Posted 14 March 2011 - 02:18 AM
The physical phenomena involved in weeping through a sieve tray are so complicated that nobody has been able to perform an analysis from first principles. The only way to get an answer, as is true of many operations in chemical engineering, is to resort to empirical correlations (as pointed out by golden_gate). This results in dimensional equations like this one from C&R.
A hole velocity of 14 m/s is not unreasonable. The hole area will typically be around 10% of the cross sectional area of the column. Even less sometimes. The superficial velocity is then about 1,4 m/s up the column. This is probably towards the upper end of the usual velocities, but is not obviously wrong.
I have seen other weeping correlations that are much more complicated than the C&R one, but they don't seem to offer any more accuracy. The most difficult part is to define the onset of weeping. Even in a tray operating well there will be some small leakage from the trays above. As the gas flowrate decreases the weeping will gradually increase. There is no magical point where it suddenly goes from 0% to 100%. So it is hard to say how accurate a weeping correlation really is.