Flood trays pressure

This problem, as with flooding, also impairs product quality. No fractionation occurs in the dry section, so the temperature difference decreases. However, unlike flooding, the pressure drop decreases and stays very steady at the ultimate minimum value. This problem is usually easier to handle than flooding. The problem is caused by either insufficient liquid entering the section or too much liquid boiling away. The problem is solved by reversing the action that caused the dry trays. 

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 the spray regime, flooding (usually called jet flooding) is caused by excessive entrainment of liquid from an active area to the tray above. It increases the tray pressure-drop, and the entrained liquid recirculates to the tray below. The larger liquid load in the downcomer and the increased tray-pressure-drop together cause the downcomer to overfill so the tray floods. 

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. 

And when the volume of a vapor flowing through a tray increases, so does its velocity. Any increase in vapor velocity through a tray results in higher tray pressure drop. And what is it that causes trays to flood Why, it is high tray deck pressure drop. 

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. 

Please note that HOLHA is the total hole area in ft2 on a single tray deck. It is used in Eq. (3.91) to calculate sieve tray jet flood and will be used to calculate sieve tray pressure drop as well. 

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. 

The internal flow of liquid and vapor must be re-evaluated from the standpoint of column capacity, both in the design and performance studies of columns. The physical dimensions of a column can handle only limited ranges of vapor and liquid flow rates. The objective of this chapter is to evaluate the hydraulic aspects of fluid flow in trayed columns. The column performance is examined with regard to factors such as flooding, entrainment, pressure drop, mass transfer, and tray efficiency. 

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

At high pressures, the difference between vapor and liquid density becomes smaller, and separation of vapor from liquid in the downcomer becomes difficult. Because of the more difficult separation, downcomer aeration increases, raising both downcomer frictional losses and froth backup in the downcomer. High liquid flow rates also increase tray pressure drop, tray liquid level, and frictional losses in the downcomer. For this reason, downcomer flooding is favored at high pressures and high liquid rates. 

Similar guidelines apply when column capacity restriction is primarily related to column vapor load [e.g., a downcomer backup restriction when backup is primarily due to dry tray pressure drop (199) or in packed columns in which flooding is induced by excessive vapor loads (195)]. The guidelines do not apply when column capacity restriction is primarily caused by excessive liquid load (e.g., downcomer choke or downcomer backup restriction when the backup is primarily... 

LPG Lean oil stripper Column was pressured up throu a connection in the overhead system while liquid circulated throu its valve tr. The gas could not travel downward, causing mechanical damage to top 12 trays. This later resulted in premature flooding. Always pressure columns fn>m the bottom up, espedaUy when column contains valve trays. 

A tray column is flooded if the liquid level in the outlet downcomer is approximately equal to the distance between two trays. Then the tray pressure drop is the hydrostatic pressure of the liquid level in the outlet downcomer. The maximum liquid load which leads to flooding of a column may be derived with this [0.1, vol 2]. 

---