Hold-Down Plates and Bed Limiters Retaining Devices

3 Hold-Down Plates and Bed Limiters (Retaining Devices) 

Hold-down plates are not used with metal and plastic packings to avoid crushing the metal or compressing the plastic. To prevent fluidization with these random packings, bed limiters (Fig. 8.7c,cf) are used. Bed limiters do not rest on the packing instead, they are fastened to the column wall by means of a support ring or bolting clips. Alternatively, they can be suspended on tie-rods from the liquid distributor. 

Random packing fluidization during even relatively minor upsets (such as column flooding) is common when a retaining device is absent. In large-diameter columns, packings tend to fluidize at random spots rather than imiformly over the entire top surface. Fluidization 

With structured packings, fluidization is unlikely, but in large-diameter columns sections of the packing may be dislodged during upsets. To prevent this, hold-down ben s are often used. These are bars fixed at the column shell and are perpendicular to the packing layers. In large-diameter towers, it may be desirable to use bona fide bed limiters rather than hold-down bars. 

Guidelines for the design and operation of hold-down plates and bed limiters are listed below  

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