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 of the weir of the tray below. This is called a positive downcomer seal. Without a positive downcomer seal, vapor will flow up the downcomer and displace the downflowing liquid. This will cause flooding due to excessive downcomer backup.
On the other hand, if the bottom of the downcomer is too close to the tray below, then the “head loss under the downcomer” will be excessive. Typically, a minimum downcomer clearance is 1.5 to 2 in. Too small a downcomer clearance will result in restricting the liquid flow from the downcomer. This will also cause excessive downcomer backup and flooding. Check the correct downcomer clearance on the vender tray drawings prior to the tower inspection.
Source : Working Guide to Process Equipment
Related posts:
- Seal Pans Downcomer The downcomer from the bottom tray is submerged in a seal pan (see Fig. 8.3), to preserve its downcomer seal....
- 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...
- Inlet Weirs Most trays have outlet weirs devoted to maintaining the downcomer seal. But some trays have inlet weirs too, or inlet...