Tower Draw-off Modification

[sshep]

Friends,

We are making some repair/modifications to a tower which requires (re)installing a 3" nozzle taking liquid draw from a chimney tray (tray is a total trapout with overflow). The 3" draw-off was located on the tower wall of chimey tray. I am being asked to approve a modification which would locate the nozzle below the tray. The proposal would then connect via pipe between an internal vessel flange (to be installed with new nozzle) to a nozzle to be installed on the tray deck. I probably won't know until monday why they can't put it back to original design, but now they are in a rush for approval of the change and I want to be ready for questions.

Column internals are almost never connected directly to the column wall in this fashion and I am sure it is with good reason. In my 20+ years as a process engineer, I have seen a few tower modifications of this type have unexpected consequences (especially differential expansion). I do not want to approve such a change without the approval of the tray vendor (may take time), but I have no hard reasons of my own to object. I think I have seen articles or been in seminars advising against this exact change, but I can't remember where or what the reason was.

It will help me greatly if you can give some comment on the proposed change.

best wishes always,
sshep

 

[epoisses]

Hmmm... with only 7 yrs experience I feel like I'm asked to explain Lance Armstrong how to pedal...
As you wrote, differential expansion would be the first thing that would come to my mind, with tray deformation as a probable cause, but would that be a serious problem in case of a chimney tray? As long as the tray stays intact, of course.

 

[epoisses]

cause -> result, sorry

 

[Milutin]

Hi sshep,

Installing pipe from tray to draw off nozzle can create week point because you introduce new flange (or connection) on tray deck, which can leak. This is only reason, which I can remember.

We have vacuum column in which we changed trays with packing, for new packing column shell was grossly oversized so we have pipes from LVGO, HVGO and wash section chimney trays to outlet nozzles. Column is 4 years in operation and we did not have any problems with this arrangement. Pipe should be self draining you can also thing about vortex breaker.

Regards,

Milutin

 

[0707]

One consideration is the placement of the draw nozzle. A flat chimney tray can be used with the nozzle located at tray floor level, or a portion of the tray floor can be lowered to form a sump and the nozzle located at the sump floor. Both designs require the same liquid head to force the design flow rate through the nozzles. Therefore, locating the nozzle in a sump lowers the liquid level on the chimney tray by an amount equal to the head requirements. This reduces the weight which the tray must support, but has little effect on residence time since that portion of the liquid depth corresponding the head requirements should not be considered as residence time in most cases.

Under most conditions rectangular chimneys are more economical than round ones. If rectangular chimneys are proposed by the tray fabricator, they should be accepted since this reduces costs.

A chimney height of 6 - 12" is normally adequate for low liquid flow. 12 - 18" is usual for high liquid flow if the draw nozzle is located in an inlet sump. All chimneys should have hats located a sufficient space above the chimney to give a peripheral area of 1.25 times the chimney area. The hats should extend at least 1" past the chimney on all sides and should be turned down slightly (15 ) angle starting 1" from the edge of the hat, if rectangular) to prevent liquid from running back underneath the hat and down into the chimney. If a leak-free design is required, the inlet sump should be seal-welded and gasketing used on the chimney tray floor. A center sump is preferred over a side sump, as some flow restriction can occur at very high liquid rates due to the shorter weir lengths of side sumps.

 

[sshep]

Gentlemen,

Thanks for the advice. While waiting for response from the internals vendor, the mechanical engineers reevaluated the nozzle stresses, etc, and informed us that they could keep the nozzle in the original location. This was the best possible result (cost, piping work, etc).

Thanks again.

best wishes,
sshep