Sieve Tray

In a sieve tray, the vapour passes through a large number of "holes" known as perforations (sieves) and emerges through the liquid in a vertical direction.

The sieve tray has the simplest design among the various tray types. There are no mechanical moving parts. There is no liquid seal and it is the passage of vapour that effectively prevents the loss of liquid through the sieves. A phenomenon known as weeping can occur at low vapour flow and/or high liquid rates when the liquid height on the tray exceeds the tray pressure drop.

Typical layout for sieve tray follows either a square hole pitch or equilateral triangular hole pitch. Hole pitch is the centre-to-centre hole spacing.

Typical sieve sizes used industrially range from as small as 1/16-inch to 1-inch. Small sieves are not suitable for fouling or corrosive services as they may be susceptible to blockage, leading to excessive pressure drop and premature flooding.

Small holes have better turndown characteristics because they reduce tray weeping and therefore increase tray capacity. In terms of cost, larger holes are cheaper as the holes can be punched, and there are fewer holes to be punched. Smaller holes are more expensive as drilling may be required.

Tray Blanking

In certain applications, there can be varying vapour and liquid loadings in the column, and the hole area requirements can vary at different sections of the column. Rather than manufacturing different trays to meet these needs, it is usual practice to use the same tray layout, but the required hole area is adjusted by means of blanking strips.