Sieve Tray Weeping

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]. 

With increasing vapor rate, the oscillations become more violent, and liquid entrainment increases up to 70%, decreasing the tray efficiency. On sieve trays, extra weep ing occurs up to 150% compared to a stable tray. Full-wave oscillation is represented by a peak wave forming along the center of the tray with a trough at each wall. This position then reverses itself, and is called full-wave oscillation. The full-wave occurs at lower vapor rates than halfwave oscillation. Increases in entrainment and weeping also occur, and are most likely to be characteristic of medium- to smaU-sized columns, particularly those operating at reduced pressure. 

As liquid level rises from 75 to 100 percent, override A5 closes the feed valve. At a level transmitter output of 15 psig, A5 has an output of 3 psig. With steam/feed ratio control, this would also cut off steam. But it is desirable to maintain enou boilup that the trays (sieve or valve) do not dump or weep. For this we provide an override (not shown) that is a minimum steam flow controUer. In most cases we have found it advisable to adjust override biases so that a high level pinches feed before steam. 

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

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. 

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.

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. 

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- ... 

Sieve tray hole velocity to avoid weeping >12... 

In plate columns the two phases are intensively mixed on each plate and separated between each plate (Fig. 6.7-5). For the distribution of the light phase through the liquid a lot of devices were developed. The simplest one is a perforated sieve tray, where the supercritical phase can pass through. To avoid weeping of the liquid through the holes different devices like bubble caps or valves (Fig. 6.7-6) were developed. 

Sieve trays (Fig. 14-18a) are perforated plates. The velocity of upflowing gas keeps the liquid from descending through the perforations (weeping). At low gas velocities, liquid weeps through the perforations, bypassing part of the tray and reducing tray efficiency. Because of this, sieve trays have relatively poor turndown. 

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

Weep Rate Prediction Lockett and Banik (loc. cit.) and Hsieh and McNulty (Chem. Eng. Progr., July 1993, p. 71) proposed correlations for predicting weep rates from sieve trays. Colwell and O Bara (Paper presented at the AIChE Meeting, Houston, April 1989) rec-... 

The terms in Eqs. (14-123) to (14-126) are in English units and are explained in the Nomenclature. For sieve trays, m= 1.94 and C = 0.79. Note that the constants are a slight revision of those presented in the original paper (C. L. Hsieh, private communication, 1991). Clear liquid height is calculated from Colwell s correlation [Eqs. (14-115) to (14-122)]. The Hsieh and McNulty correlation applies to trays with 9 percent and larger fractional hole area. For trays with smaller hole area, Hsieh and McNulty expect the weeping rate to be smaller than predicted. 

Two types of trays are most common sieve trays and valve trays. A sieve tray is a simple perforated plate. Gas issues from the perforations to give a multiorifice effect liquid is prevented from descending the perforations or weeping by the upward motion of the gas. At low gas flow rates, the upward gas motion may be insufficient to prevent weeping. 

In valve trays, the perforations are equipped with valve units (Fig. 19). At high gas rates, the gas force opens the valves, thus providing area for gas flow. At low gas rates, there is insufficient force to keep many of the valves open, and these close, preventing the liquid from weeping. Sieve and valve trays show comparable capacity, efficiency, and... 

Fractional hole area (sieve trays). Eight to 10% is generally considered optimum. Higher area may enhance capacity at the expense of more weeping at low gas flow rates. 

Figure 6.17 Sieve tray weeping mechanisms, [Reprinted with permission from M. J. Lockett and S. Banik, Ind. Eng. Chem. JVoc, Dea. Dev, vol, 25, p. 561, Copyright (1986) American Chemical Society.]...

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