Trays turndown problems with

In comparison with tray towers, packed towers are suited to small diameters (24 in. or less), whenever low pressure is desirable, whenever low holdup is necessary, and whenever plastic or ceramic construction is required. Applications unfavorable to packings are large diameter towers, especially those with low liquid and high vapor rates, because of problems with liquid distribution, and whenever high turndown is required. In large towers, random packing may cost more than twice as much as sieve or valve trays. 

Several experiences of severe weeping from valve trays have been reported (1,71,75). A well-designed valve tray is unlikely to have too many valves, but trays with light valves are common in an effort to reduce pressure drop. To avoid the turndown problems, manufacturers often specify a valve tray with two valve weights (Fig. 6.19e). When the light valves open, the heavy ones are still shut, which reduces the ratio of slot to active area and avoids weeping. This practice is discussed in detail elsewhere (1,71). 

Vapor load near the column top was three times lower than near tire column bottom, causing turndown problems. Successful resolution was achieved by different modifications to top 40% of trays. Modifications included (a) blanking half the valves, (b) using leak-resistant valve units, each containing a lijditwei t plate located below the normal disk (c) retraying with bubble-cap trays. 

Minimum wetting. As with a tray tower, this turndown chack is performed at the lowest loaded stages (8 and 9 ses Table 6.10) at turned down conditions. As per Example 6.1, the problem requires turndown to 60 percent of the expected design loads. Therefore, the vapor and liquid loads shown in Table 6.10 for stages 8 and 9 are multiplied by 0.6 for the minimum wetting check. 

To correct this problem, it is necessary to retray the upper 40% of the stripper with trays having a superior turndown ratio, or less vapor capacity. Some of the alternatives I have seen used with success are ... 

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