With 30 years fire-fighting experience and 7 years working as an Emergency Response Consultant I was confused and a bit amazed when I heard about second flashovers and how some materials can protect you from this deadly occurrence, WOW how fabulous…, what second flashover?. Talking with Fire-fighters at the sharp end of the business they are as mystified as me as to how this can happen.
My intention here is to discuss how compartment fires work and give you an insight into why claims by some, whilst interesting, and at times downright hilarious bear little relevance to what you as a fire-fighter may actually need or want.
BACK TO BASICS
There has been a lot written about the development of fire and more specifically about the development of fire in buildings, but all here relates to compartment fires and for our purpose let’s call them Rooms.
People generally think that a room fire starts and progresses like a bonfire in a garden. That is a piece of fuel gets lit, the fire burns , the heat from that piece of fuel heats a piece adjacent to it, to its ignition point, which ignites and the fire spreads to the next piece of fuel and so on until the whole mass of the bonfire is burning nicely and the smoke rises up and away, easy, everyone knows how fires work….. But It doesn’t happen like that in a room. Here I will give the scenario of a simple but decent bedroom or lounge fire, fire-fighters bread and butter. They call them house fires, but house fires are usually only one room, they may involve a hallway or a landing, but basically are confined to one room.
Smoke is not, in this instance the waste product of fire which is a nuisance albeit a hot nuisance, and although you can’t breathe it and it stops you seeing what you’re doing, it’s relatively harmless. This is a popular misconception.
So let’s watch our bedroom fire developing nicely in a T.V or chair in the corner of the room, door and window closed. The fire builds up using up the available air in the room yet the smoke cannot rise up and away as in a bonfire so it hits the ceiling and a layering of the atmosphere in the room takes place. Post fire, fire-fighters have often seen the black lines around the walls marking the layers.
The three layers.
TOP OVER PRESSURE LAYER
MIDDLE NEUTRAL LAYER
BOTTOM UNDER PRESSURE LAYER
While we’re talking about it we call it smoke, but it’s a combustible gas and is probably a fire-fighters greatest enemy. Energy rich gases like those from polyurethane foam, plastics, and paint can have an energy content similar to that of methane gas, so would you be so self assured wandering around the house and fighting a fire knowing that the house was full of methane gas?
Anyway, back to our bedroom fire. The three layers have now developed and the fires burning brightly using up the available oxygen and already something has started to happen.
A LEAN, MEAN FLASHOVER
Flames start chasing up the smoke column curling and chasing across the ceiling, some Fire-fighters call these “dancing angels” this may cause wallpaper to burn and flake off, or curtains away from the original fire, to catch and drop to the floor, (maybe even starting a second seat). This does happen, yet how can the flames from our TV/armchair reach that far? They don’t it’s the smoke (combustible gas) that’s burning. Now this would be a good time to intervene but especially with today’s high fire loaded double glazed well insulated houses fire-fighters are probably only just on their way.
So now we have two fires, the original fire, and a gas pillow fire but it’s using up all the available oxygen and slowly dies back, but loads more flammable gases were released from the ceiling and walls when they were heated by a lean flashover and by the radiated heat from that flashover down onto the room furniture and even the floor coverings. Our TV/chair, the original fire, dies back to a smoulder, the air temperature falls and thus contracts and fresh air is sucked into the room. The fire momentarily flares up; using up this new air producing more combustible gas for the gas pillow, again and again this happens fire-fighters know this as the pulsation cycle. The fire is now beginning to breathe and move off to its next stage.
THE SLEEPING DRAGON
Now is not a good time to throw open the door with the hose reel in jet mode and make like John Wayne entering a saloon in a rough part of town. Yet fire-fighters have to get in, if the door is opened or a window fails the consequence is much the same the rush of air feeds the fire on a much larger scale than described above. The smouldering fire bursts back into life and ignites the combustible gas layer in the room. The heat generated by the flames working into the gas layer is colossal and produces even more combustible gases due to the decomposition of the room contents.
Now the opening has made a basic change to the mechanism of the fire, most of the combustible gases are leaving the room by the over pressure layer this may be exiting the window or onto the landing and fresh air is entering by the under pressure layer, the movement of air is called the “air track” by fire-fighters and is the constant supply for the combustion process raising the neutral layer and allowing the flame front to work it’s way from the original fire back into the combustible layer. Now this layer is remember, full of combustible gases close to or higher than their ignition temperature and rapidly being diluted to it’s ideal mixture. The flame front reaches the upper part of the window or hallway and the flames (those combustible gases lighting progressively) connect to the outside of the room terminating in this type of flashover which is known as a “BACKDRAFT” leaving behind in its wake a full room fire.
