READERS’ FORUM: POSITIVE-PRESSURE VENTILATION
Our Readers Speak Out on a Timely Issue
The following letters were prompted in part by the editor’s Random Thoughts column of August 1989. Also refer to “Positive-Pressure Ventilation in a Test Setting” and Positive-Pressure Ventilation on the Fireground,” pages 56 and 60 of the December 1989 issue.
Throughout my 35-plus years in the fire service—volunteer, paid, and industrial—I have seen a recurring problem that has plagued our profession for a long time. Positive-pressure ventilation and its proponents are only a symptom of something that has been wrong for many, many years.
1 entered the fire service right after World War 11. At that time the fog nozzles, developed mostly by the Navy during the war, were just making their appearance in the American fire service. They were touted as the cure-all for our fire problems. In some departments the solid stream nozzles were removed from the apparatus and relegated to the closet in the station. The very first time that a fire was encountered that demanded a reaching capability for the fire streams, the limits of the tog nozzle became very apparent. In a large mill fire in New England, I can recall the officer-in-charge sending a utility vehicle to the stations during the height of the fire to retrieve the solid stream nozzles, while the fire grew in size and the main part of the mill was destroyed.
The next cure-all that came along was high-pressure fog. We were told that it was only necessary to make a small opening high up in the building, introduce small amounts of water, and wait for the stream to put out the fire. This rarely worked, more because of the reticence of the man-in-charge to stake his reputation on the method than the method itself. The system worked well in very controlled conditions but usually failed on the real fireground, as the incident commander was criticized as being afraid to enter the building and face the fire down. Or he didn’t have enough confidence in the approach to let the building stay closed long enough for the desired effect.
Next came a proliferation of foams. If you introduced high-expansion foam into the building you could fill it up from bottom to top, and the fire would be put out without men even having to enter the building. Once again, it worked well in controlled situations— almost always those in which it was possible to vent the building high to allow the products of combustion to escape as the foam filled the building. Then it occurred to those charged with the fiscal management of the fire department that if you could effectively vent the building, a much cheaper and just as effective attack could be made with 1 ½” lines and personnel equipped with selfcontained breathing apparatus.
In addition to high-expansion foam, there was “Wet-Water,” which was going to make water application so effective that conventional attack methods would pale by comparison. Then came “Rapid-Water,” which would cut down the friction loss so dramatically that it would significantly influence the fire, making many other tactics unnecessary. Then it was “Light-Water,” which was so good that it would revolutionize fire attack on flammable liquid fires. Granted, all of these things had their place in the attack inventory of a fire department, but most were touted as cure-alls.
In the more modern vein we have the Incident Command System, positivepressure ventilation, and higher education. The highly acclaimed Incident Command System seems to me to be exactly w hat the more highly respected chief officers have done over the years. They didn’t need vests or desks or special titles for each sector of the fire. They didn’t need a safety officer, because the safety of their men was one of their major considerations. They established themselves in a highly visible position, received reports, and issued orders, all the w hile making the safety of those involved in the attack a major consideration. They communicated, talked to the press, quieted the fears of the owners or occupants, and did all that they felt was necessary to control the situation. The way that the modern Incident Command System is touted by its proponents, one would expect the fire to just surrender the minute the incident command was established.
Everything cited is a tool. These tools, used properly, will have an effect on a fire situation, but none of them is the cure-all that many think they are. Consider a little bit of history. The fire department under Tiberius Caesar was so good that it lasted, virtually unchanged, for a thousand years. Can the same be said for the fire service of today?
Harry A. Lefebvre
Retired Battalion Chief Warwick (RI) Fire Department
As we have all read in most fire service training manuals, ventilation is the “planned and systematic” removal of heat, smoke, and fire gases. This definition does not include how to accomplish this task. PPV must be sized up and implemented according to a plan. That plan can only come about by studying and training with PPV so that firefighters can learn what it can and cannot do.
You state that PPV pushes heat and smoke toward victims. With careful planning and the knowledge of fire behavior, you can direct heat and smoke up and away from victims by opening and closing priority doors and windows. You also mention “unnatural fire behavior” as a result of PPV. I personally have conducted many live fire training burns, using PPV, and have never witnessed any such behavior caused by PPV. It is extremely important that firefighters study and learn fire behavior thoroughly and how the application of PPV can positively influence fire behavior during firefighting operations.
Two things that must be mentioned are flashover and backdraft. It has been my experience that PPV does reduce the probability of both. If you’ll examine the elements that cause both of those behaviors you will notice that PPV, properly applied, will reduce those elements.
