Weeping weirs

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. 

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. 

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.

In this simplified model, it is assumed that liquid may leave the plate, either by flow over the weir Ln(weir) or by weepage Ln(weep)- Both these effects can be described by simple hydraulic relations, in which the flow is proportional to the square root of the available hydrostatic liquid head. The weir flow depends on the liquid head above the weir and hence... 

A liquid level is maintained with an overflow weir while the vapor comes up through the perforated floor at sufficient velocity to keep most of the liquid from weeping through. Hole sizes may range from 1/8 to lin., but are mostly l/4-l/2in. Hole area as a... 

The main factor that affects weeping is the fractional hole area. The larger it is, the smaller the gas pressure drop and the greater the weeping tendency. Larger liquid rates and taller outlet weirs increase... 

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. 

Taller weirs (28,48,63). Lockett and Banik (56) observed that taller weirs increased the weeping tendency, except (1) at high liquid rates and high weirs (2) at high vapor rates, low liquid rates, and low weirs. In both of these exceptions, weir height had little effect on weeping. 

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. 

Sieve trays, troth regime. Most dear liquid height and froth density correlations (35,68,81-86) are based on the Francis weir formula. A correlation by Colwell (68), based on a model of froth flow over the weir, was demonstrated to agree with experimental data better than other published correlations. Colwell s correlation is recommended by the author and by Lockett (12), and was successfully used as a building block in weeping correlations (56,63,69) and in froth regime entrainment correlation (40). Colwell s correlation is... 

Provision for draining the liquid from a bubble-cap tray when the unit is not in operation is made by the use of weep holes. These holes are usually located near the overflow weir so that any delivery during operation follows approximately the same path as the overflow fluid. The weep holes must be large enough to prevent plugging, but they should not deliver an excessive amount of fluid during operation. A size in the range of to f-in. diameter is usually adequate. 

Now, as the weir wall brick grow and expand, the brick can release thermal stress by sliding outward on the Teflon film under them, and slide into the brick slots in the wall, compressing the foam. This curved design provides for both the thrust against the wall from upstream and the expansion of the brick in the weir wall. The designer should bear in mind that like other all-brick structures, this wall will not be liquid-tight and will weep into the downstream side. It will, however, act to trap any suspended material that settles out. 

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