What should you not lock out when a boiler is shut down for inspection?

When a boiler is shut down for inspection, it's essential to ensure that safety measures are appropriately implemented to prevent any hazards. In this context, the top tri-cock or boiler vent is the correct choice not to lock out because it serves a critical function in maintaining safety during inspection.

The top tri-cock, or vent, allows for the escape of any steam or pressure that may be trapped within the boiler. Locking this out would negate its ability to function properly during inspection, potentially creating a dangerous situation if pressure builds up in the boiler. It is essential during maintenance that any remaining steam or pressure can be safely vented to prevent accidents and ensure that the inspector can carry out their duties safely.

In contrast, the other options serve purposes that are more directly related to maintaining the integrity of the system when the boiler is not in operation. The bottom drain valve, feedwater pump, and main steam valve are components that should be secured (locked out) to ensure that no inadvertent actions could lead to unsafe conditions while the boiler is under inspection.