I’m not saying that PPV is the panacea for all ventilation operations. However, it is another “tool” for our toolbox of firefighting knowledge.
Training Officer Kings County (CA) Fire Department
As a deputy fire chief in a fully paid, 187member department, I have had the opportunity to utilize the positive-pressure ventilation concept many times and discussed its use with my fellow firefighters around the state. I was so impressed and convinced of its value to the firefighting forces that I became a distributor for a major manufacturer.
Positive-pressure ventilation does not put out fires. Fires are extinguished by firefighters utilizing a combination of resources. The sequence of fire attack is usually determined by the first-arriving firefighters. This could range from the lowest firefighter on the totem pole to a chief officer. Whoever it may be has a responsibility to all to set the stage for the upcoming fight. It’s usually at this point that a determination is made to ventilate or not and where and how.
Caution—the person making the decision must observe the fire and know the telltale signs it’s giving. He would no more want to employ positive-pressure ventilation if there were a potential backdraft than he would order firefighters into a building without first relieving this potential danger through overhead ventilation. Your comment “If we pop the enclosure of die vertical shaft at the roof level, positive pressure will be lost” is not totally true! Positive pressure is not restricted to horizontal ventilation applications. If your selected opening is overhead, that’s where die products of combustion are going to go. I would never advocate setting up for positive-pressure ventilation unless a line were in place and ready to go.
In theory the exit opening should be no larger than 1 ‘A times the entrance opening. Unfortunately, we in the fire service do not operate in a controlled atmosphere where theories w ork. Once the fight begins, we make exit openings where needed in an attempt to relieve the immediate area of heat and smoke so we may advance to the seat of the fire. The more openings we make, the less pressure we can build within the structure. Introducing large volumes of air into a building will still force the inside air out, but because of the excessive amount of openings we have no control of the exit points. Positive pressure works when we are able to effect a seal at the entrance point and control the size and location of the exit point.
Addressing the concern of positive pressure spreading the fire throughout the building, I have not found this to be the case. On a number of occasions we have set up for positive-pressure ventilation during a fire and were able to enter and get to the seat of the fire for extinguishment without forcing the fire into unburned areas. 1 feel this was accomplished by introducing large volumes of cool air into a location where the superheated air was about to bring surrounding materials to their ignition temperature. By bringing the temperature down we eliminated the possibility of flashover. I will concede the possibility of moving the fire somewhat if the exit opening is immediately beyond the fire and we are delayed in knocking it down for whatever reason. Consider, if you will, the effect of not cooling this atmosphere down and being delayed in finding the seat of the fire because of the heat and smoke!
Your concern for those victims or firefighters on the other side of the fire is a legitimate one. As you pointed out, conditions in this area will certainly worsen if we force the heat, flame, and products of combustion at them with a hose stream toward a horizontal vent behind them. But consider the effect that could be realized if we “selected” an exit point away from their location — such as overhead —and were able to force the products of combustion and heat to this location instead of at them. The forced introduction of large volumes of cool air will provide them w ith a cooler environment and better visibility and buy them precious time. Understandably, this must be done in conjunction with an aggressive attack on the seat of the fire in order to get a line between the fire and the victims.
One other use for positive-pressure ventilation not addressed in your article is at the scene of a hazardous-materials incident. Our department had just put one of these units into service when we w ere called to a spill of hydrogen sulfide. There were 300 people evacuated from the building and 50 of those were transported to local hospitals because of possible exposure. We set up a gasolinepowered fan at a chosen location and “selected” the exit point to remove this toxic gas from the building. Within minutes the building was purged of the toxic fumes and the situation mitigated. Without utilizing this concept it would have taken hours to clear the building.
I would be remiss in not pointing out that there are some negative factors that come with the use of positive-pressure ventilation If a department wanted to retain the use of electric-driven units, it would have to stack them to get the same cubic feet of air movement as a gasoline-powered unit. This means they would have to own more units and provide storage space for them. Gasoline-driv-en units add to the noise level on the fireground. They’re loud, and communicating in their immediate area is difficult. Both types of units w ill drive loose sand and debris in the direction they are pointed. In this area of the country, one must consider the “windchill factor” that is present during cold weather. If a firefighter has any water on his gear and passes in front of a fan, he will immediately turn to ice. This includes his SGBA regulator and mask!
Continued on page 26.
