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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Does anyone know if pvc is safe in liquid oxygen? I will have 21% liquid oxygen/78% nitrogen and I need to transfer it out of a tank. I have been trying to find out if it will ignite in this environment, but I am having trouble finding an exact answer. I may have to use metal, but I am trying to avoid materials that will conduct heat. This has been the most helpful so far
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
"The most frequently used plastics are the semicrystalline types like polytetrafluoroethylene (PTFE Teflon®), fluorinated ethylene propylene (FEP Teflon®), polychlorotrifluoroethylene (PCTFE), Kel-F® 81, and amorphous polymers like polyimides (Vespel® SP21). Polytetrafluoroethylene is commonly used in oxygen systems because of its resistance to ignition by mechanical impact, high AIT, high oxygen index, and low heat of combustion. Unfortunately, PTFE has poor creep resistance; therefore, it is often replaced by polymers that are less compatible with oxygen."
No mention of PVC here, so It's not one favoured by NASA, presumably.
Presumably you've considered small bore (1/4") stainless, surrounded by insulation?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I would stay away from it, thin walled stainless would be a much better idea, or even glass if you do not mind working with it. My understanding is that pvc is not acceptable for any cryogenic applications, oxygen containing or not.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Nice replies, but this is information I know. PVC is not great because of thermal shock as pointed out, but it is a cheap solutions. As the link points out, PVC is not listed as a suitable material for LOX environments; teflon is the material of choice.
I am thinking of making an NPT connector out of teflon. I can then connect that to a copper pipe. this should offer a gap for the thermal conductions. Making an NPT connector is going to be tricky, though.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
IamSmooth wrote ...
I am thinking of making an NPT connector out of teflon. I can then connect that to a copper pipe. this should offer a gap for the thermal conductions. Making an NPT connector is going to be tricky, though.
I've made NPT type connectors out of stainless before, on a lathe (the tapered threads were tricky, but not impossible), but in this application, I'd try to use the same material throughout, due to expansion/contraction rates.
I'd also avoid anything that might become brittle at those temperatures, which I suspect is the reason that PVC is not on NASA's list.
I believe that stainless is commonly used for cryogenics, but I'm not certain which grade is preffered. I'd guess 316 is adequate.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
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I agree that the same material is preferable, but I need to resolve the thermal conduction issue. Right now I have a reservoir in the center that is surrounded by insulation. If I have a metal pipe going to the outside I am afraid it will conduct enough heat to make extreme cooling difficult. The pipe would act like a wick and transfer the heat through my insulation directly to the reservoir.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
IamSmooth wrote ...
I agree that the same material is preferable, but I need to resolve the thermal conduction issue. Right now I have a reservoir in the center that is surrounded by insulation. If I have a metal pipe going to the outside I am afraid it will conduct enough heat to make extreme cooling difficult. The pipe would act like a wick and transfer the heat through my insulation directly to the reservoir.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615
Make your take off pipe in metal, but make a thermal barrier in it by coiling a long length in insulation. That way conduction along the length is reduced, especially if you use thin wall tubing.
mixing pure O2 with organics is generally regarded as a bad idea - i would avoid using plastic or hydrocarbon grease around pure oxygen.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600
Dunno about PVC but polyethylene is at least semi-compatible.
Apparently it won't absorb LOX and detonate, and if it's full of LOX it's much harder to light.
However you have two legs of the fire triangle, and if you do manage to get it burn, it will burn really, really, really well.
Still, so does aluminium for example; if you hit aluminium in a 100% oxygen atmosphere with a hammer, some non zero percentage of the time it will catch fire.
PET will be somewhat brittle in LOX, it's below it's brittle transition temperature, but it's still apparently quite strong.
I think PVC may be much more brittle; try sticking it in a freezer, it may shatter quite easily.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567
I agree. I eventually plan on building a PSA for nitrogen generation, but in the meantime there will be some LOX in my collecting system. I think I can get a teflon fitting made to go to the outside; I'll use copper pipe off the reservior tank.
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liquid oxygen and pvc
