In what circumstances should the main steam stop valve be secured during a boiler outage?

The correct choice highlights important safety protocols in boiler operations. Securing the main steam stop valve during a boiler outage is essential, particularly when there is potential for working on the boiler itself or if there are issues with the non-return valve, such as leakage.

When maintenance or repairs are to be performed on the boiler, it is critical to ensure that steam cannot inadvertently flow into the boiler or the system, which could pose significant safety risks to personnel. If the non-return valve is leaking, this could allow steam pressure to re-enter the boiler system despite it being shut down, which could lead to dangerous situations including potential accidents or injuries.

By securing the main steam stop valve, you effectively isolate the boiler from the steam supply, preventing any unintended pressure buildup and ensuring a safe working environment for maintenance personnel. This approach aligns with standard safety practices in engineering operations, emphasizing the importance of preventing any unintentional releases or pressures during a boiler outage.