Loss of Downcomer Seal Due to Leaks

The vertical edges to the downcomers are bolted to bars welded to the vessel wall. These are called, “downcomer bolting bars.” Gaskets are often used to tightly seal the edge of the tray downcomer to these bars. If the bolts are loose or if the gaskets are missing, vapor will blow into the downcomer and displace the descending liquid. Downcomer backup and flooding may result.

The area underneath the downcomer is called the downpour area. If a tray deck corrodes, it often first holes through in the downpour area. This will cause flooding due to downcomer back-up.

The bottom edge of the downcomer will be somewhat flexible in larger diameter towers. If the width of the tower is less than 5 ft, then the downcomer bolting bars prevent flexing of the bottom edge of the downcomer. However, if the width of the downcomer is over 5 or 6 ft, then downcomer bracing brackets (see Fig. 8.1) are required. The bottom edge of the downcomer should be immobilized by attachment to the bolting bar or bracing bracket every 4 to 5 ft, of downcomer width.

Recall that the pressure outside the downcomer is slightly greater than the pressure inside the downcomer. Therefore, a force will push the downcomer toward the vessel wall and reduce the open area of the downcomer. This restriction promotes downcomer backup and flooding. Don’t expect to see this deformation of the downcomer during your inspection. Once the vapor flow through the tray stops, the downcomer will spring back to its design position.

Source : Working Guide to Process Equipment

Related posts:

1. Improper Downcomer Clearance The bottom edge of the downcomer from the tray above should be 0.25 to 0.5 in below the top edge...
2. Seal Pans Downcomer The downcomer from the bottom tray is submerged in a seal pan (see Fig. 8.3), to preserve its downcomer seal....
3. Tower Final Inspection At a Gulf Coast refinery, the reboiler thermosyphon circulation could not be reestablished after a turnaround. The tower was reopened...
4. Inlet Weirs Most trays have outlet weirs devoted to maintaining the downcomer seal. But some trays have inlet weirs too, or inlet...
5. Shear Clips If a tower has a history of tray deck damage due to pressure surge or high liquid level, the mechanical...
6. Kettle Reboilers Reboilers are sometimes inserted into the bottom of a tower. These are called “stab-in” reboilers. It is not a terribly...

This entry was posted on Tuesday, March 29th, 2011 at 9:15 am and is filed under Tower. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.