CGA S 1.2 - U value for superinsulated cryogenic vessels at "Fire" conditions 649 Deg C 2

[SMcI]

I would be very grateful if someone could tell me if they have a value for "U" as described within CGA S1.2 "Pressure relief device standards part 2 - Cargo and portable tanks for compressed gases" cl 6.3.3. for super insulated cryogenic vessels. The insulation system consists of approx 35 layers of aluminium foil with glass fiber separating paper, and a vacuum space of around 60 mm. I have a "U" value as determined by Test, but these tests were carried out at ambient temperatures, not the elevated temperature of 649 Deg C as I do not have he means to test at this temperature.
I would be most grateful if someone would help me.
Thank you.

 

[hacksaw]

Aluminum foil won't take the temperature, so you have to assume the insulation falls off.

 

[SMcI]

Thank you. I assumed 660 deg for melting point of aluminium. I wondered if the 649 deg was somehow chosen for this reason? I am aware we are very close to the melting point but wondered if the multiple layers of separating layers would keep the insulation in place and reduce heat transfer to the layers located "away" from the walls of the vacuum jacket, which would be the direction of heat? Could I ask, have you worked to this standard before? maybe with an approval body who insisted the sections used were for loss of insulation as you advise?
Thank you again.

 

[EdStainless]

Well, the foil sort of works. I saw one after exposure to this and it looked melted, but not dripping. It does continue to block a lot of radiant heat transfer, but as it heats there is a lot of outgassing and the overall heat transfer increases a lot.
I had thought that the CGA published a guide to this?

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P.E. Metallurgy

 

[SMcI]

Thank you too EdStainless. I will look to see if there is any guidance provided in the form of CGA documents. In terms of the outgassing, the S 1.2 standards requires that the calculations are carried out for the condition of loss of vacuum with fire engulfment. Which is pretty onerous, but understandable. I have test results, albeit not laboratory conditions for loss of vacuum - measuring liquid nitrogen boil off over 48 hrs but so far can not find anything which advises overall heat transfer coefficient for the conditions as required by this S 1.2 standard.

Thank you for the advice as to how the insulation looks after being exposed to high temperature.

Thank you very much.

 

[EdStainless]

The foil crinkles up and mostly turns to fragments and dust.
I recall hearing that in the old days the N2 and Ar tanks often had dump valves in the bottom that that hand thermal fuses in them, in a fire the tank would literally dump its contents. Helps fight the fire but can be tough on fire fighters.
The tank of ours that had the most elaborate venting was the oxygen, that crap scares me.

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P.E. Metallurgy

 

[SMcI]

This is what I am sizing relief valves for at the moment - LOX tanks. I have some information which has been passed down over the years, albeit from one of the big gas companies here in the UK, but is very old and it does not provide values for the elevated temperatures as required by S 1.2. Thankfully I have seen many LOX tanks over the years, and as with the likes of LNG Tanks, for competitors tanks, always been concerned at how small the RVs are. It give me a little more confidence that the valves I have sized are larger than theirs! Not really a scientific assessment!

Once again, thank you very much for your help, time and advice.

 

[hacksaw]

The Al foil oxidizes at temperatures or 300-400C, and given that most relief conditions are sized for worse case conditions when fire is involved, you may not be able to claim any benefit from the insulator.

 

[georgeverghese]

Since this is not a fire rated insulation, agree with zero credit for any reduction of fire input to these vessels due to insulation. Would suggest using the API guidelines for fire case heat input for bare vessels, which is based on the external surface area exposed at max normal liquid level - it couldnt be any worse than this. Latent heat of vaporisation for LO2 can be abstracted from thermo tables for oxygen in the L-V region.

 

[csk62]

Read S-1.3 6.3.4. I have assumed that outer steel jacket remains in place, but the MLI degrades at 1200F and no credit is taken for the insulation, rather the annular space is filled with the more conservative thermal conductivity of the gaseous landing (in this case oxygen) or air. The heat transfer via conduction can be calculated based on the annular gap between the inner and outer vessels. Radiation HT may be approximated based on concentric cylinders at specified temperatures and known emissivities.