FOG STREAMS AND PPV: THEIR EFFECTS ON TWO FIRES

FOG STREAMS AND PPV: THEIR EFFECTS ON TWO FIRES

BY BILL GUSTIN

Fog streams and positive-pressure ventilation have a strong influence on the movement of fire, smoke, and heat within a structure. Used correctly, fog nozzles and ventilation blowers can hasten fire attack and limit property damage by rapidly purging a structure of products of combustion. Incorrect application of these powerful forces, however, can spread a fire and force contaminants into uninvolved portions of a building, rendering these areas untenable for trapped occupants and firefighters.

Let`s look at two fires and examine the strategy and tactics employed at each incident. Further, we`ll see how fog streams and PPV were used and judge if their application was effective and appropriate.

THE HOUSE FIRE: Photo 1

The first fire involves a ranch-style home in a quiet suburban neighborhood. Fire clearly has possession of the living room and front bedrooms. In addition, the fire probably is already in the attic, judging from the volume and pressure of smoke pushing from roof soffit vents.

The engine company has chosen to attack the fire “head-on,” directly where it is showing, instead of taking the line to the rear and advancing from the uninvolved side. But attacking fire from the uninvolved portion of a building is advocated in some widely accepted training manuals. The merits of this tactic, as explained in the textbooks, are quite valid: Attacking a fire from the uninvolved portion of a building helps to protect occupants and property in unburned areas by pushing the fire back where it is burning. However, there can be a vast difference between firefighting theory and fighting fire in the real world. Understaffed companies, consisting of only two or three firefighters, often must choose between “textbook” and expedient firefighting. Consider the additional time, hose, and personnel it would take to stretch a line to a rear door and maneuver it around corners in the utility room and kitchen and then, possibly, up a hallway to the fire. Also, there is no ironclad guarantee that you can always advance a hoseline from front to rear or rear to front. For example, as a neighborhood deteriorates economically, single-family homes are often divided into single-room occupancies or efficiencies. Adding partition walls in these dwellings virtually eliminates the option of attacking a fire from the uninvolved side. As a practical matter, therefore, this company probably made the right call by attacking the fire from the front door. They must realize, however, that operating from this position requires extremely careful application of the stream so that they do not push fire into the rear, uninvolved portions of the house.

The wide fog pattern directed into the front door almost completely fills this opening. This exerts a pressurizing effect, pushing the fire toward the rear of the house. Additionally, operating the fog stream at a distance from the doorway allows it to entrain a tremendous amount of air, fanning the flames issuing from the front windows.

Photo 2

Firefighters now direct their fog stream into the front windows, driving the flames back into the structure. Most fire recruit-trainees are familiar with the technique of using fog inside a building to ventilate a room after a fire has been knocked down. Directing a fog stream two or three feet from the window entrains smoke and steam and draws them out of the building. Air drawn in behind the nozzle replaces the contaminants and further helps to improve conditions. Directing a fog stream into a window also moves volumes of smoke, steam, and possibly fire but in the wrong direction! Instead of expelling contaminants, a fog stream directed into a window can push a fire and products of combustion into uninvolved areas.

Fire in the attic pushed by the fog stream blows out of a vent in the roof gable. Immediate steps must be taken to pull ceilings to access this fire. The nozzle team could begin this action by narrowing its discharge pattern to a straight stream. The force of a straight stream stands a good chance of penetrating the plasterboard ceiling and reaching the fire in the attic.

PhotoS 3, 4

The nozzle team moves in. The advance, however, is hampered by volumes of hot, expanding steam, a result of water broken by the fog nozzle into finely divided particles that vaporize shortly after being discharged. Their attack would be less punishing if side and rear windows were vented, allowing smoke and steam to escape ahead of the nozzle. Fog works well when it can force heat and smoke out of a ventilation opening close to the seat of the fire. No matter how big the vent opening is, however, there still is no guarantee that the nozzle team will escape being scalded by volumes of expanding steam.

In this case, however, the rear bedroom windows are remote from the fire, increasing the likelihood of serious injury or death to occupants trapped between the fire and these ventilation openings. Under these conditions, the nozzleman would do well to narrow the pattern to a straight stream. It would have less tendency to push fire and contaminants to the uninvolved areas and will produce less extraneous steam, which does little to suppress the fire. In addition, the number of firefighters pushing the line in the front door (photo 4) quickly reaches the point of diminishing returns because it can interfere with the nozzle team, whose members may need to momentarily reverse their advance to extinguish fire behind them.

