This seemed more to foamover, that VR carried over, There are other symptom has been observed?, if there is not another factor that increases the temperature in the bottom of the fraccionator (temperature of the fresh feed, presence of HCGO of washing zone in the bottom, etc) .
But the probable cause maybe a foamover, because you said that problem is observed during steam injection. Are you observed more coke in the bottom filter?Are you obeserved some cavitation in the heater charge pump?
You can get from this forum, some guide, please, search the word “foamover” and you can found several discussion and paper, that can help you
Best Regards