A peculiar problem is happening in my plant, we have a distillation column whose reboiler uses 17 KG steam about 330 degree Celsius. as the opening of the control valve is increased from 0 to some value, the steam to reboiler increases and hence the temperature increases, but at about 80% MV the temperature decreases increasing the MV further decreases the temperature. I think it might be a problem with the steam trap sizing.

 

Check your temperature measuring inst. most of the cases it is the cause...