As they advance, they must be vigilant of fire breaking out behind them, particularly when using a fog pattern. A short burst of fog into a well-involved room almost instantly darkens down the fire and fills the space with steam. This can give inexperienced firefighters the false impression that they have knocked down the fire enough to move on to the next room. But as the steam condenses and dissipates, air flows in, and the room can light up behind the nozzle team.

Photo 5

A positive-pressure blower is operated at the front door. PPV can rid the structure of smoke, heat, and steam and replace them with cool, fresh air. But PPV should not be implemented before careful consideration of the following:

Placement of the blower. It`s too close to the doorway. In addition to blocking access, the blower in its present position will “churn” or recycle contaminants back into the structure. The blower will be much more effective if it is taken off the porch and placed six to eight feet from the doorway. This will give the air stream distance to expand and seal the entire opening.

Will placement of the blower directly at the area of fire involvement force contaminants into uninvolved areas? Similar to fog streams, PPV works best when it can force smoke and heat out a ventilation opening close to the seat of the fire. When vent openings are remote from the fire, firefighters must determine the path that contaminants must take to leave the building and be certain that no occupants are trapped between the fire and the vent opening.

Will PPV spread fire in the attic? In the Sunbelt, attics are vented extensively to reduce the heat that builds in this space. In South Florida and other areas, residential roofs have a continuous vent opening in the underside of the roof soffit that allows air to flow freely through the attic. Fire originating in or extending into the attic can spread quickly toward these vents when positive pressure is applied. Smoke issuing from soffit vents is a cause for concern: Is it residual smoke from a fire that is completely extinguished or an indication of fire running the attic? If you`re not absolutely certain, don`t start PPV before thoroughly checking the attic. All fire attacks must be preceded by a size-up, which, at this fire, involves a walk around the house to check for occupants trapped at rear bedroom windows. An advanced fire in the kitchen or living room of this style of house would likely block the escape route out of the front door and trap occupants in the rear bedrooms. Usually the bedroom windows opened or broken by occupants trapped by security bars or casement or awning window frames are among the few ventilation openings in a structure otherwise closed to contain heating or air-conditioning. Application of PPV or use of a fog pattern prior to search and rescue can make conditions much worse for these unfortunate occupants because smoke and heat will be driven toward the windows where they are trapped. Never implement fire attack with fog streams or PPV without first considering the effect on occupants who may be trapped.

* * *

SHIPBOARD FIRE: photo 6

Our next scenario involves a fire in the engine room of the 200-foot M.V. Seismic Explorer. This type of vessel is quite common. Its open main deck is used to carry materials to offshore oil rigs and as a work platform for marine construction.

PHOTOS 7, 8, 9

Fire companies arrived in the early morning hours to find smoke issuing from ventilators and watertight door openings in the vessel`s twin stacks. The incident commander immediately assigned a rescue company to board the vessel, investigate, and locate the ship`s fire plans, which are typically kept in a weatherproof container at the main gangway entrance to the wheel house. No members of the ship`s crew were present to guide firefighters during the initial stages of this incident. Fire officers, therefore, relied heavily on the ship`s plans for orientation with the layout of each deck and the location of bulkheads, doors, hatches, passageways, and ladders. They also learned that the vessel was not equipped with a fixed extinguishing system, which would have enabled them to flood the engine room with CO2 or halon. The incident commander`s initial strategy was to confine the fire and search for any crew members who may have been below deck.

A fire on a ship can spread by convection, through openings to the compartment on fire, and by conduction, through steel decks and bulkheads. To confine a fire, shipboard firefighters must establish “fire boundaries”–that is, close all openings to the compartment on fire, cool the decks and bulkheads that surround the space, and remove any flammables in contact with hot surfaces.

Companies stretched hoselines to the main deck, which was directly above the engine room. Paint on the deck and stacks blistered from the heat. Fog streams, of course, were most appropriate for cooling these surfaces. Their small water droplets instantly flashed to steam when they contacted the hot steel. The outer hull sides of the engine room were cooled dockside by firefighters and waterside by the crew of a 41-foot Coast Guard boat, which operated its bow monitor on the hot steel. A hatch was found at the stern, which accessed the steering gear (large hydraulic rams that move the rudder). Fog streams were directed into this opening to cool the engine room`s aft bulkhead.

