VENTILATION
Ventilation is a must in any treatment of shipboard fire fighting because the thinking has changed since World War II. Even though industrial fire fighting ashore has taken this into account, practices in fire fighting aboard ship have lagged behind. In the initial stage of shipboard fire fighting, the compartment is closed tightly and the ventilation shut down. The thinking behind this is that the fire may die out from oxygen starvation.
The problem with buttoning up and shutting down ventilation is that the fire continues to run its course within the compartment. The fire progresses from the first or beginning stage on through the third or smoldering stage with heavy production of unburned gases. Conditions become critical as the third stage of burning is entered into since temperatures within the compartment are superheated and above what a human can withstand. Worse is that in the third stage of burning, unburned products of combustion are being formed and deposited throughout the room. Heat within the compartment is being carried away into compartments with the result that eventually flammables in those compartments may reach their ignition temperature. In this stage of the fire the real problem develops when fresh air enters the room when a door is opened for attack on the fire. As the fresh cool air enters, the oxygen in this parcel of air immediately is involved in rapid combustion with the unburned gases that are spread throughout the compartment. Instantaneous combustion occurs with results that look similar to an explosion. Also, the cool air that enters the compartment will be heated and expand to ten times its original volume when it comes into contact with the superheated gases. Since the compartment is sealed up except for the open door, the gases find that they have only one way out in order to relieve the pressure buildup in the compartment and that is out the open door. It is possible that the ball of flame and superheated gases that force their way out the open door will burn all personnel within the adjacent compartment. In any case no fire fighter will be able to enter the compartment once the thermal boundary has been severely disrupted.
Since air is going to enter the compartment anyway when the entry door is opened and since the fire fighters are going to find that they are not going to be able to enter the compartment due to existing conditions, it would be wise to establish controlled conditions in the compartment that will lower temperatures, vent unburned gases and superheated gases to the weather deck, and improve visibility within the compartment. Modern shipboard construction practice involves the use of mechanical supply ventilation as well as mechanical exhaust ventilation. In the initial stages of the fire the supply ventilation only is shut off to the compartment on fire to eliminate a source of fresh oxygen to the seat of the fire. Rather than shutting of the exhaust ventilation as has been traditional practice, the exhaust ventilation is kept running at full speed. Rather than hot gases collecting within the room, they are pulled off as fast as they are generated in the overhead. No thermal boundary is allowed to develop within the room; no unburned gases of combustion are allowed to collect; no gas pressure buildup is allowed to develop.
When the fire fighters open the access door to this compartment a draft will be exhausted through the door and into the compartment to the exhaust ventilation system. The fire fighters have hose in hand, and are ready to extinguish the fire on entry so any fanning of the fire due to the door opening and the draft will be only temporary in nature. Since temperatures are down and unburned gases at a minimum in the compartment the chance of explosive combustion due to the draft is at a minimum. Visibility is improved in the compartment due to this procedure since smoke is being pulled out of the compartment.
The key to shipboard fire fighting is to have drills and a plan for all ship areas so you are always ready in case a fire does break out.
