Fastening of Trays, Downcomers, and Support Beams

Trays may be clamped (Fig. 7.2a) or bolted (Fig. 7.26) to their supports. The overlap of the tray plate on the support is usually V4 to 1 in (86). Similarly, downcomer panels are usually clamped (Fig. 7.2c) or bolted (Fig. 7.2c ) to the vertical downcomer support bars. The downcomer support bars are welded to the shell. The fastening of downcomer panels to their supports should permit some on-site adjustment for ensuring correct setting of clearances under the downcomer. 

Downcomer support bars should be compatible with conunercial downcomer adapters. DoAvncomer adapters allow downcomer enlargement in a future revamp without the need to weld new downcomer support bars to the shell. These adapters are clamped to the existing downcomer support bars and to the new downcomer panels (274), and their use can dramatically lower the cost of a future revamp. Downcomer enlargement is one of the most effective and most popular revamp techniques in foaming, high-liquid-load, and high-pressure services. 

It is a common (and desirable) fabrication practice to supply loose nuts, bolts, and clamps and to have these fastened inside the column. 

An alternative is to have the tray fabricated with all nuts welded to the tray pieces. This alternative saves both assembly costs and installation time, and the savings easily pay for the added fabrication costs (128, 274). It is estimated (274) that a loose-bolted tray takes 15 percent more time to install than one which has no loose bolting. Often, a loose-bolted tray assembly requires two assemblers (one to screw the bolt and the other to hold the nut), while only one assembler is required when the nuts are welded. Despite the cost and time disadvantages, loose-bolting is almost always preferred to welded nuts, because it enables easy removal of corroded or worn parts in the fiitme. Welded nuts are only justified in clean, noncorrosive services, and when installation time is critical. Sometimes, loose bolting is specified between the tray floor pieces, and welded nuts are specified for the downcomer and tray inlet areas (274). 

Special care should be exercised in systems where previous experience has indicated tray vibration problems. When tray vibration is expected, bolted-through tray connections should be used, along with lock washers or self-locking nuts, rather than frictional fastening. 

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