Most trays have outlet weirs devoted to maintaining the downcomer seal. But some trays have inlet weirs too, or inlet weirs, but no outlet weirs. A sketch of an inlet weir is shown in Fig. 8.2. Note the horizontal distance between the downcomer and the inlet weir (dimension x). This distance ought to be equal to or greater than the downcomer clearance—that is, the vertical space between the tray floor and the bottom edge of the downcomer. Unfortunately, a small deformation of the downcomer may push the downcomer quite close to the inlet weir. The resulting reduction in the horizontal clearance between the inlet weir and the downcomer will restrict the liquid flow. This will cause downcomer backup and tray flooding of the trays above.
Often, there is no process reason for the use of inlet weirs, especially at higher liquid rates. Then, the inlet weirs may be removed. But some tray types, such as “Exxon Jet Tab,” trays or total trap-out chimney trays with no outlet weir, absolutely require the use of inlet weirs.