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Figure 6.13 Values of downcomer residence time tR to be used in the Koch correlation and in the Nutter correlation for maximum downcomer velocity [Eq. (6.26)). Data based on Koch Engineering Co. Inc. Design Manual-Flexitray, Bulletin 960-1, Wichita, Kansas, 1982 and on Nutter Engineering Float Valve Design Manual, Tulsa, Oklahoma, 1976.

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Figure 6.13 Values of downcomer residence time tR to be used in the Koch correlation and in the Nutter correlation for maximum downcomer velocity [Eq. (6.26)). Data based on Koch Engineering Co. Inc. Design Manual-Flexitray, Bulletin 960-1, Wichita, Kansas, 1982 and on Nutter Engineering Float Valve Design Manual, Tulsa, Oklahoma, 1976.

Kister (1) surveyed the multitude of published criteria for maximum downcomer velocity. He pointed at the poor accuracy and inconsistency of these criteria, then incorporated them together with his own experience into the single set of guidelines shown in Table 6.5. The values in Table 6.5 are not conservative, and some may even be slightly optimistic. For a conservative design, a value from Table 6.5 can be multiplied by a safety factor of 0.75.

Residence time in downcomers. Sufficient residence time must be provided in the downcomer to allow adequate disengagement of vapor from the descending liquid, so that the liquid is relatively vapor free by the time it enters the tray below. Inadequate removal of vapor from the liquid may choke the downcomer.

Two different definitions are used for downcomer residence time (3).

table 6.5 Maximum Downcomer Velocities

Clear liquid velocity in downcomer, ft/s

Clear liquid velocity in downcomer, ft/s table 6.5 Maximum Downcomer Velocities

Foaming tendency

Example

18-in spacing

24-in spacing

30-in spacing

Low

Low-pressure (<100-psia) light hydrocarbons, stabilizers, air-water simulators

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