Process Equipment and Plant Design
The function of manholes provided in the shell of a distillation column is to

Guard against foaming & entrainment by dumping antifoaming agent through it
Give access to the individual trays for cleaning, maintenance and installation
Keep a check on the liquid gradient over the plate by direct visual observation
All of these

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Process Equipment and Plant Design
A stuffing box is used for

Absorbing the contraction/expansion of pipeline due to temperature changes
Reducing the resistance of fluid flow
Facilitating smooth opening and closing of a valve
Prevention of fluid leakage around moving parts

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