumping increases when (77-79)... 

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