Have a thermosyphon reboiler on our amine regenerator that has not been flowing properly since turn around.  The bottom sump of the regenerator has a auxiliary baffle and a preferential baffle.  In fig 19-37 of the GPSA manual, it is baffle orientation b.  We think it might be pluggage, but we can't conclusively verify that.steam wide open with chest pressure at steam supply pressure, but no steam flow? By pluggage you hypothesize material blocking the inlet tube sheet or in the nozzle/inlet line. Other potential causes are loss of level on the reboiler side of the baffle due to leakage (raise bottoms level), foaming (shake test of bottoms material), film boiling (usually there is a cycling), a layer of floating hydrocarbons in the bottom (drain the tower).
A bit more on the symptoms might be nice as a "flow problem" is pretty non-specific. Opening up the system is likely to be disappointing, I have several times had this come up with nothing useful found inside of Butterfly Valves. On the otherhand shooting some steam (or N2, steam is nice for an aqueous system) just long enough into the reboiler to intitate the thermosyphon, opening equalization between the bottoms compartments, closing the inlet butterfly valve (if you have one) and boiling down the reboiler then opening it, or other kicks to the system have mysteriously started the thermosyphon with the root cause remaining unexplained.

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