Downcomer exit velocity
Downcomer exit velocity
(OP)
Hi there,
I am working on tray hydraulics for process water stripper in a steam cracker plant.
One of the vendors for the column internals (valve trays) has come up with a design change to increase the downcomer clearance to a higher value 65 mm (instead of original value of 60 mm) to be within the max. liquid velocity at downcomer exit(1.5 ft/s or 0.45 m/s).
I understand the reason might be due to the correlation between DC exit velocity and DC outlet area (Vud α 1/ DC outlet area).
Strangely, the vendor did not quote any change in DC outlet area in his revised rating sheet. So, I am wondering if there is any other correlation that i am not aware of.
Awaiting your insightful responses.
Cheers.
I am working on tray hydraulics for process water stripper in a steam cracker plant.
One of the vendors for the column internals (valve trays) has come up with a design change to increase the downcomer clearance to a higher value 65 mm (instead of original value of 60 mm) to be within the max. liquid velocity at downcomer exit(1.5 ft/s or 0.45 m/s).
I understand the reason might be due to the correlation between DC exit velocity and DC outlet area (Vud α 1/ DC outlet area).
Strangely, the vendor did not quote any change in DC outlet area in his revised rating sheet. So, I am wondering if there is any other correlation that i am not aware of.
Awaiting your insightful responses.
Cheers.





RE: Downcomer exit velocity
Increasing downcomer clearance and weir height usually results in reduced turndown - e.g. from 20% with the original dimensions to 30% with the new dimensions. Also worth verifying with the supplier.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Downcomer exit velocity
Coming to my original query, I was going through tray design manual from NORTON which has provided standard ratios of tray dimensions and area relationships.
Say for a given ratio of downcomer width to tower dia (H/D), downcomer area to tower area (Ad/At) is fixed.
So, does it explain why the downcomer area has been unchanged or is it supplier specific?
Regards,
Avani
RE: Downcomer exit velocity
Downcomer cross-sectional area remains the same because it is independent from downcomer length. The D/C area is what you see when looking at the tray from the top perspective.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Downcomer exit velocity
RE: Downcomer exit velocity
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
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