Liferaft inflation gas

[MM5AHO]

Which gas is best for inflating life rafts?
Is it worth considering nitrogen rather than air as some use in tyres?

 

[maby]

Do you really mean "liferaft" - I thought they were all inflated from standard cylinders like a larger version of what goes into a lifejacket and assumed they were all CO2.

 

[MM5AHO]

I think that they are CO2 in life rafts, not air as I said above. But why CO2? Why not N2?

 

[KenMcCulloch]

I believe it's a mixture of CO2 and Nitrogen. Straight CO2 will cause the valve to freeze. No idea about the proportions.

 

[maby]

I think that they are CO2 in life rafts, not air as I said above. But why CO2? Why not N2?

I think it is down to packing density - I believe you can get more CO2 into a given volume under pressure than N2. It liquefies at relatively low pressures - far lower than N2 - which gives you a much better packing density - imagine how much steam you get by boiling a cup full of water.

 

[MM5AHO]

Surely any gas depressurising at that rate will absorb a lot of heat (cause the valve to freeze).
I know it does this with O2, N2, CO2 and Ar gasses in bottles.
That is a good point, but doesn't really differentiate why CO2 in a life raft?

I wonder if its molecule size? (H2 could easily leak for example, though I can think of other reasons not to use it!)

 

[maby]

I think it is mostly down to boiling point under pressure. Gasses like oxygen and nitrogen need very high pressures to liquefy them at room temperature. I think you'll find that industrial N2 cylinders are shipped at about 200 bar and contain compressed gas, not liquid. So you would need a cylinder with a volume of about one two-hundredth that of the liferaft tubes - a relatively large cylinder. If you use a gas that will liquefy at a manageable pressure, you can get away with a significantly smaller cylinder. Butane would work, but has obvious dangers.

 

[cliff]

why CO2 in a life raft?

CO2 will liquefy at comparatively low pressure and perhaps more important it is cheap and non flammable. In addition the heat of sublimation is comparatively low so any "icing" of the valve will not last long as the "ice" will be porous "solid" CO2. The water ice on the outside of the valve will last a lot longer.

 

[winsbury]

I believe it's a mixture of CO2 and Nitrogen. Straight CO2 will cause the valve to freeze. No idea about the proportions.

Definitely its a mix but I dont know why for certain. N2 proportion is quite low, somewhere about 5-10% if I remember correctly and I heard it's there to help expel the CO2 at the max rate. If you ever let a cylinder off by accident its quite astonishing how violently they empty (I speak from personal experience ) and quite different from letting off a CO2 fire extinguisher.

 

[VicS]

I think it is mostly down to boiling point under pressure. Gasses like oxygen and nitrogen need very high pressures to liquefy them at room temperature. I think you'll find that industrial N2 cylinders are shipped at about 200 bar and contain compressed gas, not liquid. So you would need a cylinder with a volume of about one two-hundredth that of the liferaft tubes - a relatively large cylinder. If you use a gas that will liquefy at a manageable pressure, you can get away with a significantly smaller cylinder. Butane would work, but has obvious dangers.

i think you will find that gasses like oxygen and nitrogen cannot be liquefied at room temperature.

Any gas must be cooled below its critical temperature before it can be liquefied ( -147C for nitrogen, -119C for oxygen, -140C for air but +31C for carbon dioxide)

 

[Seajet]

Taking another angle on this, I have sometimes wondered if in extremis Co2 / dry powder fire extinguishers could be used to inflate a dinghy ( ie no liferaft available ) or would it freeze the material and cause even more problems ?

I know the powder is corrosive but I'm thinking of an urgent emergency.

 

[maby]

Taking another angle on this, I have sometimes wondered if in extremis Co2 / dry powder fire extinguishers could be used to inflate a dinghy ( ie no liferaft available ) or would it freeze the material and cause even more problems ?

I know the powder is corrosive but I'm thinking of an urgent emergency.

I guess a CO2 extinguisher would work - if you could find a way to couple it to the dinghy without excessive leaks. I doubt a dry powder extinguisher would contain enough gas - it's just a propellant for the powder, not there to function as a fire inhibitor.

