HDPE permeabilty 1

[alleak]

Hi;
I want to know what is the range of permeability of HDPE liners in CNG4 vessels in different pressures.is there any plots showing this.any help would be appreciated.
thanks

 

[Demon3]

Permeability to what? Water, oils, oxygen? How thick is the liner? Will your supplier tell you those numbers?

 

[layfieldman]

If you want information on HDPE liners I'm sure GSE (probably the worlds largest manufacturer) would be happy to assist you.

 

[alleak]

Hi Demon3;
Permeability to CNG(I mentioned CNG4)

 

[Demon3]

Aha, thanks. Well I have no idea what CNG4 is chemically so that's why I didn't get it the first time.

 

[alleak]

Hi Demon3;
There are 4 main catagories in CNG(Compressed Natural Gas)vessels.Type 4 wich is called CNG4 has non-metallic liner(typicaly HDPE(High Density Polyethylene)).and what i want to know is HDPE permeability in different CNG pressures.
Thanks

 

[Demon3]

Aha, now I am enlightened. Thanks a lot. Well, first I have to state I am not an expert in that field. On the other hand I can tell you that PE will be very permeable to natural gas because they have the same polarity and so the solubility of the gas in the polymer will be high.

For the same reason it is know that car fuel tanks (PE) are very permeable to gasoline (petrol) and lose the gasoline way too fast. To prevent that the standard process is to react the inside layer of the PE with fluorine gas. This gives a thin PTFE-like coating that is much less permeable to alkanes like methane, gasoline etc.

Fluorine is not nice stuff to work with but I know that the gasoline tanks are treated that way on a huge scale so it must be economical to do so and manageable. Maybe you could do something similar.

 

[alleak]

Hi Demon3;
thanks for your consideration,but my question is remained.I know that HDPE is currently used in CNG4 vessels(vessels with non-metalic liner overwrapped with composite filaments) but i don't know what process is done for manufacturing these liners.my question is the permeability range in standard manufactured CNG4 vessels.
thanks

 

[Demon3]

Sure. Well, I have no value, the manufacturer should be able to tell you though, that's their job. My answer is that the permeability of HDPE to natural gas will be high.

 

[patprimmer]

I agree with Demon3.

I would expect that, the smaller the molecule the higher the transmission rate, so CNG will be quite high.

Major reputable raw materials manufacturers should be able to advise. They normally have this sort of data in their technical departments, although this might not be available to the general public, so you will need to talk with a tech service person.

Regards

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[alleak]

so what is the difference beetween high density polyethylene and polyethylene

Cheers
alleak

 

[patprimmer]

Not to be trite, but High Density Poly Ethylene (HDPE)has a higher density than MDPE or LDPE.

This is due to a more linear molecule allowing a higher degree of crystalinity.

The higher degree of crystalinity has a pronounced effect on various properties including permeability and flexural modulus.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Demon3]

Yes, the crystals are impermeable to gases and so the higher crystallinity of HDPE lowers the permeability somewhat. However, I really think that the permeability of any type of PE to alkanes will be very high. Afterall PE is just the same type of molecule as the natural gas only longer. Like dissolves like so the gas will go straight through the PE.

 

[ARenko]

HDPE is the preferred pipe for natural gas distribution. See page 6 at the below link for some numbers on permeation.

http://www.cpchem.com/enu/docs_pipe/PP30168003408HDPE.PDF

 

[Demon3]

Nice article. Thanks for the link djv. It says that PE is permeable to hydrocarbons but the level is too low to affect the amount of natural gas leaking as you transport the gas through a pipe. In as static vessel as alleak described, and under pressure too, the leakage could be significant.

I read that the vessels are PE on the inside only then wrapped with an epoxy or polyester / carbon fiber composite. That outer layer probably is what gives the barrier properties as it is polar and the natural gas is not so it will not diffuse through that layer as readily.

 

[ARenko]

Yeah, I'm not sure how it would relate to a static vessel, but it did give a permeation at 60 psi of just HDPE. Probably like others have said there are other/ additional barriers. Maybe the inside surfaces are treated with a flourination or sulfonation process. Or maybe the liner is multilayer, with a barrier layer other than HDPE. Are we really wanting to know the permeability of CNG through HDPE, or trhough a tank that happens to have an HDPE liner, but may have additional forms of permeation prevention?