closures, expansions and clearances gen.

[Lonely Knight]

Помогите понять, что имеется в виду под closures и expansions.
Контекст скуден, к сожалению. Инструкция к огневому подогревателю. Вот весь раздел целиком.

Check closures, expansions and clearances.
For the whole start-up period, these details should be continuously checked. Bear in mind that eventual flue gas leakages could cause a quick damage of the panels, while, on the contrary, air leakages into the firebox could cause on efficiencies decrease.

[AVLS]

крышки и компенсаторы?