Beating the Backdraft

Beating the Backdraft

STRATEGY AND TACTICS

One of the dangers of forcible entry is the confined fire area’s explosive potential. Four safety measures apply.

A round midnight, an engine and a ladder truck arrive at a new shopping center. Smoke fills the parking area in front of a row of 10 new stores. Each is 20 feet wide, 40 feet deep, and 2 stories high, with retail areas on the first floor and offices on the second.

Protecting each store’s large display window is a steel-mesh security gate rolled down from its horizontal metal housing. It’s held in place by four padlocks—two on each side.

Large amounts of smoke are pushing out the top of the display windows of the center five stores.

“Ladder 6 to communications center,” the truck company officer calls into his apparatus radio. “Transmit a second alarm for a working fire in the Kings Point shopping mall.”

The pumper lays a supply line from a hydrant in the parking area, and the firefighters stretch a 1¾inch preconnected attack line to the front of the stores. The ladder apparatus is placed away from the front of the stores, upwind of the fire. The ladder company members spread out, looking into each smoke-filled store and trying to locate the main body of fire.

On one of the stores, a blackgray film, streaked with condensation, coats the inside of the two large, plate glass display windows. The windows are pulsating in and out, and smoke puffs out around the glass door set back between the two windows.

“Hey, Cap, I found the fire! Here it is,” shouts a firefighter standing in front of that store.

All the firefighters regroup there, and the captain orders them into action. “You,” pointing to one, “get a ground ladder; check out the floor above. You,” pointing to a firefighter who has a portable radio, “get to the rear of this store. See if you can vent. When the engine gets the line in position, I’ll give you the word.” Turning to the remaining firefighters, the officer orders, “Force that security gate open.”

The firefighter carrying the axe and halligan tool sizes up the gate. The padlocks are cheap and can be forced easily. He hands the other firefighter the eight-pound flathead axe, places the point of the halligan in the bow of the padlock on the left, and slides his gloved hands down to the forked end of the tool.

Pulling at the outstretched padlock, he orders the other firefighter to swing the axe. The back of it strikes the halligan tool behind the point placed through the padlock bow. Bang! The padlock breaks away from the staple and lock pin which it held secure and falls to the sidewalk.

Quickly moving to the other side of the security gate, the team repeats the procedure. Bang! They knock the other padlock off.

The officer’s radio blares. “Ladder 6 rear to Ladder 6.”

“Ladder 6. Go ahead.”

“Captain, there’s no way to vent the rear. It’s a solid brick wall.”

“Ladder 6, 10-4.”

Walking over to the engine officer, the truck captain says, “When we get this gate up, we’re going to take out the display window. Let the fire blow, then you guys can move in. There’s no way to vent at the rear or above, so watch it.”

Inside the recessed entrance, the glass door suddenly cracks from the heat in the store and crashes to the floor behind the security gate. Smoke momentarily billows out and envelops the firefighters, but almost immediately, it retreats, sucked back through the broken door.

Boom! The captain is lifted off his feet, blown backwards several feet, and slammed on his back, skidding and sprawling in the road.

Slowly, very slowly, he rises up on one knee, shaking his head. Staring at the front of the store, he can’t believe his eyes. Flames shoot out a gaping, black hole where the display windows were seconds ago. Several firefighters are getting up off the ground and running over to the hose line, where a firefighter without a helmet is directing the nozzle into the flaming store.

Small pieces of broken glass are scattered in a 50-foot radius in front of the store. The 20-foot metal ground ladder has been blown over and lies twisted and bent 90 degrees at its center, one end sticking crazily straight up in the night air. The entire security gate, with its frame and roll bar housing, has been blown off the front of the store in one piece, and now it’s half on the sidewalk, half in the road.

“I’m alive,” the captain thinks. “What an explosion!”

“Captain, captain!” A firefighter runs over to him. “What happened? I was blown off the ladder, but I’m all right. God, this is something!”

“It must have been a backdraft,” the officer manages, still dazed. “Where’s the forcible entry team?”

Just then, they hear a cry. “Help! Help! Get this off me!”

The captain and the firefighter run over to the security gate and lift it several feet. A firefighter crawls out from beneath, his face covered with blood. He tries to stand up.

“Joe’s still under there,” he whispers.

“Quick, get that portable ladder and wedge it under this gate while I hold it,” the captain orders the other firefighters. They turn the bent ladder on its side and place it under the gate. The captain crawls under the metal screen, drags out the unconscious firefighter, and rolls him over. Blood pours from a wound in the man’s chest.

“He fell on the halligan tool; the point punctured his chest. Get an ambulance. Hurry!”

Question 1. Which of the following is true?

  1. A backdraft can occur during the growth stage of a fire’s development.
  2. A backdraft can occur during the fully developed stage of a fire’s development.
  3. A backdraft can occur during the decay stage of a fire’s development.
  4. All the above.

Question 2. Which of the following is true?

  1. A backdraft is a combustion explosion.
  2. The triggering event of a backdraft is the same as the triggering event of a flashover.
  3. Protective equipment is of no value once a firefighter becomes caught in a backdraft.
  4. A firefighter can avoid the blast from a backdraft if the person detects a warning sign seconds before the backdraft.

The term backdraft is used only in the fire service. It refers to a specific type of explosion which most often occurs during the initial entry into what had been a tightly sealed fire area, after the fire has burned for a long time—long enough to pass through the first two stages of combustion development and into the third. (See Figure 1, opposite.) Over hours or days, it has first smoldered, then become a fully developed fire which consumes most of the combustibles in the area, then diminished in size and intensity.

