Downcomer exit velocity

[A1nee]

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.

 

[EmmanuelTop]

Downcomer area has probably remained the same since inlet weir height needs to be increased for the same amount as downcomer clearance, in order to maintain liquid seal on the trays. The best thing is to check directly with the vendor.

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

 

[A1nee]

I checked the exit weir height, which has't been increased corresponding to the downcomer clearance. I think as long as the minimum static seal (typically 13 mm) is maintained downcomer clearance could be increased without any adverse effect on the tray efficiency. But, thank you for pointing out the that it has an negative effect on the turndown.

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

 

[EmmanuelTop]

I would go to the supplier with regards to this query. Please note that inlet weir height must be increased if downcomer clearance is increased, in order to maintain the liquid seal. If there is no inlet weir and one increases downcomer clearance above the height of the outlet weir, I don't see how the seal can be maintained. It will work OK at high loads, but at reduced throughput there will be not enough liquid to maintain the hydraulic seal.

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

 

[mfelzien]

Have you considered the problem of frothing and seal heights as reasons for design changes?

 

[katmar]

The D/C bottom clearance affects the pressure drop for the flow of the liquid through the downcomer. It may be that the supplier believes the liquid in the D/C is backing up too high, and increasing the clearance will allow that level to drop slightly. Apart from this additional consideration , I agree with all that Dejan has advised.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"