Backup downcomer

Liquid flows across a tray deck toward the outlet weir. The liquid overflows the weir, and drains through the downcomer to the tray below. 

Vapor bubbles up through the sieve holes, or valve caps, on the tray deck, where the vapor comes into intimate contact with the liquid. More precisely, the fluid on the tray is a froth or foam—that is, a mixture of vapor and liquid. In this sense, the function of a tray is to mix the vapor and liquid together to form a foam. This foam should separate back into a vapor and a liquid in the downcomer. If the foam caiuiot drain quickly from a downcomer onto the tray below, then the foamy liquid or froth will back up onto the tray above. This is caWed flooding. 

If the pressure in the vapor space of tray j is Pj, then the pressure in the vapor space of tray j -i-1 may be calculated as 

4+1 - 4 Pigh, and combining with the above equation, an expression may be 

The actual downcomer backup, or the height of the froth in the downcomer, is calculated as where (f) depends on the foamability of the fluid and must 

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Downflow Flooding Columns can flood because of their inability to handle large quantities of liqmd. For crossbow plates this hmit on liquid rate Is evidenced by downcomer backup to the plate above. To avoid downflow flooding one must size the column downcomers such that excessive backup does not occur. 

Downcomer backup is calciilated from the pressure-balance equation... 

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... 

GPM = Column liquid loading, gal/min Hj,. = Downcomer backup, inches of liquid hi = Condensing side film coefficient, Btu/hrft °F H,(j = Head loss under downcomer, inches of liquid H v = Weir height, ins. 

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. 

In current design practice for downcomers, three parameters are commonly considered liquid residence time, liquid velocity, and downcomer backup. 

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 is calculated from the pressure balance... 

The flooding mechanism to which the derating factor applies (entrainment, downcomer backup, downcomer choke, or all these) must be specified. 

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.)...

The other two parameters, small clearance under the downcomer and small downcomer top area, have little effect on entrainment flooding, as they are associated with the downcomer only. Downcomer clearance affects downcomer backup, but not downcomer liquid velocity, while downcomer area affects the velocity, but has little effect on downcomer backup. 

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... 

This head loss, required for downcomer backup calculation (Sec. 6.2.7 , is calculated for segmental downcomers from (2-5,18,31,32)... 

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-... 

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