What causes flooding in distillation columns and how is it prevented?

Flooding occurs when vapor velocity is too high, preventing liquid from flowing down the column - liquid accumulates and is carried upward with vapor. Causes include excessive vapor rate, high liquid load, or fouled internals. It is detected by rapid pressure drop increase and loss of separation efficiency. Prevention involves proper column sizing (75-80% of flood velocity), monitoring pressure drop, and maintaining clean internals. Downcomer flooding occurs when liquid backup exceeds tray spacing.