Take credit for column condensing when reboiler is overboiling?

[carol2005]

Working on sizing a PSV for a column. I am considering the excessive reboiler heat input scenario. In a thermal-siphon reboiler, the process flow is heated in the tube side with steam on the shell side. The control vale on the steam supply senses the column temperature to control the steam flow. An overpressure scenario potentially can be created when the control valve fail-wide-open, passing excessive steam to the reboiler, generating excessive vapor in the column. Although it is likely the steam trap can not remove the consequent excessive steam condensate, eventually paralyze the reboiler operation, it is better to be conservative not to take credit for it. The column top pressure is controlled with reflux flow.
When calculating the relief rate, I assumed "steam control valve failure" with "reflux failure" is a double jeopardy. I checked the reboiler heat transfer capacity and determined the excessive heat input can exceed the condenser's capacity to condense or otherwise absorb the build-up of pressure. Therefore, I subtract the condensing duty from the reboiler duty at upset condition. Based on that heat input, I calculated the required relief rate.
Would you agree with my approach? Should I take more conservative way not to count on the condensing capacity? Please share your experiences and insights. Any comments are very welcome.

 

[CJKruger]

carol2005, my view is you only have to relief the additional vaporization due to the control valve failing open, which seem similar to your approach.

The difficulty is deciding how much additional steam will condense in the reboiler. I would calculate it from the reboiler area, clean U value, and relieving temps, and not the change in control valve Cv.