What is a through stay in the context of boiler construction?

In the context of boiler construction, a through stay refers to a threaded rod that extends across the width of the boiler, typically held in place with nuts on either end. This design is crucial for maintaining the structural integrity of the boiler against the forces generated by the steam pressure. The use of a threaded rod allows for adjustments in tension by tightening or loosening the nuts, which helps in accommodating thermal expansion and preventing distortion of the boiler's structure during operational conditions. This adaptability is essential for ensuring the safe and efficient operation of the boiler.

The other options do not accurately capture the characteristics or function of a through stay. A rod with a fixed length does not account for the necessary tension adjustments; a bolt running between two tube sheets is more indicative of a different structural component in the boiler system; and a flexible stay would not provide the necessary rigidity and support that a through stay is designed to deliver. Thus, the description of a threaded rod secured with nuts effectively encapsulates the critical role of a through stay in boiler construction.