Chimney Tray Leakage

It is desirable to place the draw-off nozzle in a portion of the tray floor that is lowered to form a sump. This lowers the height of liquid on the chimney tray by an amount equal to the nozzle diameter and reduces liquid leakage and the mass of liquid that the tray must support. The outlet nozzle should be flush with the sump floor to ensure adequate drainage. This practice is particularly fruitful in leak-tight applications, because it eliminates th> need for weep holes. 

A technique which can positively eliminate leakage is using a welded internal coliunn head instead of a chimney tray for liquid collection (Fig. 4.11c, d). The vapor can travel from the column section below the head to the section above either through risers or via an external pipe. This technique is expensive, and its use is restricted to situations where no leakage below the collection point can be tolerated. 

In some services (e.g., refinery fractionators), vapor approaching the chimney tray is hotter than the chimney tray liquid. Heat will be transferred from the vapor to the liquid. If the vapor is condensable, some will condense on the bottom face of the chimney tray. The net result is analogous to leakage. The author is familiar with situations where refractory was installed on the bottom face of the chimney tray. In all these cases, steps were also taken to minimize leakage, making it difficult to independently assess the effectiveness of the refractory. For multicomponent, partially condensable vapor condensing on an uninsulated bottom face of a chimney tray (e.g., in a refinery fractionator), a typical heat transfer coefficient is 15 Btu/(h ft °F) (237). 

In services where leakage is to be minimized and pressure surges tend to occur, it is often recommended to specify heavier gage for chimney trays and trapout pans (232). 

Only specific incidents were included. For instance, a statement such as "Leakage from chimney trays in refinery, vacuum columns can be reduced by seal-welding tray sections does not constitute a case history. On the other hand, a statement such as "One vacuum column experienced severe chimney tray leakage at low-rate operation. Seal-welding tray sections reduced leakage to acceptable levels does. 

An all-welded chimney tray can eliminate tr xnit tr leakage. 

Since this draw tray is a chimney tray and performs no fractionation, the temperature of the condensed liquid leaving the tray is estimated as being the dew point of the hydrocarbon vapor from the flash zone at the hydrocarbon partial pressure above the draw tray, the leakage air and the bottoms ste being defined as inerts. The temperature at this pressure is read from the vacuum region EFV curves. The vapor temperature leaving the grid is estimated in the same way but referred to the total pressure at that point. 

A leaking valve tray in trapout service was replaced by an aU-welded chimney trEQT, which was seal-welded to the tray ring. Leakage was eliminated. 

Gas redistribution chimney(s) installed properly with flow area dimensions as in drawing Draw-off trays — insiailed, sealed and secured correctly — satisfactory leakage rate test carried out... 

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