Readers’ Forum: Positive-Pressure Ventilation
As you say, “Positive-pressure ventilation is a new and innovative tool.” If we understand its concept, experiment with it, train with it, and control its use, we will have made a giant step forward.
Framingham (MA) Fire Department
My department has used positivepressure ventilation on initial attack for eight years. We have also conducted tests with the China Lake Naval Weapons Center Fire Department that included installing thermocouples and carbon monoxide sensors in each room of several buildings that were then burned and extinguished using PPV on initial attack. The Kern County Fire Department has never experienced any of the adverse conditions you mention.
Positive-pressure ventilation in our department consists of using gasolinepowered fans capable of producing 8,000 to 10,000 cfm air flows. These are frequently used in tandem to produce air flows up to 20,000 cfm. The fan is started but not directed toward the fire building until the incident commander or the ventilation officer has determined the location of the fire and then, if the fire has not already vented itself, the fan ventilates the fire. If this is not possible, then the use of positive-pressure ventilation is reevaluated. This operation is constantly monitored by either the incident commander or the ventilation officer. This operation, if used correctly, rapidly expels heat and fire gases out the ventilation outlet. In the tests mentioned above, the temperature rapidly dropped to 300° at the hottest portion of the fire building (which was directly above the fire). Several fires were lit in hallways and could not be spread by using remote ventilation outlets.
To relate your concerns to my experiences:
“it makes rescues easier.“ If used properly, PPV rapidly clears the fire building of heat and smoke. This allows the building to be searched quicker and the fire to be located quicker, in an atmosphere under 300°. As you state, “But these are the victims that are always found, in one state or another, by all interior attack firefighters. In this case, their removal is just that—a removal.” While it is, sadly, true that most of these victims are already dead, the ones that would benefit by being found half a minute or more quicker deserve that chance.
“Where are the rescues’!PPV will rapidly force any heat and smoke (also any steam produced by firefighting operations) out the ventilation outlet and replace them with fresh air. This is because the horizontal thermal balance is rapidly replaced with a vertical thermal balance, with the hottest area directly over the fire and at the ventilation outlet (in one test the curtains on the window used for the ventilation outlet did not catch fire since the hottest temperature recorded was below their ignition temperature). Also, the smoke and heat are not just removed in line from the pressurization point to the ventilation outlet. A building under positive pressure can be compared with a balloon in that all the air is proceeding toward the ventilation outlet, just at different velocities. So while the area from the pressurization point to the ventilation outlet will clear the fastest, the area behind the fire will also clear of heat and fire gases.
Firefighters searching a building in temperatures over 600° and charged with smoke and fire gases without a charged line become victims.
“Unnatural fire behavior. ” The success of PPV, like any other tactical operation available to the firefighter, depends on the proper application at the right time. Improper, or inappropriate, application can have anywhere from mediocre to disastrous results. Positive-pressure ventilation will not force the fire to behave unnaturally. Fire is governed by the laws of physics. It will not force the fire through the building. PPV will force heat and smoke out the ventilation outlet (or outlets). It should not be confused with forced horizontal ventilation.
“Vertical ventilation.” Positive-pressure ventilation is not limited to either horizontal or vertical ventilation. It also should not be limited to the entire structure but can be used to pressurize individual rooms, floors, or stairwells. The act of creating a ventilation outlet is not as simple as popping the window in the fire room or opening the top of a stairwell. For PPV to be successful, several things have to be considered.
- You have to force more air into the building than can escape to the outside. This raises the pressure in the building above the outside. When this happens, all the air in the building will move toward the ventilation outlet (or outlets). If you cannot control the size, or the number of ventilation outlets, then PPV should not be attempted.
- More heat has to be ventilated to the outside of the building than the fire produces. If you cannot overcome the heat output of the fire by increasing the amount of heat ventilated to the outside, then the fire will continue to expand. It is up to the incident commander, or the ventilation officer, to determine if PPV, in combination with interior attack, should be attempted. In any case the fire intensity will be reduced by the number of Btu’s forced out the ventilation outlet.
- Positive-pressure ventilation is not static. If a room or stairwell is cleared of smoke, then ventilation outlets can be closed and new ones opened.
Positive-pressure ventilation is not a cure-all for every fire. Properly performed, combined with ventilation outlet management, and under the constant supervision of experienced fire officers, it can allow firefighters to search for victims and locate and extinguish fires quicker and with better visibility in a more tenable environment than using natural or forced ventilation. In the instances where PPV can be used, it can increase the chances for victims to be saved and allows firefighters to work in a safer environment.