Seal Pan Below the Bottom Tray

The downcomer descending from the bottom tray of a tower (or from the tray above a chimney tray) must be sealed to prevent vapor upflow through it. A case where improper sealing of chimney tray downpipes could have been responsible for poor column performance is described in Sec. 4.10, guideline 15. The guidelines for recessed seal pans also apply to seal pans below the bottom tray (or above a chimney tray). Additional considerations are 

The clearance between the bottom downcomer and the seal-pan floor should minimize pressure drop and restriction to fluid flow. 

This clearance should be greater than the normal clearance imder the tray downcomers, and should be at least 2 in, because the bottom seal pan is one of the most sensitive areas for solids accumulation. 

Submergence of the downcomer within the bottom seal pan should be approximately the seune as the clearance between the bottom of the downcomer and the seal-pan floor. One designer (307) recommends that the submergence be made at least 2 in. 

The horizontal distance between the bottom-seal-pan overflow weir and the downcomer bar should be somewhat larger than the clearance under the downcomer in order to minimize pressure drop. 

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