Seal pans downcomer, clearances

Both curves in Fig. 6.17 (especially the upper curve) are very sensitive to some of the downcomer layout design parameters, particularly to the clearance under the downcomer, and the design of inlet weirs and seal pans. Figure 6.18 illustrates the effect of reducing the clearance imder the downcomer, from 1.5 to 1.0 in on the startup-stability diagram shown in Fig. 6.17. 

Inlet weirs (Fig. 6.21a) and recessed seal pans (Fig. 6.216) are primarily used for achieving a downcomer seal in cases where a potential sealing problem exists and clearance under the downcomer is limited by one of the design criteria previously cited (Sec. 6.19). These devices provide a positive seal on the tray imder all conditions and ensure that the second and third sealing criteria (Sec. 6.18) are always satisfied. Sometimes it is argued that these devices improve liquid distribution to the tray, but this function is usually performed satisfactorily by the downcomer outlet (48, 172, 257, 404) and can rarely justify using either device. One exception is when the downcomer is circular... 

The cross-sectional area for liquid flow at the bottom of the downcomer (between the downcomer and the bottom of the seal pan) and the upflow area of the seal pan should be as close to the same as possible to avoid restricting liquid flow. It has been recommended (73) that a seal pan depth of 4 in be normally used, but this should be increased to 6 in for high-liquid-load applications. A deeper seal pan and a greater clearance between the bottom edge of the downcomer and the pan floor may also be justified when some sedimentation is expected. 

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. 

Dimensions of small magnitude, such as clearances under downcomers, weir heights, notch dimensions, seal pan widths, distances between downcomers and inlet weirs, downcomer widths (for narrow downcomers), clearances between chimney tray or redis-... 

Narrow openings, such as clearances under the downcomer, seal pan widths, and distances between the downcomer and inlet weir, have not been blocked on any tray. Often, a fabrication error will cause this. In one case (2066), an inlet weir intended for the top tray was located five trays below, effectively closing off the downcomer. 

Clearance between bottom of downcomer and top of tray or seal pan below shall not deviate from the ... 

Downcomer horizontal clearances, measured from the bottom edge of downcomer to recessed seal pan or inlet weir, shall be within +/- 1/8 in. (3mm) of nominal. 

Clearances between the seal pan and downcomer are double normal those that would be used for the downcomer and the tray deck. 

The downcomer from the bottom tray is submerged in a seal pan (see Fig. 9.3) to preserve its downcomer seal. I always set the horizontal dimension between the over-flow lip of the seal pan (dimension y) and the downcomer at 4 inches, so I never have to worry about restricting liquid flow from the bottom tray. This horizontal dimension should be equal to or greater than the vertical clearance between the downcomer and the seal pan floor (dimension x, which is typically 2 to 3 inches). If a deformation of the downcomer reduces the horizontal clearance between the seal pan overflow lip and the downcomer. 

Figure 9.3 Reduction of seal pan to downcomer clearance can flood tower.

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