Recessed sump

Install recessed sumps under the downcomer outlets. In effect, this modification increases the open area under the downcomer and this reduces downcomer pressure drop. 

One thing I never do in fouling services is to use recessed sumps below the downcomers. What s to prevent these sumps from filling up with sludge ... 

Finally, the inlet weir is also a dirt trap (like the recessed sump). So I would be afraid to use it in fouling or corrosive services. 

Below the downcomer of the bottom tray is the seal pan. The seal pan, like a recessed sump, is also a potential dirt trap. I ve crawled... 

Sometimes for large liquid rates, the downcomer skirt clearance is increased by recessing the bottom end of the downcomer into a sump built into the tray floor. Such a sump increases tray cost and may trap debris. 

All tray sections, draw-off sumps, recessed downcomer inlet sumps, seal pans, chimney trays, and other tower internals must be selfdraining. As a column is heated on start-up, water can expand to steam with explosive force. Especially for lower pressure and larger diameter towers, tray panels are frequently dislodged due to flashing of trapped water. My experience indicates that drain holes of less than 0.5 inch, drilled in the bottom of a pan are likely to plug off due to fouling. The better way to place a drain hole is in a vertical wall of a pan, flush with the pan floor, as indicated in Fig. 9.3. 

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