Tray damage

Avoid vapor entry close to a liquid level. Reboiler vapor should enter the bottom of a fractionator a distance of at least tray spacing above high liquid level. Tray damage can result if the liquid is disturbed. 

Whatever the cause of the tray damage, however, it is often hard to prove tray damage without column shutdown and inspection, especially if damage is slight. Besides poorer fractionation, a damaged tray section will experience a decrease in temperature difference because of the... 

Bottoms Level. Trays are particularly vulnerable to damage during shutdown and startup operations. Glitsch. Inc., (Reference 14) provides several good tips to minimize the possibility of tray damage during such periods. 

If tray damage may occur, blow by connecting hose to PI isolation valve. In this case, close isolation valve at the column. 

Special care must be exercised with columns containing valve trays. In other types of columns, vapor can freely flow upward or downward through fixed apertures as well as through the downcomers (Fig. 11.2a). In valve trays, the valves seriously restrict downflow (i.e., they act as nonreturn valves that close for vapor downflow. Fig. 11.26). Excessive pressure drops, which may cause serious tray damage, can therefore occur at relatively low downward flow rates in valve trays. This problem is most severe if the downcomers and/or clearances under the downcomers are small. 

Water washing is sometimes used (7,23) as the last step of cooling a column, often from temperatures of 200°F or lower down to ambient. The wash water (or a cool wash liquid) should not be admitted unless the column is sufficiently cool and "heat pockets are absent. In one case (23), premature introduction of water caused tray damage. Guidelines 1 to 4 in Sec. 11.7 also apply to washing hot columns. 

Figur 11.6 Tray damage caused by a large step increase in cold water rate during a steam-water operation.

Assuming the sample of incidents is representative, one out of two tray damage incidents can be prevented by avoiding excessive liquid level in the column (Sec. 13.2). Six out of seven incidents can be prevented by also preventing vacuum formation in sections of the column (Sec. 11.2, 11.7, 11.8, 12.6, 12.9, 12.11) and by adequate installation and inspection (Chap. 10). 

TABLE 13.1 Summary of Tray Damage Causes In 17 Columns... 

Several incidents have been reported where trays were lifted off their supports due to such slug (49, 107, 231, 296). The author is familiar with some additional similar experiences. Ellingsen s survey (107) suggests that over half the tray damage incidents in the chemical industry are caused by deep liquid levels in columns (Sec. 13.1). 

In addition, Ellingsen (107) has suggested the following techniques for preventing tray damage due to excessive liquid levels ... 

In services using water-cooled condensers, a tube leak can lead to water being refluxed into the coliunn. In one case (358), this caused a pressure surge and severe tray damage. The effect of a condenser tube leak should be examined if a pressure surge may result, water removal facilities should be provided at the reflux drum. 

At shutdown, any hot liquid lines (e.g., bottom line from another column) must be blinded and any hot liquid drained prior to steaming out. In one incident (3), hot liquid leaked into a column and contacted condensate that formed when an upstream unit was steamed into the column. This caused a pressure surge and extensive tray damage. 

Effects of heat exchanger tube leaks into the column include off-spec products and/or undesirable chemical reactions. In some cases, this reaction may lead to rapid corrosion or plugging. It is important to realize that material leaking at the condenser or at an intermediate exchanger may travel to the column base and decompose there. In one vacuum column, water leaking at the condenser reached the reboiler and caused a pressure surge and tray damage (358). 

Surging may also occur when the column bottom contains water-insoluble components along with a small quantity of water. Because of steam distillation, the water acts as a light component. A case where severe tray damage was caused by this mechanism has been reported (358). The author is familiar with another experience where smdl quantities of water caused reboiler surging. The problem was eliminated by elevating the reboiler liquid offtake about a foot (Fig. 15.4c), and converting the section below the offtake into a reservoir which constantly supplied a small amount of water to the reboiler. 

An automatic steam cutout on hi level was installed in the stripper section of the tower. It saved the tower many times from tray damage. 

Ellingsen, W. R., "Diagnosing and Preventing Tray Damage in Distillation Columns, DYCORD 86, IF AC Proceedings of International Symposium on Dynamics and Control of Chemical Reactors and Distillation Columns, Bournemouth, U.K., Dec. 8-10, 1986. 

---