Thanks fellas for all the help... I appreciate all your help and through experience will one day pass it forward. .... After reading your response, I would agree with katmar. I have a diesel reboiler stripper type that is boiling and flashing gasses off the top, going through a condenser and into a reflux drum. the reflux drum in itself is a flash drum sending both liquid back into the stripper top and flashing vapors off the top. Since the process is boiling it would be safe to say that the pressure in the vessel is equal to the vapor pressure.