As mentioned previously, establishing fire boundaries also involves closing all openings to the space on fire. This helps to contain the fire within the space and reduces the flow of air feeding the fire. It is critical, however, that closures are made in the proper sequence–from the bottom up, starting at the lowest decks and working progressively toward the upper levels of the vessel. Fire, smoke, and heat will take the path of least resistance. Their natural tendency is to rise by convection and vent through topside openings. Closing topside openings first will force fire and its products to push out of openings below deck. At this point, it was not known if the watertight door to the engine room on the lower deck was closed. It was decided, therefore, not to close the doors in the stacks that were directly above the engine room. Further, no streams were directed into these openings for fear that smoke and hot, expanding steam would push out of the engine room and contaminate the lower deck.

As companies attempted to confine the fire, a team of firefighters began searching for crew members. Using search ropes and orientation taken from the ship`s plans, the firefighters entered the galley and descended a ladder to the lower deck. Finding no victims, they made their way aft through three watertight bulkheads and managed to close the door to the engine room. Now, while companies were organized for an attack on the fire, the topside doors to the engine room in each stack were closed, depriving the fire of oxygen and reducing its intensity. (Vent openings in the stacks, however, could not be secured because they lacked covers or damper controls.)

Reaching a fire burning on the lower deck of a vessel can be an extremely punishing ordeal. Before going below, select an opening and path from which to attack the fire and designate other openings for ventilation. Because the watertight doors in the stacks led directly into the engine room, they certainly would be the shortest, most direct route down to the fire. But the updraft of heat made it impossible to descend by this route. These doorways, however, made ideal ventilation openings because they were directly above the fire. To attack the fire, two engine companies paired up to advance a 134-inch hoseline forward into the galley, down to the lower deck, and then aft to the engine room. Firefighters were positioned along the line to maneuver it around corners and keep couplings from catching on the raised sills at each watertight bulkhead.

PHOTO 10

Hoselines were advanced aft below deck. Firefighters were positioned at each watertight bulkhead to keep couplings from catching on raised sills. Firefighters advancing hoseline followed the search line (to the left of the hose) that was played out by the search team and tied off at the entrance to the engine room.

The incident commander considered that his companies could encounter a considerable amount of diesel fuel burning in the bilge. This might be the result of the failure of a hose, connection, or sight glass to the “day tank,” located in the upper level of the engine room, that supplies fuel to the main engines and generators by direct gravity feed.

Foam, of course, is the agent of choice to suppress fire involving diesel fuel. But it was decided to precede foam application with a fog stream for two reasons: First, water fog gently applied to burning diesel fuel, a combustible liquid, may successfully extinguish the fire because No. 2 diesel has a relatively high flash point. Second, water fog applied in advance of AFFF would make a foam attack easier and more effective by cooling the fuel, thus reducing its rate of vaporization, and by quenching the hot steel surfaces, which degrade a foam blanket and reduce its vapor-sealing ability.

When companies advancing the hoseline reached the watertight door to the engine room, they ordered the doors in the stacks opened to provide topside ventilation. In addition, a PPV blower was operated into the galley. This had the effect of pressurizing the lower deck passageway, which kept it clear of smoke and heat when the door to the engine room was opened.

From this doorway, the nozzleman directed a fog pattern into the upper levels of the engine room. This produced voluminous clouds of steam, which were captured by closing first the lower and then the upper doors to the engine room. Conditions here were ideal for an indirect attack with water fog: tremendous heat (sufficient to vaporize the small, divided drops of water) and confinement (to contain the steam, which had a smothering effect on the fire).

The attack would prove to be successful. Black smoke, pushing from vents in the stack, was soon replaced with gray, then white clouds of condensing steam. Ventilation was later reestablished. Cool pressurized air flowing from behind the firefighters helped to push less buoyant residual smoke up and out of the openings above. Firefighters could now enter the space to overhaul the fire, which now consisted primarily of smoldering electric cable insulation. At this fire, firefighters effectively used the cooling and suppression capabilities of fog streams to their best advantage. Further, they properly channeled the flow of positive-pressure ventilation by correctly selecting the air inlet, the exhaust outlet, their path of attack, and the path of smoke and heat to reach ventilation openings.

PHOTO 11

In the aftermath of this extremely hot engine room fire, note that fire completely consumed the fire hose on the overhead reel (top of photo). n


House fire photos by Al Griffin.






Ship fire photos by George Izquierdo.






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