BY BRIAN ARNOLD
The Oklahoma City (OK) Fire Department introduced positive pressure ventilation (PPV) during the late 1980s and early 1990s. It began removing from the apparatus old smoke ejector fans, which moved approximately 3,200 cubic feet per minute (cfm), and replaced them with new, gasoline-operated positive pressure fans that move between 15,000 and 23,900 cfm. Our department has had some success in our newfound tool; it is considered only one of several ventilation options for truck crews arriving on-scene.
But over the past several years, I have seen a trend developing in many departments that may one day result in a catastrophe. PPV is sometimes the only ventilation method they use on well-developed fires that would be better handled using vertical or horizontal ventilation. Some consider PPV the only way to ventilate at many incidents, instead of just one option among others. I see fewer structures vertically or horizontally ventilated; the PPV fan is dropped off and turned into position while firefighters are still stretching the attack line through the front door. Using PPV in this manner is completely contraindicated for several reasons.
Several National Institute of Standards and Technology (NIST) studies compared several room-and-contents fires ventilated using PPV with those that were naturally ventilated. Two of these specifically addressed single-family dwelling (SFD) compartment fires: NISTIR 7342, “Full-Scale Evaluation of Positive Pressure Ventilation in a Fire Fighter Training Building,” and NISTIR 7213, “Effect of Positive Pressure Ventilation on a Room Fire.” Note that firefighter PPV experiences across the country have spawned the term PPA (positive pressure attack); NIST makes no distinction between PPV and PPA. I will use the term PPV for the remainder of the article. I hope that future studies comparing PPV use with PPA will assist firefighters in determining in which situations the fan is most effective and safe to use.
NIST 7213 reinforces the need for applying PPV in the proper setting. Future NIST studies of PPV in SFDs are in progress and should provide us with even more information on which to base some of our ventilation decisions. Some of the findings from these studies are as follows:
- Both NISTIR 7342 and NISTIR 7213 studies show that there is a significant increase in heat release (up to 60 percent greater) once the PPV fan is started before the positive effects of the fan could overcome the internal fire and pressures it produced.
- Both studies showed PPV upsets the thermal balance that the fires in these structures naturally create. Upsetting this thermal balance not only reduces visibility down at the firefighters’ living space but, more importantly, increases the temperature in this same space and subjects fire attack companies to undue punishment. Think about trapped victims in this situation!
- Both studies showed that the effects of the PPV take approximately 90 to 120 seconds to fully reverse the pressures of the internal fire. One study recommended that fire attack crews delay their attack approximately one to two minutes until total PPV has taken hold. This will allow the heat and smoke layering to rise, thus reducing extreme conditions for firefighters.
- Both studies noted increased flame extension out of ventilation openings and stressed the need to evaluate exposures properly.
- Both studies also noted that employing horizontal ventilation alone allowed the fire to grow to the same heat levels as the PPV after a relatively short time.
- Both of these studies also used a single room-and-contents fire with an adjoining corridor to simulate that of a full-scale SFD; they did not use a full-scale single-family dwelling structure.
Although the NIST laboratory tests are invaluable, the studies themselves state that they do not address all aspects of the actual structures within which we fight fires. For example, neither of these studies addressed the addition of multiple rooms with even more fuel loading (smoke), which propagates more British thermal units (BTUs, or heat). Each study noted that both the naturally ventilated and the PPV fires generated a significant amount of smoke and heat. However, in neither study was smoke allowed to remain in the structure before or after flashover; the smoke and heat vented from the experiment room to the outside environment. If the smoke and heat were transmitted to other rooms within the structure, the videos would have been even more dramatic, showing a fireball erupting from all exit openings.
Three excellent experiments that demonstrate heat and smoke production are available on CD from NIST. The first is the video that demonstrates the rapidity of flashover development when a dry Scotch pine Christmas tree is involved. The second video features the sofa/living room of a simulated SFD, again demonstrating how quickly a flashover can develop in an SFD. The third video shows an open floor plan office workstation fire, demonstrating how quickly a flashover can develop in a wide open floor and ceiling space.
The common factor in the buildup of all three of these fires is the amount of smoke and heat generated. When a significant volume of smoke (fuel) is generated, it needs only a single element, heat, to create a flashover. When we deploy the PPV fan at the front door before extinguishment, we are literally “fanning the flames,” accelerating the fire growth and allowing it to reach flashover more quickly. Once the smoke (fuel) does ignite, it in turn generates even more Btus, which generate more Btus, and so on, until the fire becomes too difficult for interior attack companies to extinguish without the addition of multiple or larger handlines.
Examples of the misuse of PPV and its effects are not a new phenomenon and can be seen on the Internet. These videos provide multiple examples of PPV being employed with near-catastrophic consequences. The Fire Engineering Community Web site recently posted a similar video showing the effects of improper PPV. I’m sure in the coming months we will see even more instances.
PPV CAN BE A POWERFUL TOOL
PPV can be a powerful tool when used properly or a deadly one when used improperly. When PPV was introduced to many departments, it was only meant to be one additional ventilation tool among others in our toolbox, not the only ventilation tool! All fire department personnel must understand that a fan on the rig does not take the place of a properly trained truck company. Use the proper tool for the job!
What I have seen within many departments is common across the nation—fewer fires mean less job experience. If we are not responding to many working structure fires nationwide, we must train our personnel about the proper time and place to employ PPV instead of vertical or natural ventilation. Listed below are some general ventilation guidelines. These guidelines are not all-inclusive; the incident commander and ventilation officer must consider them together with experience, size-up, training, and gut feeling. These guidelines are based on the experiences of multiple engine and truck company officers from around the country who are sharing their firsthand knowledge, close calls, and sometimes excellent videos that drive the points home.
