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[simon84]

Hello guys

 

Since unit startup 10 years ago, level measurement in gas oil side-draw stripper doesn't work properly. 

The problem is, that approximately each 1.5-2 hours spike occures on this measurement. It behaves always

in the same way. At first level suddenly decreases in at 8-10% during approximately one minute and then it

returns back in 1-2 min without overshoot when it is in manual. During these upsets intensive bubbling sound is present in stripper. 

 

Level is measured by dp sensor and it is controlled by bottom product flow. Measurement was checked multiple times and no faults were found. Inlet flow from main column is controlled according to temperature bellow gas oil draw tray in main column. Pressure in stripper is around 42 KPA and temperature around 240 DEGC. Level setpoint is 50% most of the time. We tried to run it with all controllers around in manual mode, but level behaviour didn't change. Stripping steam valve is a little bit sticking but level peaks aren't correlated with its movement. There is field pressure measurement on stripper but it doesn't change before or during level peak.The same is with all process variables around - there is no change which can trigger  peaks on level. Our current explanation is, that kerosene draw is missing in main column (gas oil is only side-draw, overhead is heavy naphtha) and kerosene periodically boils from gas oil stripper.However when I increased stripping steam three times, there was no change in level behaviour. Thats why I doubt about that explanation and if you have some experiences with such issue, please share it.

 

Thanks!