Backdraft

[hmckay91]

Interesting that there are fairly fundemental differences in some of these posts. I would have thought that there would be an established "procedure" for such conditions

I also find it interesting that there seems to be such a wide variety of opinions on this.  I always thought it was fairly straight forward concept.  

A backdraft is a situation which can occur when a fire is starved of oxygen; consequently combustion ceases but the fuel gases and smoke remain at high temperature. If oxygen is re-introduced to the fire, eg. by opening a door to a closed room, combustion can restart often resulting in an explosive effect as the gases heat and expand.

A flashover is the near simultaneous ignition of all combustible material in an enclosed area.

I will throw this material out to be considered.  Check out

http://www.firetactics.com/

CHECK OUT THE PPV SAFETY NOTES AT THE BOTTOM

Backdraft

A backdraft is an explosion that occurs when the heated gases of an oxygen-starved fire are mixed with oxygen and ignite suddenly.

Why Does Backdraft Occur?
•   Except under carefully controlled conditions, combustion is rarely complete.

•   Some of the combustion elements are not consumed but pass into the surrounding atmosphere in the form of unburned combustible gases.

•   Many combustibles begin to smoke before they actually burst into flame.

•   Either there is insufficient available oxygen to support fire or the ignition temperature of the material has not been reached.

•   The gases and carbon being emitted are flammable.

•   Improper ventilation at this time, such as opening a door or window, will supply the missing link, oxygen.

•   As soon as the missing oxygen rushes in, the stalled combustion resumes with devastating speed. A backdraft occurs – a virtual explosion

Signs of Backdraft
The following conditions may indicate a backdraft or smoke explosion condition:
•   Smoke under pressure exiting through small building openings

•   Black smoke becoming dense, greyish yellow

•   Confinement and excessive heat

•   Little or no visible flame

•   Smoke leaving the building in puffs and being drawn back in

•   Smoke stained and/or rattling windows

•   Muffled sounds

•   Sudden, rapid movement of air and smoke inward when an opening is made

•   Hot or warm outside walls with little or no fire evident

If visibility through windows is practically nil but no fire is evident (or perhaps only a faint glow can be seen through the smoke), it is reasonable to assume that a fire has been smouldering for some time.

Under these conditions extreme caution must be exercised because a backdraft danger exists. Horizontal ventilation should not be attempted if another option is available.

Backdraft Prevention
If the conditions are recognised upon arrival, every effort must be made to prevent the backdraft from occurring. Prompt and proper ventilation is in order, but even this procedure cannot absolutely prevent a backdraft. There is no sure method of prevention.

Vertical ventilation is usually the best way of mitigating the backdraft condition. Unfortunately, it may not always be possible.

Each enclosed section of the building must be treated as a separate potential backdraft area.

If it is decided that the space cannot be allowed to cool before making entry, horizontal ventilation may be the only available option. Ventilation must be performed in such a manner as to maximise your safety and minimise property damage.

Once backdraft conditions develop within a confined space, you have only a few safe courses of action.
•   It may be necessary to simply do nothing but monitor the space until the smouldering fire goes out due to lack of oxygen or fuel.

•   Allow the gases within the space to cool below their ignition temperature

•   Allow the backdraft to occur, but control its path by vertical ventilation

•   Use a piercing nozzle to penetrate the space and accelerate the cooling process

Protecting Yourself from Backdraft
You should not stand in doorways or directly in front of doors, windows, or other openings when the possibility of a backdraft exists. It is safest if you do not stand anywhere within the V-shaped force near an opening. The gaseous products of the backdraft will expand as they come through the opening because of the lesser pressure of the atmosphere outside the building.

It is possible that backdraft may not exist upon arrival but will develop in confined spaces within a building. If you overlook the warning signs, you may find yourself inside the building when backdraft conditions begin to develop.
 


Flashover  is the sudden transition from local burning to widespread burning of all exposed fuels. It occurs when all the contents of a compartment reach their ignition temperatures in a very short span of time, usually seconds. It happens because hot gases are radiating and convecting heat to all the exposed surfaces. Not all fires reach the flashover stage.

Victim survival is highly unlikely. Heat in the compartment goes from 300?C to approximately 1200 to 1400?C in seconds.

Your escape time from this environment is about ½ to 2 seconds before gear failure and serious injury.

•   Prior to flashover
?   temperature increases rapidly.

?   additional fuel becomes involved.

?   fuel packages in the compartment give off combustible gases.

Positive Pressure Ventilation Safety Precautions

1.   PPV should not be used if backdraft conditions exist.

2.   A prolonged application of positive pressure without a ventilation (discharge) hole may cause the fire to flashover because of the volume of air being injected.

3.   In heavy fire load situations, PPV may lead to rekindle.

[nearlythere]

Before I joined the Brigade many moons ago I was in the Royal Navy. Then, as part of initial training, we all had to undergo ship fire training.
To enter a compartment we used a water wall and jet through 2 x 70mm supplies. The water wall was created with a special branch which formed a 360 degree perfectly formed wall of water which could extend out numerous metres from the branch. This was used to ensure a constant protective barrier when opening a door or hatch. When the door or hatch was fully opened the water wall was used to maintained a cooling barrier between a fire and the operators. The jet could then be played through the water wall and as long as the branch itself did not obstruct the wall the effective was maintained. It was a very good system and I was extermely surprised when I joined the Fire Brigade to find out that it did not use such a technique.

[firedodger]

The kind of jet pattern you describe used to be available on our standard issue branch, the AWG, with a solid jet in the centre and a water curtain around it (this is 20 years ago in an LAFB).  While I can see it would be useful in dealing with the enourmous heat build up in a ship fire I am not sure if it could be used to prevent or deal with a backdraught.

As far as flashovers are concerned there is some debate about whether or not modern hosereels and the branches now commonly used with 45 mm actually deliver enough water to reduce compartment temperatures in order to forestall flashover. This has been particularly questioned in high-rise fires where compartment design (concrete box, sealed unit double glazing, possible high wind speeds) prevents heat dissapation by  means other than the front door.

Therefore it could be that your recollection of the efficiency of large jets may well be more than lamp swinging ;-)

Paul Grimwood is the absoloute source on this kind of thing.

website :    http://www.firetactics.com/PAUL-GRIMWOOD.htm