Valve Tray Weeping

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. 

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 8-133. Weeping performance comparison. (Valve tray also gives a lower weep rate at a liquid flow rate of 50 gal/min/ft of weir.) Used by permission. The American Institute of Chemical Engineers Hsieh, C-Li. and McNulty, K. J., Chem. Eng. Prog. V. 89, No. 7 (1993), p. 71, all rights reserved.

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]. 

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 point correlations for valve trays were presented by Bolles (loc. cit.) and by Klein (Chem. Eng., Sept. 17,1984, p. 128). Hsieh and McNulty (loc. cit.) gave a complex extension of their weep rate correlation to valve trays. 

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... 

Bolles (71) extended Fair s sieve tray weep point correlation (31 Fig. 6.18) to investigate weeping in valve trays. Some results are depicted in Fig. 6.19. The axes of Fig. 6.19 are identical to those of Fig. 6.16. Each dashed line is the locus of weep points predicted from Bolles extended Fair correlation. The heavy lines are the "operating lines, i.e.. 

Figure IW Weep point prewure balance for sieve and valve trays, (a) Sieve tray (6] well-designed valve tray, (c) valve tray with too many velvet or with valves that are too light (d) valve tray with too many valves, but fewer than in C (a) well-designed valve trey with two valve weights. (From w. h. Haifa, Chem. Eng. Png., 72 (B), p 43 (September 1876), reprinted courtesy of the American Institute of Chemical Engineers.) 305... 

Several experiences of severe weeping from valve trays have been reported (1,71,75). A well-designed valve tray is unlikely to have too many valves, but trays with light valves are common in an effort to reduce pressure drop. To avoid the turndown problems, manufacturers often specify a valve tray with two valve weights (Fig. 6.19e). When the light valves open, the heavy ones are still shut, which reduces the ratio of slot to active area and avoids weeping. This practice is discussed in detail elsewhere (1,71). 

Tests by Banik (72) and Zhang et al. (70) show that weeping from valve trays is nonuniform. In Banik s 4 ft x 2 ft rectangular simulator, most of the weep issued from the inlet half of the tray at low liquid rates (< 3 gpm/in of outlet weir) and from the outlet half of the tray ax high liquid rates (>10 gpm/in of outlet weir). The nonuniformity appeared to escalate as weir height increased. This pattern of nonuniformity is similar to that observed by Banik and Lockett (56) on sieve trays. In Zhang et al. s (70) 5 ft x 1 ft rectangular simulator. 

Unlike sieve trays, valve trays were observed to experience substantial weeping from the inlet row of valves (72). An inlet weir about 1 in tall was shown (72) to roughly half this inlet weep at higher liquid rates (> 5 gpm/in) but to be less effective at lower liquid rates. The installation of such an inlet weir ( interrupter" or breaker bar) is a common design practice on valve trays. 

Factors affecting weep. The mechanical design of the valves has a large impact on the weeping tendency. This is discussed elsewhere (1). In addition, most of the factors that increase weeping tendency in sieve trays also increase weeping tendency in valve trays. These include... 

Weep prediction, The weep point of valve trays can be calculated from the Bolles extension (71) of Fair s weep point correlation (31). The same correlation (Fig. 6.18) is used, except that the sieve fractional hole area is substituted by the ratio of valve slot area to tray active area. An alternative weep point correlation for valve trays was presented by Klein (73). Hsieh and McNulty (63) extended their sieve tray weep rate correlation (Sec. 6.2.12) to valve trays. The extension is complex, and discussed elsewhere (63). 

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). 

This means both vapor and liquid loadB are raised and lowered simultaneously. Increasing vapor rate reduces efficiency, while increasing liquid rates raises efficiency. The two effects normally cancel each other, and efficiency is practically independent of load changes (assuming no excessive entrainment or weeping). Figure 7.106 shows a typical dependence of tray efficiency on vapor and liquid loads for a commercial-scale distillation column. Anderson et al. (97) show a similar dependence for several different valve trays. 

Banik, S., "Weeping from Valve Trays, Paper presented at the AIChE National Meeting, Houston. April 1989. 

---