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Entrainment Flooding Capacity
Figure 6.11 The Smith, Dresser, and Ohlswager entrainment flooding correlation. (R. B. Smith, T. Dresser and S. Ohlswager, Hydrocarb. Proc. & Pet. Ref., 40 (5), p. 183, 1963, reprinted courtesy of Hydrocarbon Processing.)

mend using the value of 25 dyne/cm in Eq. (6.12). Both CSB and wfloo<1 are based on the net area AN and are calculated from Eqs. (6.12) and l6.9), respectively. The correlation applies for nonfoaming systems at tray spacing greater than 14 in. Other restrictions are listed in Table 6.3. There are four advantages that make this correlation superior to those described above:

■ It gives a close approximation to the effects of physical properties, operating variables, and tray geometry on the flood point. This is a major improvement compared to the previous correlations above.

■ It was shown to predict most of the presently published sieve-tray and valve-tray entrainment flood data to within ± 15 and ± 20 percent, respectively. This is an improvement compared to the previous correlations above.

■ It can be expressed in terms of dimensionless groups that describe table 6.3 Recommended Range of Application. The Kister and Haas (15) Entrainment Flood Correlation

Flooding mechanism: Entrainment (jet) flood only Tray types: Sieve or valve trays only Pressure: 1.5-500 psia (Note 1) Gas velocity: 1.5-13 ft/s

Liquid load: 0.5-12 gpm/in of outlet weir (Notes 2, 3, 5)

Liquid density: 20-75 lb/ft3

Surface tension: 5-80 dyne/cm

Liquid viscosity: 0.05-2.0 cP

Hole diameter: Vi-l in

Fractional hole area: 0.06-0.20 (Note 5)

Weir height: 0-3 in

NOTES:

1. At pressures above 150 psia, downeomer flood is often the capacity limitation. This limitation is not predicted by the correlation. Caution is required.

2. At high liquid loads (above 7-10 gpm/in), downcomer flood is often the capacity limitation. This limitation is not predicted by the correlation. Caution is required.

3. Equation (6.70) does not apply for liquid loads lower than 0.5 gpm/in of weir (35). For this reason, this correlation must not be extended to lower liquid rates.

4. At lower tray spacing, entrainment flooding may be related to lifting of the froth envelope and to froth rather than spray height. This correlation must not be extended to lower tray spacing.

5. The correlation does not apply when the following three conditions occur simultaneously. (a) Ratio of flow path length to tray spacing is high (>3); (5) liquid rate is high (>6 gpm/in of weir); and <c) fractional hole area is high (> 11%). Under these conditions, entrainment flooding is related to vapor channeling and vapor cross flow rather than spray height.

spray regime entrainment. It therefore has stronger theoretical basis compared to the previous correlations above.

■ It was derived from a much wider data base of commercial- and pilot-scale columns data.

Valve trays. Manufacturer literature contains correlations for entrainment flooding (7-9). The three sieve tray correlations above are also applicable to valve trays. Of the three, the author recommends the Kister and Haas (15) correlation because it was specifically extended for valve trays and because it possesses the advantages listed above. When applying this correlation to valve trays, Af is the fractional hole area when all valves are open, i.e.,

Af - (Total area of opening of all valves when fully open)/As

while dH is taken as the equivalent hydraulic diameter of a fully open valve opening given by

H (Wetted perimeter of opening of one fully open valve)

For a round valve with a diameter dv at the narrowest opening, the expressions for Af and dH are

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