Weeping trays

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)... 

The problem we have been discussing—loss of tray efficiency due to low vapor velocity—is commonly called turndown. It is the opposite of flooding, which is indicated by loss of tray efficiency at high vapor velocity. To discriminate between flooding and weeping trays, we... 

Plate towers can be utilized in high removal efficiency applications, even at medium H2S concentrations. The preferred design for Stretford applications uses weeping trays, thus avoiding the chance of downcomer plugging. For I0W-CO2 gases, each tray is reported to be equivalent to 0.6 transfer units. 

Fractionating coiumn totai cross section vapor veiocity 1.0-1.5 Sieve tray hoie velocity to avoid weeping >12... 

At low vapor rates, valve trays will weep. Bubble cap trays cannot weep (unless they are damaged). For this reason, it is generally assumed that bubble cap trays have nearly an infinite turndown ratio. This is true in absorption processes (e.g., glycol dehydration), in which it is more important to contact the vapor with liquid than the liquid with vapor. However, this is not true of distillation processes (e.g., stabilization), in which it is more important to contact the liquid with the vapor. 

Flexibility Not generally suitable for columns operating under variable load, falling below 60% of design. Tray weeps liquid at low vapor rates. 

If inlet weirs are used they should have at least two slots %-in. by 1-in. flush with the tray floor to aid in flushing out any trapped sediment or other material. There should also be weep or drain holes below the downcomer to drain the weir seal area. The size should be set by the type of service, but a minimum of M-in. is recommended. 

A general recommendation [5] is to provide four square inches of weep hole area per 100 ft of net open liquid tray area in the tower. This latter refers to the total of all trays in the tow er. 

The importance of the downcomer seal is to prevent vapor from the tray from bubbling into the downcomer (see Figure 8-63), whether the trays are bubble cap, valve or sieve types. If a seal weir is not included in the tray design, then operation problems to avoid flooding, weeping and unstable performance, including pressure drop, are increased, particularly during the start-up phase. 

Weeping A condition occurring when the vapor rate is not large enough to hold all the liquid on the tray, so that part of the liquid flows over the outlet weir while the rest falls through the holes. 

Tests have indicated that the hydraulic gradient is negligible or very small for most tray designs. Ustial design practice is to omit its effect unle.ss the alue o( A is expected to be greater than 0.75 in. If hydraulic gradient is appreciable, then tbe holes nearer to the tray inlet (liquid) will tend to weep before those nearer the tiay outlet. 

For weeping sieve trays, see Figures 8-131 and 8-132, and example in later paragraph. 

Figure 8-131. Weeping correiation for sieve trays with downcomers. Used by permission, Hugh-mark, G. A. and O Conneli, H. E., The American Institute of Chemical Engirteers, Chem. Eng. Prog. V. 53. (1957), p. 127M, all rights reserved.

Figure 8-132. Weeping sieve trays. Used by permission, Smith, B. D., Design of Equilibrium Stage Processes, Chapter 15 by J. R. Fair, McGraw-Hill Book Co. (1963), all rights reserved.

The weep point is considered to be the minimum vapor velocity that will provide a stable tray operation, pre-... 

Hsieh and McNulty [210] developed a new correlation for weeping of sieve and valve trays based on experimental research and published data. For sieve trays the estimation of the weeping rate and weep point is recommended using a two-phase countercurrent flow limitation model, CCFL. 

The weep point for sieve or valve trays is the vapor rate at which the liquid weeping rate is diminished to zero. Thus, J L approaches zero asJ G is increased [210]. For a vapor rate that leads to J g higher than the weep point value, then there should be no weeping. 

The higher the value of W ndex> the more confidence that there will be no weeping [210]. At a constant weep point, J G then, the higher the percentage opening of the tray, and the higher will be the vapor volumetric flow required to satisfy the weep point criteria. 

Figure 8-135. Weeping rate of Type-T veilve (Koch) vs. sieve tray. Used by permission, Hsieh, C. Li. and McNulty, K. J., The American...

Hsieh and McNulty [210] (also see section on Sieve Trays) show that the weeping rate for 14.3% open area valve Koch Type-T (Figure 8-72) is nearly an order of mag- ... 

The weeping rate of the sieve tray is strongly influenced by the gas flow rate, that is, the weeping rate will increase as the gas flow rate reduces below the weep point, i.e., where the weeping starts. Note the comparison of sieve and valve trays during weeping, Figure 8-135 [210]. 

This is based on the correlation of Mayfield [45] where hjt (weep) = dry tray pressure drop at tray weep point, in. liquid. 

Set minimum design dry tray pressure drop 30% above the value of hdt (weep). 

Unstable liquid oscillations on a tray have received only limited examination when compared to perhaps tray weeping, flooding and froth build-up. Biddulph [87] pro-... 

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