Sunday, December 10, 2006

Tank pressure test success...

I did not have a small enough threading tool or tap to make the threads in the tank end cap. When I got a pipe tap I broke the tank when twisting it. So I'm doing it all again now that the fittings are threaded.... This give me the opportunity to take more pictures and weigh things....
The end caps:

The end cap weigh 40 gm for the blank end and 61 gm for the end with the fitting and pipe nipple.
The aluminum end caps weigh more than the whole rest of the tank. While the origional short tank had a mass ratio of 3.5. An 8 foot version constructed identically would have a MR of 6.7 .
To put this in persepctive, an aluminum sphere with 31000 psi UTS will have a MR of around 6.4. After I threaded the fittings I built a new tank that was about 18 inches long to try and pressure test. I took it by my friends at FloMetrics and we pressure tested it to 1500 psi. It did not fail at 1500 psi, that was just the limit of the water pump we were using to presurize it. The NPT threads around the pipe nipple leaked, but the tank held at 1500 PSI! Since I had used the pipe nipple as a winding post I could not tighten it to make the leak go away. I've been told that aluminum NPT pipe threads leak badly, but some things you need to learn yourself. If anyone has recommendations on a type of pipe fitting that can be used in aluminum and won't leak please let me know. Next time I get a dewar of LN2 I'm going to hook a pressure transducer to the tank and burst it at low temperature. I'll keep everyone posted.
I may also try fabricating some larger PET tubes and building a 4 or 6" tank.

A picture of the whole tank:I did not use any glue on the main part of the tank I only used "gorrilia" glue on the ends to secure the windings.


Anonymous said...

Very cool. Other than soda bottles and flourescent light covers, are there any other common, inexpensive PET tubes in other sizes?

Paul Breed said...

I can't find any larger tubes. I can find it in 4' x 8' sheets. If I can figure out how to make the seam work it would be perfect.

Anonymous said...

Hi. I've been following your blog with great interest. I've decided to do a Lander project myself. See my blog for details.

Your tanks are awesome ! As for your end caps being heavy you can machine them out quite a bit I'd think. The idea looks sound. Its really nice when something that looks good on paper actually works !

For a good pipe seal in Alu use a straight pipe thread with an o-ring or crush gasket face seal. These fittings are often used in hydraulics. And don't forget an anti-seize compound. Alu will gall really fast without.

Anonymous said...

Is the tank still flexible at cryogenic temperatures?


Paul Breed said...

Is the tank still flexible...
Yesterday I would have said yes, today I don't think so I thought the florescent tube cover was PET, I've since learned its polycarbonate. If its polycarbonate it will be brittle and shatter. so Now I have to find a source for PET tubes and try again...

Anonymous said...

PET and PETG are not the same material, the Glycol ( G ) modifys the properties. Also FYI soda bottles are multi-layer with different layers having different properties. Most bottles are blow molded from "parisons" ( test tube shaped objects with the screw threads on top )which is why they shrink when heated.
Have you tried polycarbonate tube? PC is not brittle, it actually has a very high impact resistance.
As far as your end caps, have you considered vacuum forming them?


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