Water Mist Systems (Fire Suppression)
 

Hi all,

Something that has been discussed while I've been studying at BCFT is the alternative methods of fire suppression systems used other than Halon.

One particular method that seems to be discussed frequently are water mist systems. I understand that the use of Halon 1301 is the most common form of fire suppression, but it appears to me from reading various sources that more "eco friendly" methods such as water mist are more appealing.

However, why is it that water mist systems have not been installed as standard on aircraft now if it is such a desirable method? Furthermore, does the water mist system have the capability of being both automatically and manually operated?

Any help would prove invaluable.

Kind regards,
Dan

Although water mist systems were shown to be very good at supressing fire AND SMOKE the big drawback with them is the risk of accidental activation. Flooding your aircraft with water plays havoc with electrical systems!

I would also suggest weight would be a very large issue. Water is (obviously) incompressible unlike halon and would require a large storage tank.

I have had experience with hi-fog systems (I believe that is just a trade name) when working at sea and even allowing for scale and space available on ship they took up a fair bit of space. The installations tended to be manually activated with guarded activation points inside and outside machinery spaces. They are very effective - on training courses I was shown videos of actual fires where the systems had been used and they knocked the fire out very quickly.

On a ship or fixed installation where weight is not a massive marginal issue they are clearly an easy engineering choice; they also have the massive advantage over (for example) CO2 systems of being survivable if personnel are in the compartments when activated. I suspect for aviation applications weight and electrical interference are the two primary drawbacks.

Not entirely sure that injecting water over a heat source in a pressurised environment is a sound idea.

I've managed to find a couple of drawbacks to the system, however the method has been tested before and has shown promise. But after the various tests that have been completed the whole process seems to have gone very quiet

Actually, the FAA recommends using water for fire suppression in some instances.

Water = weight.

Water + fire = steam.


Steam is not good, it expands massively. Over 1500x increase in volume iirc.

This vid will give you an idea of the efficacy of water fog.


https://www.youtube.com/watch?v=24PdBM-phVQ

I was told by a national marine officer (serving on small to medium size surface vessels) that water spray (as in droplets) can be used in fires of electronic equipment without collateral damage.

However I guess the volume of machinery needed to turn water to droplets would be massive for an aircraft.

Couple of issues...
 

For those noting the weight of water: 1 kg of water weighs exactly as much as 1 kg of Halon 1311. (I'm not being snarky; I'm pointing out that what matters is "how much fire extinguishing power does 1 kg of water have relative to 1 kg of halon 1311?") I don't know, and a 30 second scan of a couple of articles didn't enlighten me.

For those noting the concern of water turning to steam in a closed cabin and creating overpressure: The same is true of halon 1311 - it comes out of the cylinder as a liquid and rapidly expands to a much higher volume of gas, same as water does when it encounters heat.

There were extensive studies carried out by the FAA, CAA and industry in the late 1980s into water spray fire suppression systems. The conclusions as I understand why it was not taken forward were related to cost benefits and concerns over inadvertent discharge in flight.

http://www.fire.tc.faa.gov/pdf/fsr-0001.pdf

But how much does it expand by?
I was under the impression water to steam was an unusually large expansion compared to other liquid-gas state changes

BEA or BA did a trial on a Trident IIRC...possibly after the Airtours fire either in the 70s or early 80s.
Rumour had it that the weight and costs stopped it going ahead.

The steam would condense back to water pretty quickly as it vented away from the actual fire or contacted a cooler surface. (If you watch the "steam" come from a tea kettle, you're actually looking tiny water droplets -- the steam is the invisible part between the kettle and what you can see.)

Also, even though water expands around 1000 times in volume when it turns to steam, the flow rates can't be all that high from a mist system. If you multiplied by 1000, I suspect you'd only reach a small fraction of the outflow valve capacity.

After a bit more digging it transpires that it's certainly been looked into as being a "green" alternative but as of yet it still doesn't exceed the ability of Halon. As many of you have said the drawback seems to be focussed mostly on its weight, plus a few shortcomings in particular tests.

Water/steam ratio is around 1820.


What should be remembered is not all the water will turn to steam thereby providing cooling. Another mechanism of the micro spray is the exclusion of oxygen.


A suitable system would extinguish a fire before it ever became established.

If you were a passenger on a commercial aircraft that experienced a fire in flight, either in the cabin, the engine nacelle, or the cargo hold, would you really give a darn whether the type of extinguisher used was "eco friendly"? Or would you prefer that the most effective type of fire suppressant was used?

Gases are much more effective for the hand-held extinguishers typically used for cabin fires. And Halon is safer than CO2 since it is less likely to suffocate passengers.

The assignment is more directed at what the airline industry can do to provide an eco-friendly alternative that still provides the same effectiveness as halon, or an even better alternative.

So far that alternative hasn't been found yet, hence why the deadline to replace Halon on new aircraft has been lengthened several times.

There are alternatives. The question becomes, "why are they not choosing to adopt the best alternative?"

Apart from the extra weight penalty of water mist systems (as previously mentioned) another reason that would mitigate against their use is that in Cargo Holds any fire inside a ULD container wouldn't be tackled with a water mist system - in cases like that only a gaseous system works.