MFA #27: Intended Consequences - The two facets of Tactical Ventilation

Previous postings on this subject addressed the need to reassess our approach to ventilation as a fire control tactic in light of our newfound awareness of the significant increase in heat production it can cause (http://community.fireengineering.com/profiles/blog/show?id=1219672%...) and then provided an alternative basis for categorizing the different circumstances in which it is utilized, specifically related to whether or not combustion will be affected (http://community.fireengineering.com/profiles/blog/show?id=1219672%...).  This submission will examine those two situations and related considerations in order to provide a foundation for evaluating the varying scenarios we might face, and thereby improve our ability to carry out this technique in the most efficient and effective manner.

Knowing as we now do that performing ventilation of a compartment fire can increase its intensity more than it releases the resulting products of combustion, and yet faced with a continued need to remove those same heated toxins in order to accomplish our goals of protecting life and property, we must re-evaluate and revise our use of this tactic in order to ensure that it continues to serve our purposes.  Whether ventilation in a particular circumstance will raise temperatures and worsen interior conditions should be one of our first considerations regarding its implementation.  In my last post on this topic, Vent Fire was the term I proposed to indicate that combustion would be increased by our actions, while Vent Smoke would signify that only the products of combustion were being released, without affecting the burning process.  

We Vent Fire when our actions provide air flow to a compartment that contains uncontrolled burning (with “compartment" in this context referring to any enclosed space, regardless of size, shape or configuration).  An example of this would be opening the "A" side (front) door of a house in which a fire is located in the "C/D" corner (right rear) bedroom, as long as every door between remains open, as that action would, eventually, affect the intensity of combustion.  Depending upon the distance from the front entry to the bedroom, among other factors (e.g., wind direction, presence of open window in the bedroom, stage of burning, other flow paths, etc.), the resulting effect on the fire could occur in a matter of seconds, or be delayed for a few minutes.  Similarly, breaking a window to the room on fire or opening the roof of a burning building are interventions that meet the Vent Fire definition, and which typically cause the immediate increase of a fire’s heat release rate, regardless of other circumstances.

Vent Smoke, on the other hand, is intended to indicate that opening up will not affect the burning process, as when the fire has been controlled, or the location of the fire does not communicate with the ventilated area.  Removing the remaining smoke and heat after a fire has been knocked down can be an urgent matter, such as to facilitate search, or a less time-critical activity, as in overhaul.  The key point in the “fire under control” determination is that the fuel has been sufficiently cooled so that increased airflow will have no significant effect on combustion.  This might occur almost immediately after water flow is applied, as in a fire that has remained confined to a single room, or may require completion of overhaul in order to reach and thoroughly soak all of the hidden areas into which a fire has spread.  As with so much in firefighting, arriving at this point is a process, rather than a single step, and its accomplishment varies with conditions and circumstances.  A great example of the difficulty in determining when fuel is in a ventilation limited situation is when a novice firefighter throws a wet, smoldering mattress out of a window during overhaul, and it erupts into flames before it even hits the ground.  Of course, depending upon the amount of water applied vs. the amount of fresh air provided by ventilation tactics, almost any extinguishment can, eventually, be reversed, as the fuel is dried and remaining embers are fanned back into flames.  

Looking for examples of when we might Vent Smoke from an area separated from the fire, probably the most basic is the VEIS (Vent-Enter-Isolate-Search) maneuver, where the searching firefighter immediately locates the door to the room entered and ensures it is closed, and only then opens the window more fully to clear the smoke to facilitate the search and victim tenability.  An equally appropriate, but more robust, operation that also suits this term would be when bulkhead doors, roof scuttles, or skylights are opened to allow clearing of common areas in a multi-family dwelling fire, while the door(s) to the involved apartment(s) remain(s) closed.  In such instances, ventilation allows firefighters to advance and occupants to evacuate, while the compartmented fire will not be affected by the inflow of fresh air.  Between such extremes as carefully opening a single room for a quick search, or multiple floors of a common area to facilitate the movement of persons therein, there are many other circumstances when ventilation can be used to clear an area that is smoke-filled, yet isolated from the fire.  Sometimes a building's features (doors, walls, windows) have already provided this separation, and other times we can create equally effective barriers by closing doors or placing smoke curtains.   

Despite being the originator of this schema to categorize situations that respond so differently to ventilation, I have to admit (before someone else points it out) that the real world is not so cut and dried, even regarding such a basic “Yes/No” choice as to whether or not our actions affect the combustion process.  For starters, divining from the exterior the presence and location of a fire and its flow paths can be quite difficult, and at times impossible.  Breaking a window already darkened by hot smoke will obviously feed the fire within, but the next window, immediately adjacent, might open to a space that is separated by a closed door, which can be a remarkably effective barrier to smoke and heat, and the removal of that second window may result in no immediate effect on the fire.  

Secondly, burning buildings are extremely dynamic environments, with flow paths opening and closing by our actions, or by the fire reaching new sources of fuel and air by its spread.  While the size-up process is intended to inform us, among other things, about where a fire is located and where it’s going, and even if we are able to ascertain that information, the flames are not going to wait long before adding to and/or changing directions.  And, as mentioned above, a situation can transition from one of Vent Fire to Vent Smoke as we accomplish fire knockdown, yet revert back to Vent Fire if there is a rekindle before we complete extinguishment.  The distinction between the two, then, is important, but tenuous.

Finally, even when we decide to perform ventilation maneuvers, the potentially significant delay in their effects, when coupled with a fire crew’s ability to quickly stretch and flow a hoseline, can render the difference between the two situations meaningless.  For instance, the UL tests showed that a fire in a second-floor room with an open window did not reach untenability for firefighters (flashover) in response to an opening of the ground floor front door until after 3 minutes (200 seconds, on average), while a first floor fire reacted as much in just over 1 1/2 minutes (100 seconds, on average).  Any respectable crew of firefighters should be able to make either stretch with time to spare, though that would be a race that carries a potentially deadly penalty if lost.  (Of note, vertical ventilation over a fire can result in increased combustion in just 10 seconds, and breaking a window to a room on fire has been shown to cause almost instantaneous transition to flashover!) 

We have in ventilation a process that is a complex mix of risks and benefits, can be difficult to control, and is carried out in what is probably the most dangerous and chaotic setting that is regularly found in the civilian world.  Yet, most firefighters praise its utility, implement it almost reflexively, and have little fear of its hazards.  So, we need to reconcile, on the one side, the caution and reality that has resulted from fire dynamics research with, on the other side, our own enthusiasm and skills with this tactic.  Harnessing the action that is ventilation into a tactical tool will be the next topic of this series. 

MJC

The author can be reached at markjcotter@comcast.net

Next: MFA #28: Balancing Act - Controlling ventilation