What is a potential change that must be made to a boiler’s valves and fittings when using superheated steam?

When using superheated steam, one important consideration involves the material properties of the system's components, particularly valves and fittings. The correct adjustment is to replace cast iron components with iron or steel alternatives.

Cast iron, while robust for many applications, has limitations when it comes to the high temperatures and pressures associated with superheated steam. It can be more brittle and may not handle the thermal stresses as effectively as iron or steel, which have superior strength and ductility at elevated temperatures.

Using iron or steel for valves and fittings ensures that they can withstand the higher thermal expansion and potential phase changes of steam without risking failure. This change is critical for maintaining the integrity and safety of the boiler system under the demanding conditions of superheated steam operation.

The other options do not address the specific needs for the handling of superheated steam effectively. For instance, bronze fittings are not generally suitable for high-temperature steam applications, as they can also have limitations similar to cast iron. PVC materials are unsuitable due to their low-temperature threshold and inability to handle the pressures involved, while merely increasing the size of fittings does not address material compatibility and strength concerns.