Bed limiters and hold down plates are retaining devices used above packed beds to prevent fluidization and restrict packing movement, which can occur during upset conditions. Bed limiters are used for metal and plastic random packing as well as structured packing. They are fastened to the column wall by means of a support ring or bolting clips. They can also be suspended on tie rods from the liquid distributor.
Hold down plates are used for ceramic and carbon packings. They rest directly on packings and prevent packings from breaking up due to fluidization when operated at high pressure drops or during temporary surges.
In place of bed limiters, anti-migration bars may also be used at the bottom of the gas risers of a distributor. They do not prevent fluidization of the bed but prevent the random packing elements from being blown up through the gas risers.
Bed limiters are designed to provide high open-area and reduce interference to liquid flow. They should be designed to withstand upward forces acting on the packed bed.
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