Flood trays weir height

Figure 8-137. Flooding capacity, sieve trays weir height is less than 15% of tray spacing low- to non-foaming system hole area at least 10% hole sizes Ms-in. to M-in. dia. surface tension = 20 dynes/cm. Used by permission, Fair, J. R., Petro/Chem. Engineer, Sept (1961), p. 46, reproduced courtesy of Petroleum Engineer International, Dallas, Texas.

Referring to Figure 8-63, the weir height, h, must always be greater than the clearance under the downcomer, i.e., between bottom of downcomer and tray floor, hdci-Always avoid too low clearance as this can cause flooding of liquid in the downcomer. There are flow conditions... 

The clear liquid backup is divided by the froth-density to give the froth height if this exceeds the tray spacing plus the outlet weir height, the tray is deemed to be flooded. 

As the weir height of the trays is 3 in, it is a safe assumption that the low tray efficiency is due to tray deck dumping, rather than flooding. As shown in Fig. 3.3, this column has no reflux. This is a typical design for strippers when feed is introduced on the top tray, there is no need for reflux. 

An increase in reflux rate, assuming that the reboiler is on automatic temperature control, increases both the tray weir loading and the vapor velocity through the tray deck. This increases both the total tray pressure drop and the height of liquid in the tray s downcomer. Increasing reflux rates, with the reboiler on automatic temperature control, then will always push the tray closer to, or even beyond, the point of incipient flood. 

LIQUID HEAD IN DOWNCOMER. If the head of liquid in the downcomer is greater than the tray spacing plus the weir height, flooding will occur and liquid will build up on the trays. In design practice, the height of liquid in the downcomer (based on clear-liquid density) should be less than 50 percent of the tray spacing. 

The weir height, 7/w. usually varies from 1 to 3 in. (2.5-7.5 cm). Two inches (5 cm) is a typical value. For heights above 15 /o of the tray spacing reduce the effective tray spacing for jet flood by the excess over 15 /o ... 

Tray type Weir height Weir length Downcomer clearance Tray spacing Foaming factor Fraction of flood Surface tension Liquid viscosity... 

Sieve trays will be used with 60 cm spacing, 6 cm weir height, 0.6 cm hole diameter, 0.25 cm tray thickness, 5 cm downcomer clearance, and hole area 10% of the total tray area. The foaming factor is 0.80 and the froth density in the downcomer is 0.5. The target fraction of flood velocity is 0.70. 

Consider the absorber of Example 3.7. Assume that the wash oil is n-tetradecane (C14H30). The absorber will be a cross-flow sieve-tray tower with dg = 4.5 mm on an equilateral-triangular pitch 12 mm between hole centers, punched in stainless steel sheet metal 2 mm thick, with a weir height of 50 mm. Estimate the number of real trays required, the dimensions of the absorber, and the power required to pump the gas and the liquid through the tower. Design for a 65% approach to the flooding velocity. 

There is available a sieve-tray tower 0.75 m in diameter, containing 6 crossflow trays at 0.5-m tray spacing. The perforations are 4.75 mm in diameter, arranged in a triangular pitch on 12.5-mm centers, punched in sheet metal 2 mm thick. The weir height is 40 mm. Assume isothermal scrubbing with pure water at 303 K. The water flow rate to be used should not exceed 50% of the maximum recommended for crossflow sieve trays, which is 0.015 m3/s-m of tower diameter (Treybal, 1980). The gas flow rate should not exceed 80% of the flooding value. 

Downcomer Backup Limit The maximum downcomer liquid height allowable is set by the tray spacing and outlet weir height. Downcomer backup flooding occurs when the liquid froth in the downcomer reaches the tray above. Assume the maximum backup limit as 80% and the froth density q) = 0.6 (Kister, 1992). Applying equation (12.23) yields... 

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