Thus the answer to Question 1 is C. If backdraft occurs, it’s during the decay stage.

At that advanced stage, the firefighters arrive and force the doors open. The air is sucked into a superheated, confined fire area which contains residual combustible gases at high temperatures, and Boom!, a backdraft occurs. The blast or shock waves it generates may create only a puff of smoke, or it may be more serious—a ball of fire, exploding glass, a collapsed security gate, or a collapsed wall.

A backdraft falls within the category of chemical explosion, so the answer to Question 2 is A: A backdraft is a combustion (chemical) explosion. A chemical explosion is the result of the chemical reaction of heat, oxygen, and fuel—the same ingredients that create a fire. (The other broad classification is physical explosion—the rupture of a cylinder or container because of excessive pressure.)

An explosion is different from a fire, though, because the speed of the reaction and the rapid expansion of gases cause shock waves. A fire doesn’t.

Not all explosions occurring at fires are backdrafts. Most are caused by natural gas leaking from broken or melted gas piping during the fire or by residual vapors of the flammable liquids an arsonist has used to start the fire.

Serious blasts from the rupture of cylinders containing liquefied petroleum gas are also becoming common, while most minor puffs of smoke at fires are caused by household aerosol containers.

Before a fire officer declares that an explosion was a backdraft, an investigation should be conducted to eliminate these other types of explosion.

Firefighters often ask what the difference is between backdraft and flashover. There are several.

A flashover isn’t an explosion. Both backdraft and flashover are rapid combustion reactions, but the backdraft chemical reaction happens faster—-so fast that the suddenly heated atmosphere expands and creates excessive pressure in the fire area. This is released or dissipated as shock waves. Flashover, the sudden ignition of combustible fire gases, doesn’t produce shock waves.

Another difference is that a flashover occurs during the first stage of fire development, the growth stage, rather than the third (decay) stage.

Finally, the triggering events are different. (See Figure 2.) For backdraft, the triggering event is the addition of oxygen, the fresh air introduced to the confined area after forcible entry. The triggering event for a flashover is the addition of heat, caused by radiation feedback to the fire area. The heat is generated by the fire, accumulated at the upper levels of the fire room, absorbed into the ceiling and walls, and reradiated into the room. There it ignites the fire’s combustible gases in the presence of sufficient oxygen.

Scientists state there are five ways to safeguard against an explosion or its effects: contain the explosion, vent its effects, quench the explosion with an extinguishing agent, isolate it from people, or “dump,” or remove, the explosive material.

Firefighters can’t remove the explosive material, but they can use the other four procedures to protect themselves against a backdraft during forcible entry:

Containment. When sizing up a door for forcible entry, a firefighter checks to see if it’s locked, analyzes the type of lock, determines if the door opens inward or outward, and feels the door to estimate the amount of heat and fire behind it. If there’s intense heat behind the door and if the door swings inward, the door must be held closed or its inward swing controlled during forcible entry to prevent air from rushing into the fire area. This could “contain” the explosive atmosphere.

To do this, a firefighter or the officer should crouch below the members of the forcible entry team, who are standing, and hold the doorknob closed with a gloved hand or a short piece of rope.

Air is sucked in, and Boom!

Venting. Backdrafts often occur in shopping centers and rows of stores which are closed up overnight or over weekends, where fires can go undetected for a long time. These are usually one-story buildings which can be vented at the roof, and vertical ventilation above a fire before the front door is forced open happens to be one of the most effective methods of preventing or reducing the explosive effects of backdraft.

Where glass skylights are present on a flat roof and are located directly over the fire, they should be immediately removed to vent the fire below. This will release any superheated gases up and out, offsetting the chemical reaction of heat, fuel, and oxygen inside the store. Even if backdraft occurs, some of the shock waves would be diverted out the skylight.

Quenching. Unfortunately, skylights have almost disappeared from the roofs of one-story retail stores over the past 30 years. Store owners have sealed up skylights which a burglar might use to enter a store, so quick venting to avoid backdraft is becoming rare. Roofs must now be cut with power saws and the ceiling below pushed down with pike poles to vertically vent a store fire. Firefighters usually force the front doors open before this slower form of roof venting can be done. Anyway, roof venting can’t prevent backdraft in a two-story structure when the fire is on the first floor.

So another protective method must sometimes be used. A charged hose line must be ready at the front door when the door is forced. A quick stream of water on a superheated fire area as soon as the door is open could cool a potentially explosive atmosphere. This isn’t as effective as roof venting, but it’s a practical alternative.

Isolation. In some instances, the fire inside a store is at the rear behind a partition wall, and the hose stream wouldn’t hit it from the front door. If there are no top or rear vent openings, the firefighters are, in effect, standing in the barrel of a loaded shotgun about to fire when fresh air enters the burning store. The full blast of the backdraft is coming out of the front entrance into the path of the advancing firefighters.

As a last resort, firefighters should break the glass display windows, then step back quickly as the fire or explosion vents out of the front entrance. At that point, they can advance a hose line—already placed and charged ahead of time—into the now flaming store. This allows the explosion to occur in “isolation” and safeguards the firefighter from the blast. A disadvantage is that the fire may spread above and into the adjoining stores. Hose lines will have to be positioned at these areas.

The tactics for dealing with backdraft are available to be used once a proper size-up recognizes backdraft conditions early on. That combination is the key to firefighter safety when forcibly entering a tightly sealed building or room where fire may have been burning for a long time.

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