Room-and-Contents Fire, Single-Story Structure
Light to moderate smoke and low-heat conditions issuing from the structure or on opening an entry point. PPV is the preferred ventilation method; natural ventilation is a good second option. Stretch and charge attack lines, and be ready to advance on the fire room before initiating the PPV fan (photo 1).
|(1) A single-family dwelling with moderate smoke showing from the eaves of the structure and visible fire in the window on the A/B corner. Depending on the rescue situation, you should consider PPV for this structure. If you do initiate PPV, closely monitor the effects to ensure a positive flow of smoke and heat toward the exhaust opening. (Photo by Tim Olk; photos by author unless otherwise noted.)|
Heavy smoke and high heat conditions issuing from the structure. Vertical ventilation is the best option to channel smoke and heat out of the structure; natural ventilation is the next best tactical choice. PPV is the last option unless fire attack companies are held at the door and delayed until the effects of PPV are seen. If PPV is performed without the noted delay, fire attack companies will be playing Russian roulette with the fire, gambling that they can make it to the seat of the fire before flashover. This dangerous gamble may result in firefighter injuries or fatalities, especially if an additional handline has not been stretched and positioned to protect the initial fire attack crew (photo 2).
|(2) Heavily charged smoke and heat are pushing out of all openings, but there is no visible fire. Vertical ventilation is the best option, followed by horizontal ventilation. Avoid using PPV until after the fire is knocked down.|
|(3, 4) A training fire in a single-family dwelling with moderate smoke and heat; heavy fire shows from two windows of a single room. Horizontal ventilation of this fire proved more than adequate; precut vertical ventilation holes in the roof were not used.|
Heavy smoke and heat issuing from the structure and signs that the fire has extended to the attic space. Consider vertical ventilation first if roof stability and building construction allow. If roof stability is in doubt, do not commit interior attack companies. In that case, the best option is a quick fire knockdown using an indirect attack with a straight or solid stream from an exterior position. Avoid PPV at all costs; it will only succeed in spreading the fire throughout the attic. Even an aggressive attack crew pulling the ceiling may not be able to get ahead of the fire, and opening the ceiling may drive the fire back down on attack crews (photo 5).
|(5) Heavy fire shows through the roof along with the horizontal ventilation that has already taken place; this and limited door access make the structure a poor choice for PPV. (Photo by Tim Olk.)|
Room-and-Contents Fire, Multilevel Structure
With any smoke showing. Delay PPV in multistory SFDs until you can verify the fire location and protect the stairways for civilian and firefighter egress. If PPV is begun before this confirmation, you could drive the heat and smoke directly to the upper floors and hinder search operations while spreading the fire (photo 6).
|(6) PPV is in use immediately after the fire is knocked down at this room-and-contents fire in a two-story SFD. Note the smoke pushing out under the mild pressure of the PPV fan from the open door on the second floor, even with the failed window of the fire room in the A/B corner.|
|(7) A 1½-story single-family dwelling with a basement. Heavy fire is showing from the B/C sides of the structure. Hold off PPV use until after the fire is knocked down. (Photo by Tim Olk.)|
WHEN NOT TO USE PPV
AEIOU is an acronym for specific fire situations in which you should not use PPV that some departments that rely heavily on PPV as a primary ventilation tactic use. It gives personnel very simple guidelines when deciding whether or not to fire up the fan.
Do not employ PPV for the following situations:
Attic fires: Fire spread throughout the attic may lead to early roof collapse.
Exhaust opening too small: Heat and smoke will not vent from the structure quickly enough to prevent rapid fire progression.
Imminent rescue: If the path of smoke and heat cannot be completely directed away from victims and path of egress.
Overpressurized occupancy: Increased interior temperatures from the fire.
Unable to locate the fire: Initiating PPV before the fire is located could lead to rapid fire progression and trap firefighters and occupants.
The above guidelines do not consider the possible fire involvement of multiple rooms and must be applied according to the conditions of each incident. However, using PPV after fire knockdown will benefit almost all situations; consider using it to supplement horizontal and vertical ventilation.
Size up interior compartment fires in multifamily dwellings, high-rise structures, and commercial occupancies before choosing the ventilation tactic. Consider the possibility of trapped victims, smoke, heat, wind, occupancy construction, the number of personnel on-scene, the distance to the seat of the fire, and many other variables. These factors will dictate whether positive pressure, vertical, or natural ventilation is the best tactic.
1. “Full-Scale Evaluation of Positive Pressure Ventilation in a Fire Fighter Training Building,” NISTIR 7342, National Institute of Standards and Technology. July 2006, www.fire.nist.gov/bfrlpubs/fire06/PDF/f06066.pdf.
“2. Effect of Positive Pressure Ventilation on a Room Fire,” NISTIR 7213, National Institute of Standards and Technology, March 2005, www.fire.nist.gov/bfrlpubs/fire05/PDF/f05018.pdf.
3. “Flashover 1” video, National Institute of Standards and Technology, www.fire.gov/publications/index.htm.
BRIAN ARNOLD is a 23-year veteran of the fire service, serving since 1991 with the Oklahoma City (OK) Fire Department, where he is a captain. He has contributed articles to Fire Engineering and is the fire training coordinator for the EOC Technology Center in Choctaw, Oklahoma, where he is responsible for recruit academies and other fire training courses.