When inspecting a boiler, main steam valves are:

When inspecting a boiler, the main steam valves should be closed, locked, and tagged to ensure safety and compliance with industry standards. This practice helps prevent accidental operation of the system while inspections or maintenance are being conducted. Closing the valves isolates the boiler from the steam distribution system, ensuring that no pressure hazards exist for the personnel involved in the inspection.

Locking and tagging the valves further enhances safety by providing a clear communication system that indicates the boiler is out of service and cannot be operated until maintenance is completed and the locks are removed. This procedure is part of a larger safety protocol known as Lockout/Tagout (LOTO), which is critical in preventing accidental energization or start-up of equipment, which could lead to injury or safety incidents.

The other choices do not reflect best practices for safety during boiler inspections. For example, having the valves open could expose personnel to dangerous conditions or steam release. Thus, ensuring the valves are securely closed, locked, and tagged is essential for a safe inspection process.