What are the two recommended valves in a double-block-and-bleed configuration?

In a double-block-and-bleed configuration, the focus is on ensuring that there is a reliable method for isolating sections of a system while allowing for safe venting or draining. This configuration enhances safety by minimizing the risk of unintentional leaks and ensuring that maintenance can be performed safely.

The correct choice identifies the use of an automatic nonreturn valve combined with an outside-screw and yoke valve. The automatic nonreturn valve serves to prevent backflow, ensuring that the pressure in the system does not reverse and compromise safety. Coupled with the outside-screw and yoke valve, which provides a reliable means of isolating a section of the system, this combination offers both isolation and protection against potential hazards.

This configuration is crucial for boiler operations because it maintains system integrity and allows for safe maintenance practices. In contrast, other options might not offer the same level of reliability or fail to provide the optimal combination of isolation and backflow prevention necessary for a double-block-and-bleed setup.