What diagram?
Here vapor pressure drop stands for what? Is it total pressure drop of vapor across the tray or it is only dry tray pressure drop due to orifice?
Neither.
It is the dynamic pressure (0.5*ρ*uh2) of the vapor exiting the sieve holes that must be sufficient to counteract the static head of the liquid on the tray.
It is not the dry tray pressure drop that matters. In fact sieve holes are punched either down or up, usually down. A sieve tray with holes punched downward will have a higher dry tray pressure drop that an identical one with the holes punched upward. But that does not mean that a tray with downward punched holes would have less weeping.
Equation (6.31.a) of the socalled Fair's model is in my opinion wrong, as dry tray pressure drop hd is not what matters, and moreover the RHS is wrong because the liquid on the tray is aerated so its head is less than its height.
Note also what Kister wrote on page 300: The mechanism of weeping is not well understood.
So unless you want to become an expert on the subject (requiring searching for, and reading of, many scientific papers on the subject), I suggest you spend your time on more interesting and more important aspects of distillation or chemical engineering.
If you would ever need to design a tray yourself you would be using dedicated software that would warn if weeping (or entrainment or whatever) is not within proper design range and recommend adjusting number and/or diameter of holes or whatever. Or you simply send your specifications (tray loadings, fluid properties, max deltaP, et cetera) to a tray vendor.
http://www.cheresour...-pressure-drop/
https://www.cheresou...llation-column/
Edited by PingPong, 07 May 2018 - 10:18 AM.