 

[Ruffles]

According to a program I was watching about the new airbus, aircraft escape rafts are inflated largely with air. The CO2 acts via some kind of venturi effect to drag air into the envelope. This way they can use smaller cylinders and save weight. No idea if life rafts are the same.

 

[Seajet]

I'm surprised if they use air for aircraft escape slides, the military aircraft I worked with all used nitrogen in the tyres for fire safety reasons - brake fires etc - and I'd have thought slides would be a good place for it; maybe there's a technical or - I hate to say it - financial reason to do with the quantity involved ?

As far as powder extinguishers to inflate dinghies, I wasn't thinking of fully inflating, just any quick source of gas would be a help.

 

[MM5AHO]

Also wondered about CO2 - if a leak in a liferaft there's a chance of asphixiation. Be a shame to survive the sinking to suffocate in a leaky liferaft?

 

[maby]

Also wondered about CO2 - if a leak in a liferaft there's a chance of asphixiation. Be a shame to survive the sinking to suffocate in a leaky liferaft?

They are not particularly air-tight - I would think that any leak fast enough to take the CO2 levels inside to dangerous levels would drown you first. Any inflation gas other than oxygen would be capable of suffocating you - CO2 is not particularly poisonous in the way that CO is - just will not keep you alive

 

[Ruffles]

Slow thinking on my part. If you used the venturi principle on a ships life raft it could end up full of water!
But this is from the Wikipedia article on AIRCRAFT evacuation slides: http://en.wikipedia.org/wiki/Evacuation_slide

Inflation systems

Both slides and slide/rafts use a non-explosive, inert gas inflation systems. The FAA requires evacuation of the entire aircraft in 90 seconds using 50% of the available evacuation exits. To meet this, all evacuation units need to deploy in less than 10 seconds. For large, wide body aircraft such as A300s and B747s a successful deployment is complete in about 5–7 seconds, depending on conditions (such as temperature and winds).
The inflation system usually consists of a pressurized cylinder, a regulating valve, two high pressure hoses and two aspirators. The cylinder can be from 100 to about 1000 cubic inches, filled to about 3000 psi with either gaseous Nitrogen, or a mixture of gaseous CO2 and Nitrogen. Once made of steel, most cylinders now are made of aluminum or alloy cores wrapped with fiberglass, or other lightweight, fuel saving materials. The CO2 is used to slow down the rate at which the valve expends the gases.
The valve is used to mechanically meter out the gas at a rate of roughly 3 - 600 psi and 4 CFM. Typically there are two high pressure hoses attached to the valve, which are connected at the other end to aspirators. These are usually cylindrical, hollow aluminum tubes with sliding cylindrical or internal flapper doors that open when high pressure gas is applied, and close when the gas stream subsides and the internal slide back pressure reaches about 2.5 - 3.0 psi. They work on the Venturi principle, and draw outside air into the evacuation unit at a rate of about 500:1. A 750 in3 (0.43 ft3) cylinder can fill a slide with about 850 cu ft (24 m3) of air to a pressure of about 3 psi in about 4–6 seconds.

By the way I use CO2 cylinders to inflate bike tyres in competitions. But the CO2 will leak out of an inner tube in 24 hours. Explanations abound on cycling forums.

 

[lw395]

I'm surprised if they use air for aircraft escape slides, the military aircraft....

A small volume of compressed gas is used to drag in a large volume of air.
Similar things are used for compressed air cooling sometimes in industry.
Some sort of venturi
A big cylinder of compressed gas would not be something you'd want in a crash.

 

[lw395]

Taking another angle on this, I have sometimes wondered if in extremis Co2 / dry powder fire extinguishers could be used to inflate a dinghy ( ie no liferaft available ) or would it freeze the material and cause even more problems ?

I know the powder is corrosive but I'm thinking of an urgent emergency.

In extremis you can inflate a dinghy orally.
But a dive cylinder with the right accessories is quite quick!