Overflow chimney tray

An excessive number of risers should be avoided, because they will obstruct liquid flow and form a hydraulic gradient on the chimney tray. With short risers, this can provoke liquid overflow into risers. Caution is required to prevent any row of risers from restricting liquid flow. The hydraulic gradient can be estimated by techniques similar to those used for estimating hydraulic gradients on bubble-cap trays (48, 257, 319, 371). 

An overflow pipe (or downcomer), with an opening located below the top of the risers, is recommended for preventing liquid from overflowing the risers at high liquid levels (Fig. 4.10). The overflow pipe should be liquid-sealed at the bottom to avoid vapor rise through it. Two experiences have been described (57, 237) where failure to provide an overflow pipe caused liquid to overflow the risers and prematurely flood the column section above the chimney tray. In a third case (334), liquid overflowing the risers caused entrainment (Sec. 8.2). 

Downcomers, downpipes (internal or external), and overflow pipes are often used to transport liquid from the chimney tray to the column section below. Undersizing these will cause liquid to overflow the chimneys, with the consequences described in guideline 14 above. 

The author experienced one troublesome case, which was also reported by Lieberman (237), where liquid overflow through the chimneys caused a severe loss of efficiency in the packed section above. The chimney tray had undersized downpipes that were not liquid-sealed either the undersizing or the lack of seal (or both) could have caused the overflow. Lieberman (237) suggests that the overflow led to entrainment and flooding, hence the loss in efficiency. However, subsequent pressure-drop measurements and other observations provided no supporting evidence for the existence of flooding, and the author believes that vapor maldistribution due to liquid overflow (guideline 14 above) caused the loss in efficiency. 

When the chimney tray is used for liquid drawoflf, liquid level on the tray should he controlled or maintained by a sufficiently tall overflow weir in order to avoid vapor in the outlet (unless downstream piping is designed to handle vapor). 

I-beam interference can be just as troublesome in the space above a chimney tray. In one case history contributed by D. W. Reay (334), this interference is believed to have led to severe vapor maldistribution in a refinery vacuum tower (Fig. 8.66). The maldistributed vapor profile was displayed as a carbon deposit on the siuTace of the bottom packing. The deposit formed an annular ring about 5 ft wide that extended about 1 in into the bed. In that case, liquid was known to overflow the chimneys for several months because of an incorrect location of level tappings. This overflow caused liquid entrainment. Some entrained droplets ultimately carbonized on the base of the bed. Had the vapor profile been uniform, entrainment (and therefore deposit laydown) would have been more uniform. It is believed that vapor from the side chimneys was blocked by the beams and preferentially ascended around the periphery. If liquid overflow (down the risers) had been uneven, the maldistribution could have been further aggravated. 

I like to establish a water level on chimney trays up to the overflow pipe. Next, I crawl under the tray to look for trickles of water. When the tower heats up, such minor leaks are likely to get worse. Also, I check the alignment of the chimney hats to make sure they are actually centered over the chimneys. After the water is drained down, I look for puddles of water that indicate a need for drain holes. If there is an overflow pipe. I ll check that the overflow pipe will be submerged in its seal pan. 

Chimneys (risers) blocking flow to draw sump forcing liquid to overflow prematurely. Flooding of trayed section below pump around. Lack of response to pump around flow changes. Design error. 

Installed and fixed Baffles Overflows Weirs Downcomers Trays Chimneys Internal distribution systems Dip pipes Vortex breakers ... 

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