I dont see the link between corrosion and hole size either.
What type of tray elements do you have in this wash section - sieve trays ?

Norton sieve trays, total number of trays 18, Tray 17 is chimney tray orientation, tray 1 at the bottom and 18 at the top.

Hole size in sieve trays is directly related to the minimum required tray pressure drop at minimum turndown vapor flow. I would suspect it was changed due to to some design oversight on tray hydraulics at turndown and not corrosion. Liquid must not weep through the holes at min turndown. There are design procedures available in the public domain for tray pressure drop - see if you can run these calcs to confirm that the 4mm hole size is appropriate for the stated column vapor turndown flow. Some simulation programs such as Pro II from Simsci can do these for you also. The tray vendor Norton may have design literature also.

Hi,
Take a look at the paper in the link, you may find pointers and calculation procedure.
https://www.researchgate.net/publication/260224803...
Note: you may want to contact Norton, they may be able to support. Norton may have kept a technical note about the change.
My 2 cents
Pierre

The trays were replaced due to corrosion.
New trays had different hole size from the original, so it was mentioned.

DHURJATI SEN
Kolkata, India