Fire doors and fire dampers: What It Is and How It Works (SOLAS)

A division holds the fire only if its openings hold it too: fire doors close the passages and fire dampers cut off the ventilation ducts.

Key facts

A door in a fire division has a fire resistance matched to the class of the division it stands in.
Stairways passing through more than one deck are enclosed by walls of at least class A-0, with self-closing doors on all levels.
Doors in the walls of category A machinery spaces are gastight and self-closing.
Self-closing doors must not be held open with hooks or other mechanical holders. The only holders allowed are remote-release ones, which let go by themselves when the release system fails or the power drops.
A fire damper in a ventilation duct maintains the fire resistance of the division the duct passes through. A smoke damper only limits the flow of smoke and gases; it gives no fire resistance.
Closing the fire doors, the dampers and the ventilation shut-offs is one of the first actions after a fire alarm. Who does it, the muster list says.

Day to day, the fire doors on the stairways and in the passages stand open, because the whole crew walks through them. Electromagnets hold them. On a fire alarm the magnets let go and the doors close by themselves.

From the deck: sea practice, not regulation. Details vary with the ship and the owner.

These are not bulkhead signs: a system is marked with its symbol on the fire control plan (IMO res. A.1116(30), table 3).

SOLAS II-2/9 (openings in fire divisions: doors, stairway enclosures, machinery space doors, dampers).

This is not a legal text: when in doubt, the current wording of the Convention and the FSS Code decides.