no Liquid level with steam on - ok/not ok

[mrtangent]

Dear sirs,

I am interested in peoples advice here as to if we have a situation to be concerned about.

We have a column which is apparently started up "sometimes" so the operators tell me. That they leave the steam onto the reboiler when the level transmitter reads zero level. I have always thought this to be an issue to to coking of the tubes however this fluid is stable to very high temperatures and does not coke. The reboiler is a thermosyphon. I think often there is level in the base otherwise why would the steam be able to flow through the reboiler trap it is just below the liquid level.

I've seen plants with process trips which prevent this but i'm not sure what nornal pratices is.

 

[lastone]

Based on your description I would say that the Column you are looking at is a thermosyphon with a partition wall in the sump. The liquid from the bottom tray, or packing, is funneled to one side of the column. It falls down into the "reboiler" sump which is separated from the main sump by a wall inside the column. The liquid then flows through the reboiler, and the liquid that is not vaporized returns to the column and falls into the main sump. Usually, the only level indication you will have is on the main sump, not the reboiler side of the partition. If the column is started up empty, the operators that know better, realize they must start steam to the reboiler before there is a level present in the sump. Starting the steam is the best way to get liquid into the main sump. The operators that don't know better, just add feed to the column until they see a level in the bottom. To get the level they had to completely fill the reboiler until it overflowed with liquid into the main sump. Both ways work, but the ones that start steam without a level are probably able to get the column started quicker, and get the bottoms on-spec faster.