While controlling a Depropanizer column, what should be done if the bottom level controller, level gauge, and bottom product flow controller all malfunction?
How can I roughly estimate the level of the column to maintain it within the desired range?
Look at your PI.
Temperature profile, thermal image?
Sight glass?
But if your flow control stops working, why would it matter? If you don't fix it, you won't be able to control it, so why do you really need to know what level it has?
--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."