Sieve tray open area determines vapor velocity through the deck, while hole diameter controls the number, spacing and fouling behavior of vapor entry points. Both must be rated with tray liquid hydraulics across minimum and maximum operating rates.

An out-of-level tray creates deep liquid on one side and shallow liquid on the other, leading to simultaneous local flooding, vapor bias and weeping. Deck, support and weir elevations must be surveyed separately against the minimum operating liquid depth.

Outlet-weir height affects tray liquid depth, froth, residence time, pressure drop and downcomer backup. The correct height must be selected together with weir length, vapor load, tray spacing and process sensitivity rather than maximizing or minimizing holdup alone.

Downcomer bottom clearance controls liquid exit velocity, downcomer backup and vapor sealing. Too little clearance can cause hydraulic restriction and flooding, while too much can permit vapor blowback and unstable liquid delivery to the tray below.

At low liquid flow, concentrated rising vapor can deflect, atomize or entrain distributor discharge and move the actual irrigation points. Local vapor velocity, outlet protection and two-phase testing should be evaluated together with liquid-head uniformity.