A Rat's vengeance

Some months ago, a tragic event occurred in Refinery. The crude unit was under shut down. The condenser had been off line for cleaning. A rat lost its life inside the equipment and was solely due to the gross negligence.

A rat, which resided from maintenance wala's toolbox, escaped and entered the condenser outlet pipe which was opened for inspection during the shut down $($refer Fig.$).$ What actually happened was, Engineer who was inspecting the internal health of process equipments, chased the rat and it crawled up the riser pipe to the reflux drum, got its head stuck in the drum's inlet nozzle. Unaware of the rodent's predicament thought it escaped outside thro some vent and recommended the vessel to be put back into service. After start up$,$ the rat must have died. Well, we all must come to that end eventually, although perhaps not quite that exact end. Such is the way of all flesh.

He had forgotten about the rat, until the poor soul took its vengeance $-$ the tower's pressure rose as high as $8k\frac{g}{c}{m}^2!$ It's $8$ times greater than the normal pressure of an atmospheric column and Engineer was greatly fired by MD$,$ as he was the process personnel who cleared the equipment for operation!

But being a cool guy, this firing dint raise the blood pressure up$.$ He coolly sat, thought about the causes that would possibly have lead to this incident that whole night and recalled back the rat incident. And thus he could find the reason and means to fix it back. Now had the Engineer not been able to find the solution, how would you apply your process engineering knowledge to help him fix it back?