What device is used to protect the boiler from expansion and contraction of steam mains?

The correct choice is expansion joints. Expansion joints are specifically designed to accommodate the dimensional changes that occur in piping systems due to temperature fluctuations, such as the expansion and contraction of steam mains. As steam temperature rises, the steam mains expand, and when the temperature decreases, they contract.

By integrating expansion joints into the system, the boiler is protected from the stresses that can result from these changes, ensuring it maintains structural integrity and operates safely. This protection is critical because excessive stress from thermal expansion and contraction can lead to leaks, mechanical failure, or even catastrophic incidents if left unaddressed.

In contrast, other devices mentioned serve different purposes. A check valve prevents backflow in a piping system, an expander is generally used for other types of mechanical purposes, and a relief valve is designed to release excess pressure to prevent pressure-related failures in the system. Thus, while each of these devices plays an important role in boiler and piping operation, they do not serve the specific function of managing the expansion and contraction of steam mains like expansion joints do.