Trays downcomer backup

The calculations include fraction of flood velocity, pressure drop per tray, downcomer backup, tray liquid holdup, and check for weeping. 

Jet Flood. Flooding generally occurs by jet flood or downcomer backup. Reference 15 gives Equations 1, 2, and 3 for Jet flood, using Ballast trays. 

Downcomer Backup Flood. For downcomer backup. Equation 4 can be used. Reference 15 states that if the downcomer backup for valve trays exceeds 40% of tray spacing for high vapor density systems I3.01bs/ft-), 50% for medium vapor densities, and 60% for vapor densities... 

Downcomer backup Assume 114-in. clearance between bottom edge of downcomer and tray floor (or equivalent depending on design of downcomer-tray relationship.) See Figure 8-63. 

Downcomer backup flooding results from pressure drop at bottom outlet of downcomer, causes liquid to backup in the downcomer and flood the tray above. Generally the cause is due to excessive tray pressure drop. 

The tray may flood. Water and hydrocarbon mixing on the tray deck, stirred up by the flowing gas, creates an emulsion. The emulsion does not separate as readily as clear liquid from the gas. Premature downcomer backup, followed by tray deck flooding, result. 

Downcomer Backup Flooding Aerated liquid backs up in the downcomer because of tray pressure drop, liquid height on the tray, and frictional losses in the downcomer apron (Fig. 14-32). All these increase with increasing liquid rate. Tray pressure drop also increases as the gas rate rises. When the backup of aerated liquid exceeds the tray spacing, liquid accumulates on the tray above, causing downcomer backup flooding. 

Downcomer backup flooding occurs when the backup of aerated liquid in the downcomer exceeds the available tray spacing. Downcomer backup can be calculated by adding the clear liquid height on the tray, the liquid backup caused by the tray pressure drop, and the liquid backup caused by the friction loss at the downcomer outlet. The downcomer backup is then divided by an aeration factor to give the aerated liquid backup. 

Tray area is usually determined from an entrainment flooding correlation. Trays are normally designed to operate at 80 to 85% of flood at the maximum expected throughput. Downcomer area is usually determined from the downcomer choke criteria. The design is then checked to ensure that downcomer backup flood does not occur. 

Figure 6.7 Common flooding mechanisms in tray columns, (a) Spray entrainment flood (ft) froth ontrainment flood (c) downcomer backup flood Id) downcomer choke flood. (Parle a and ft reproduced from Dr. D. C. Hausch, Discussion of Paper Presented In the Fifth Session, Proceedings of the International Symposium on Distiliation, the Institution of Chemical Engineers (London), I960, reprinted courtesy of the Institution of Chemical Engineers, UK- Parts c and d from H. Z. Kister. Distillation Operation. Copyright C 1990 6y McGraw-Hill, Inc. reprinted by permission.)...

Downcomer backup flooding occurs when the backup of aerated liquid in the downcomer exceeds the tray spacing, i.e.,... 

Downcomer backup. The factors that resist liquid flow from the downcomer onto the tray below are the froth height on the tray, the pressure drop on the tray, and the friction loss under the downcomer apron. These factors cause liquid to back up in the downcomer. Each of these factors can be expressed in terms of clear liquid heads. A tray pressure balance gives... 

The clear liquid height, or the liquid holdup, is the height to which the aerated mass would collapse in the absence of vapor flow. The clear liquid height gives a measure of the liquid level on the tray, and is used in efficiency, flooding, pressure drop, downcomer backup, weep-... 

Downcomer backup. For side downcomers of two-pass trays, the force balance of Eq. (6.19) makes the dear liquid height in the downcomer... 

Summary. The third trial checks well against the various hydraulic criteria. Column capacity is limited by downcomer backup flood in the bottom section center-to-side trays (i.e., side downcomers). All trays will operate in the emulsion regime. 

The downcomer backup (clear liquid height basis) should be 40% or less of the tray spacing for high-vapor-density systems (>3.01b/ft, 48.1 kg/m ), 50% or less for medium-vapor-density systems, and 60% or less for low-vapor-density systems (<1.01b/ft, 16.0 kg/m ). 

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