In distillation column design and maintenance, what is the primary function of manholes provided in the shell?\nChoose the best purpose based on standard plant practice.

Introduction / Context:
Manholes (also called handholes or access doors) are structural openings in the shell of a distillation column or tower. They are crucial for safe entry during shutdowns and turnarounds. Understanding their true purpose helps prevent misuse and improves operability, safety, and compliance with confined-space procedures.

Given Data / Assumptions:

• The equipment is a typical trayed or packed distillation column used in refinery or chemical service.
• Normal operating observation and control are handled by instruments and sight glasses, not by opening manholes during operation.
• Standard industry safety practice prohibits opening manholes while the column is in service.

Concept / Approach:
Manholes are designed for personnel access, equipment insertion (e.g., trays, packing), inspection, cleaning, and maintenance when the vessel is safely isolated, depressured, drained, and gas-freed. They are not process-control devices and are not intended for routine operational observation of hydraulics or for chemical injection during live operation.

Step-by-Step Solution:

Verification / Alternative check:
Plant turnarounds use manholes for entry, tray removal/replacement, packing changeouts, and internal inspection. Normal hydraulic checks rely on differential pressure, tray temperature profiles, and sight glasses, not open manholes.

Why Other Options Are Wrong:

• (a) Visual observation of liquid gradients under operation is performed via sight glasses; opening a manhole would be unsafe.
• (c) Antifoam dosing uses dedicated injection quills; dumping through a manhole in service is unsafe and nonstandard.
• (d) “All of the above” is incorrect because (a) and (c) are not appropriate functions.

Common Pitfalls:
Confusing manholes with observation ports or nozzles; attempting to use manholes during operation (violates safety and environmental controls).

Final Answer:
Give access to the individual trays for cleaning, maintenance, and installation.