How exposure control works
If ignored, this tactic can be the event that moves a fire from a structure of origin to a fire that has a block of origin
My previous article on rescue was the continuation of a 10-part series titled, "Coordinated Fire Attack" and is a review of those tactical processes needed to ensure a well orchestrated approach to fireground tactics for suppression activities.
Using the acronym: REC-REVOS (rescue, exposures, confinement, rapid intervention, extinguishment, ventilation, overhaul and salvage) as our tactical template, we move now to the next tactic, "exposures."
Exposure control is seen as one of the red-headed step children of fireground tactics alongside salvage. However, if ignored, this tactic can be the event that moves a fire from a structure of origin to a fire that has a block of origin.
To truly consider the idea of exposures, we need to remember that we are not just talking about the escape of fire from an original "box" of origin, but internal as well as external exposures and how fire/heat moves through a structure.
Once we remember how and why a fire moves from area to area, we can anticipate movement and implement tactics to cut off movement of fire within and outside a structure.
Number of causes
Once a fire starts, it can move and spread through a number of causes: convection, radiation, and conduction. Some textbooks use the term "direct flame impingement" or another similar definition. But I consider this as a combination of both conduction and convection, with this consideration: both events are happening at the point of fire contact as the heat of the source of fire is causing pyrolysis to the item being contacted by flames.
This flame contact initiates the pyrolysis process, whereby the item being contacted by the flames starts creating flammable vapors. As the contact point receives heat from the fire, it transfers heat (convection), which slowly changes the process from an endothermic (heat absorbing act) to an exothermic process (giving off heat) when the volume of flammable vapors have reached a point of self-sustainability.
Convection is the process whereby heat is transferred from one item (area) to another through the motion of heated matter. This matter can consist of smoke, heated air, heated fire gases and flying embers.
As fire consumes an item, it gives off light and heat as a byproduct. This heat moves from the source item outward, and can generally move based on environmental air movement, but at some point will generate sufficient energy to move upwards and outwards from the source.
As it moves, it follows the laws of thermodynamics and gives off the higher temperature energy on contact with items that are lower in temperature. This process is cumulative in nature and will continue until the items receiving the heat either reach sufficient temperature to ignite, or the process is cooled down.
This cooling down can take place through the application of water or cooler air movement such as positive pressure ventilation.
As the fire grows in size, smoke and heated air move vertically and horizontally, mushrooming at the ceiling levels of the container they are in. This mushrooming will force the smoke and heat to accumulate until it "banks" down and starts lowering temperature level in the room, or finds an outlet to escape, such as a door into the rest of a structure or a window that allows it to leave the structure of origin.
This movement of heat and gases causes pre-heating of all items in its path and spreads heated fuel (smoke) ahead of itself, assisting with self-propagation. For this reason, fire travel by convection is the most difficult firefighting problem to control.
In addition to pre-heating items in the direct path of travel, separate fires can be initiated away from the original fire based on the flammability/combustibility of items in the path of the smoke.
Movement of heat
Consider the movement of heat down a hallway that has a wood panel covering. This is an area where the deposit of grease from a near-by kitchen would obtain a coat of atomized oil on its surface and could ignite much quicker than other areas.
A kitchen wall covered with the precious artwork of a child alters the combustibility profile of a standard latex paint covered piece of drywall. The flammability/combustibility of decorative items completely alters the ability of a surface to ignite. It also increases the speed at which an existing fire will move from an area of origin, to subsequent areas.
The next method of initiating fire from one area to another is through the concept of radiation. Fire gives off radiation in the form of electromagnetic waves. These electromagnetic waves radiate out in all directions, transferring heat subsequent to the energy carried by the waves.
These waves are direct in nature and are not affected by wind. These electromagnetic waves can pass through glass and cause the ignition of drapes and cause an exposure fire. These electromagnetic waves are able to transmit their energy through water (which is why "water curtains" never worked) and explains why water must be applied to surfaces that are absorbing the radiation.
The water application removes heat from a surface faster than the electromagnetic waves can transmit their energy (heat).
The final method we will consider for manners of heat transfer in a fire is through the conduction process, which is the movement of heat through a solid body. As we mentioned in the convection explanation, the law of thermodynamics forces heat to move from an area of high temperature to an area of low temperature.
This means that heat sources transferring energy to materials that are able to transfer heat will do so - causing heat to radiate through generally solid objects to other solid materials contacting the initial wall being impinged upon by direct fire.
These materials can consist of pipes, metal girders and joists. These metal objects transfer heat readily and can cause heating of combustible materials on other floors, assisting with the spread of fire.
Consequently, structures near the fire building or interior parts of the structure of origin may ignite secondary to this heat transfer. This may also cause structural elements to weaken and may cause collapse to roof supports or floor supports.
Principles of protection
The principle of protecting exposures is that of reducing the ability of a fire to leave the confines of the structure or room it is currently in. This requires the attack process to be applied whether the fire is leaving the walls of the structure or leaving the room of origin.
The establishment of exposure coverage is generally considered only an exterior function, which is not true. Exposure control is practiced when fire exiting a doorway is extinguished and kept within the room of origin.
The concept is to ensure that materials (or structure) closest to the edges of the fire are covered first. When exposures are close, the most vulnerable areas are just above the fire.
We will consider both internal and external exposure protection concepts. Most firefighters apply the idea of exposure protection to the protection of areas outside a structure, so let's look at that first.
When anticipating the movement of fire from a structure of origin moving to another, we need to ensure that we are negating the abilities of convection, conduction and radiation from being able to transfer its energy to other surfaces, causing their ignition.
With exterior exposures, we need to evaluate how close the exposed structure is from the fire. And what type of building construction and materials are present in the structure being exposed. What is the size of the fire and its intensity in the area of the immediate fire? Do we need to get water on the fire exposure immediately to prevent extension, or do we have the 10 minutes or so it is going to take to get an appropriate sized fire attack line inside the structure of origin and extinguish the fire?
If we are not sure if we can make this happen, and we do not have rescues that need to occur, we should consider the establishment of exterior exposure lines. These can be set up for one person to operate, while another may be used to initiate a transitional fire attack.
Ask yourself this: "How much can I do with what I have?" Finally, remember that proper exposure hose line tactics has water flowing onto the structure that the fire could ignite, not the structure that is the source of the fire.
Interior exposures will be dealt with using interior attack lines. This concept is to suppress fire from its outer margins to its seat of origin when possible. Once the fire is controlled at the seat, start looking at the surrounding structure for hidden voids.
Has the fire gotten up into the attic or into a floor with truss construction support systems? Has the fire gotten into the voids of a balloon constructed house? Start looking for areas where vertical fire spread may have occurred.
Has the horizontal movement of fire caused sufficient pre-heating of other areas that should be considered with overhaul? If you believe fire may be in the walls or ceiling, open up inspection holes to confirm.
Work closely with your department investigators to cooperatively protect areas of origin or suspicious areas when possible. Keep in mind that your department investigators have a job to do later, too, so try not to make their jobs any more difficult than possible.
The conversation of exposure protection is not seen as a significantly glamorous part of the job and no one wants to sit outside on an exposure protection line when the fire is inside the house.
But, this task is critical and must be anticipated and applied when possible. Keeping fire contained to the smallest area possible is a hallmark of a fire professional. This is one of those tasks that, when performed correctly, is one of the greatest public relations tools at our disposal. Use when necessary and do it right!
- Fire Attack