7 replies to this topic

[B-2 Spirit]

Hello Experts,

I need to know the significance of the paramter "Downcomer head loss" or Head loss under downcomer or Hud.

I am using Sulcol for tray adequacy checks. And this parameter for a certain load is coming very low.
the acceptable range is 2.54 to 38 mm. In my case it is 0.34mm.
Although all other parameters are ok, I have a low downcomer flood of 12%.
Is there something to worry abt?
thanks in advance.

regards,

Alok

[abhi_agrawa]

Alok,

The liquid in the tray downcomer is distributed on to the tray inlet through the downcomer clearence. If the head loss is low, then you may have liquid maldistribution in the tray inlet leading to poor sepatation efficiency due to chanelling.

You should discuss this with a tray vendor, they will have different mechanisms to overcome this potential maldistribution.

Hope this helps,
abhishek

[Dipankarc84]

Hi Alok,

As Abhishek has already pointed out, low under downcomer head loss would generally lead to maldistribution and also more importantly you must check whether the under-downcomer clearance is less than the weir height or not. In case you have a downcomer clearance higher than than the weir height, at such low head losses, you might have vapor bypassing into the downcomer from the active panel.

Rather than looking at the head loss, it is always better to check the velocity under the downcomer. It is roughly OK to have a velocity in the range of 2 to 5 m/s (these are just rough nos.)

Downcomer Velocity = (Liquid Flow Rate (m3/hr))/ (Weir Length X Under downcomer clearance)

Hope this is helpful

Regards
Dipankar

[Rajkumar Chate]

Hello,

If u have a very less head loss in DC means you have a less liquid load or u r DC is over sized, if you do not have a limitation of pressure then increase weir height and reduce clearance.

DC head loss will create a problem of liquid distribution on tray below but this effect will not be a significant and will not hamper tray efficiency if column dia is small and flow path is less than 1 meter.

As explained by Deepankar, insted of looking to DC head loss important thing is that incase if you have very less DC head loss then there shall be positive sealing of 10-15 mm to ensure that vapors are not bypassing through the DC, if you have a 10 mm +ve sealing then check the DC clear liquid backup.

less head loss will not create a big problem in liquid distribution on tray but if you have a high head loss then it will be a problem of liquid distribution on tray. At higher head loss your DC exit velocity will be on higher side, it will create a imbalanced liquid head on tray panel at different locations and there is a high chance of vapor channeling due to this. To avoid this DC exit liquid velocity shall be kept below 0.5 m/s .


Regards

Rajkumar Chate

[Dipankarc84]

Hello,

If u have a very less head loss in DC means you have a less liquid load or u r DC is over sized, if you do not have a limitation of pressure then increase weir height and reduce clearance.
If you have very less "head loss" in DC, I guess the only parameter that would be affecting it is an oversized Underdowncomer Area. And oversized DC should not contribute to head loss. Correct me if I am wrong.

DC head loss will create a problem of liquid distribution on tray below but this effect will not be a significant and will not hamper tray efficiency if column dia is small and flow path is less than 1 meter.

As explained by Deepankar, insted of looking to DC head loss important thing is that incase if you have very less DC head loss then there shall be positive sealing of 10-15 mm to ensure that vapors are not bypassing through the DC, if you have a 10 mm +ve sealing then check the DC clear liquid backup.

less head loss will not create a big problem in liquid distribution on tray but if you have a high head loss then it will be a problem of liquid distribution on tray. At higher head loss your DC exit velocity will be on higher side, it will create a imbalanced liquid head on tray panel at different locations and there is a high chance of vapor channeling due to this. To avoid this DC exit liquid velocity shall be kept below 0.5 m/s .

Will the same UDC create an imbalance on liquid head? I think the imbalance comes into play in multipass trays where different liquid loads fall into different passes of the downcomer. The single most important effect of high downcomer head loss is an increase in downcomer backups which can contribute to DC Backup Flooding. Again, please correct me if I am wrong Rajkumar.

Regards

Rajkumar Chate

[Rajkumar Chate]

Yes only under DC clearance area is contributing to head loss, however this clearance area is not only depend on under DC clearance, it depends on clearance and bottom DC chord length. DC is oversized means bottom chord length is more and ultimatley your clearance area is going to increase.

[Rajkumar Chate]

Regarding high liquid load and vapor imbalancing.

If you have a high DC exist velocity means pressure at inlet panel on tray deck is less than the one at DC exit. more vapor will try to pass through deck close to DC inlet panel and there will be a imbalancing of vapor on tray panel.

[Dipankarc84]

Hi Alok,

As Abhishek has already pointed out, low under downcomer head loss would generally lead to maldistribution and also more importantly you must check whether the under-downcomer clearance is less than the weir height or not. In case you have a downcomer clearance higher than than the weir height, at such low head losses, you might have vapor bypassing into the downcomer from the active panel.

Rather than looking at the head loss, it is always better to check the velocity under the downcomer. It is roughly OK to have a velocity in the range of 0.2 to 0.5 m/s (these are just rough nos.)

Downcomer Velocity = (Liquid Flow Rate (m3/hr))/ (Weir Length X Under downcomer clearance)

Hope this is helpful

Regards
Dipankar