This happens quicker than it took me to write it and faster than you can read it.
Fire-fighters have seen the evidence when they clear-up, the whole room contents has been destroyed by fire but only one small area of floorboards burned in the corner where the TV/chair once stood, there never was a bonfire in the room with a fuel – flame – fuel – flame contact, the bed and other furniture has all but gone yet the fire never crossed the floor. This is how compartment fires work. So now we have an appreciation of what happens in a room when its on fire so by the very nature of Backdrafts they must use up the available combustible gas so, how do you get caught a second, You can’t, it’s as simple as that.
If you are a fire-fighter in this room you need to be dressed in the best Turn-out gear available. You need to be dressed in NOMEX®, probably the best Turn-out-gear in the world.
CHOOSING THE RIGHT PPE
There are other types of flashover, the delayed flashover, the hot rich flashover, the roll over (my favourite), however, being as this is not a technical article we will not go into here. We have to protect our Fire-fighters from this phenomenon with PPE but what should we choose?
IF Para-aramids are so good to specify 60% as a minimum in the outer-shell why don’t DuPont®, who make probably the world’s most famous Para-aramid, KEVLAR® make a 100% KEVLAR® outer shell material?, it’s easy, being the inventors of both NOMEX® and KEVLAR® with over forty years of complex fibre experience and even building a specific fire test laboratory in Geneva, and giving the industry the world’s first life size fire test manikin (much copied but never bettered) DuPont® know that KEVLAR® is a fantastic product that brings strength to materials yet DuPont® also know that Nomex® is better Heat and flame protection for Fire-fighters.
DuPont® have tried and tested many different combinations of fibres over the years and a high NOMEX® content is what DuPont® recommend. Why?, because NOMEX® has the ability to neutralise the heat energy, it stays pliable whilst under extreme fire conditions solidifying when the heat is removed, the energy is dissipated in its destruction not passed on as with Para-aramids.
Modern Fire-fighters PPE has to perform many tasks, Firstly be inherently fire resistant, maintain its look and feel, be breathable, be supple, allow freedom of movement and be resistant to abrasion, yet not create heat stress. Its most extreme use will be facing a flashover. Not much to ask is it !
If the kit is to heavy and cannot breath because of the weave and/or the amount of Para-aramid fibre or by design, the fire-fighters by just working and getting to the fire scene before the fire attack commences, may have produced more sweat than the PPE can release. If a flashover occurs when your wet inside the membrane the consequences can be extremely serious. This is what Fire-fighters call “boil-in the-bag suits” a bitter irony as this is what often kills them.
The rise in core body temperature that is associated with above also brings a silent hazard of it’s own, heat stress, the silent killer. By poor design and/or poor choice of ensemble you can create heat stress. Raise the core temperature of a Fire-fighter by one degree Celsius and 40% of even the simplest decisions he will make will be incorrect.
THERE ARE MANY NOMEX® FABRICS, HERES A WINNER
Hainsworth’s TITAN® Fabric has been specifically designed for Fire-fighters, the award winning weave which offers the NOMEX® fibre outwards to the exposed surface giving excellent flame and heat protection plus abrasion resistance, protection that Para-aramids alone cannot match, but the clever part is that the Kevlar® is kept to the back of the fabric away from abrasion, where it can perform its job, giving strength. Now if you thought that was clever (it is, you need a special loom to weave it on) the designers realised that NOMEX® & KEVLAR® have different expansion and contraction ratios so in weaving this way the fabric naturally buckles and traps air within itself when exposed to flame , now how clever is that, added to that you have high air permeability, great TPP, results and a fantastic range of colours to choose from. Thinking about it that’s probably why Hainsworth’s are known for innovation, and hold a Royal Warrant.
Which UK Fire & Rescue Services have conducted their own tests and have chosen TITAN®?.
LONDON, NOTTINGHAM, DERBYSHIRE, CORNWALL, WILTSHIRE, BUCKINGHAMSHIRE, CLEVELAND. WEST MIDLANDS, ESSEX, HAMPSHIRE, AVON, NORTHAMPTONSHIRE, LIECESTERSHIRE, ISLE OF MAN
18,000 Professional Fire-fighters ….. that’s who.
This is not meant as a technical article, if I did slip into techno speak now and again please forgive me, oh and please excuse the occasional American spelling. It is meant as a look at the way we do things and why we should remember the basics. If you would like to talk to a real person, not a salesperson, about your PPE requirements with people who know what they’re talking about, or just chat, please contact me, DuPont® or Hainsworth® direct at:
George.farenden@btopenworld.com
www.dpp-europe.com
charlotte.brandt@hainsworth.co.uk
NOMEX®…… we’ve got you covered.
