coker fractionator with tray in the wash zone and P1 tray below wash section, bring this problem (old design) this area should always keep a lower temperature to 730 F; loss of flow or low flow in this section like you say are going to coke up from the wash zone to the shed tray section, a rule can be maintain a minimum flow of 3 to 4 gpm per inch of weir
the other method is that you can simulate this section (program simulation) and calculate the flow such that at least 30% of the flow remains liquid (leaving the plate P1), because once that tray are dry and again to get wet again the HCGO vaporizes and begins to coke up
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