Dipleg bridging

Blockage or plugging of the dipleg due to an obstruction or catalyst bridging resulting from defluidization or sticky catalyst fines will effectively flood the cyclone, rendering the cyclone inoperative. 

A fully fluidized dipleg to insure against bridging of solids in descent calls for a relative velocity between the gas and the solids not less than that for incipient fluidization ... 

A particular hazard in the operation of non-fluidized diplegs handling cohesive powders is their tendency to bridge (or to arch) unexpectedly, even though the dipleg has been correctly designed for mass flow. Powder material... 

It is possible to avoid such solids bridging by modifying the flapper valve assembly so that a small amount of gas can leak into the valve and maintain some fluidization of the dipleg solids. This can be achieved by suitable modification of the flapper valve assembly. Once the flapper valve assembly has been in service for some time, normal erosive wear of the mating surfaces will automatically permit sufficient leakage to prevent solids compaction. 

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