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Old 25-07-2018, 11:58   #16
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Re: CNG Cylinders

Scuba tanks blow all the time, for various reasons, but almost always the discharge disk has been tampered with.
https://www.cnn.com/videos/us/2011/0...ath.bay-news-9
A friend lost her life filling an O2 Deco tank a few years ago.
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Old 25-07-2018, 12:01   #17
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CNG Cylinders

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Originally Posted by kmacdonald View Post
A 20 lb propane tank has a LOT more energy in it than a 80 cubic foot dive tank of CNG at 3000 psi, and I mean WAY more. I think about 5.2 times more energy in the propane tank. Now you can see why I'd like to store it at higher pressures than the standard 2000 psi tanks. CNG tanks in CNG cars are 3000 or 3600 psi. That's one of the reasons why propane is so popular---energy density. As far as safety goes, CNG is much safer. CNG has a higher ignition temp and a tighter gas to air ratio to become explosive.


Propane tanks from pressure have very little stored energy as they are relatively low pressure, now if the propane is released and allowed to mix with the right amount of oxygen, which believe it or not is difficult, then you of course have a fuel air explosive, which is phenomenally powerful, but it’s real tough to get the right mix.
You can shoot full propane tanks with a high power rifle, nothing happens except the gas is released.
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Old 25-07-2018, 12:05   #18
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Re: CNG Cylinders

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Originally Posted by john61ct View Post
Yes just like with LPG, the issues for DIYers are not technically challenging.

It is the insurance / regulatory restrictions that are hard to overcome.

Maybe - if you don't get good answers here, maybe try contacting someone actually certified to deal with these issues?

https://www.csagroup.org/search-qual...=usa&states=ny
I talked with a manufacturer rep that makes and sells CNG refueling receptacles. They won't sell the adapters that he told me to use because they aren't "certified" but he said they were perfectly safe to use. ORB fittings were what he recommended and not NPT. The different fitting sizes to prevent the wrong gas being put in a tank or the wrong tank being hooked up to the gas line by remedial workers makes sense in an environment that uses multiple gasses but for our purposes it really doesn't matter. The different fitting sizes are to prevent human error, not because they are better for that particular gas. I'd prefer to use my own equipment rather than old questionable tanks that someone chargers $75 to swap out with another old full tank. Like I said before, a 80 cubic foot, 3000 psi SCUBA tank only costs about $2 to fill.
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Old 25-07-2018, 12:06   #19
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CNG Cylinders

Steel scuba tanks of the same capacity are much lighter and smaller than aluminum tanks, especially if we are talking the 3442 PSI exemption tanks.Click image for larger version

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Left to right front row, 130 steel, 100 steel, 80 steel, 80 aluminum and 62 aluminumClick image for larger version

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Different view of an 80 cu Ft aluminum and an 80 cu Ft steel. Actually an 80 cu ft aluminum isn’t really 80, it’s smaller, I think 78 cu ft.

These capacities are at rated pressures, 3,000 for aluminum, and 3442 for the steel.
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Old 25-07-2018, 12:13   #20
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Re: CNG Cylinders

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Originally Posted by a64pilot View Post
Propane tanks from pressure have very little stored energy as they are relatively low pressure, now if the propane is released and allowed to mix with the right amount of oxygen, which believe it or not is difficult, then you of course have a fuel air explosive, which is phenomenally powerful, but it’s real tough to get the right mix.
You can shoot full propane tanks with a high power rifle, nothing happens except the gas is released.
The tank, CNG or propane, rupturing is really not a concern. Neither have a history of doing so and both have a long track record of safety. Now with the old un-maintained tanks, anything is possible.
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Old 25-07-2018, 12:18   #21
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Re: CNG Cylinders

I like the steel tanks but they are quite a bit more expensive. They come in HDG also.
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Old 26-07-2018, 07:45   #22
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Re: CNG Cylinders

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Originally Posted by kmacdonald View Post
The places that swap tanks for full ones don't seem to test the tanks.

I would be extremely surprised if you could find a place that swaps tanks that gives out cylinders that were filled after the hydro test expired.


Typically the hydro test is good for 10 years for natural gas cylinders. The shorter five-year cycle applies to aluiminum cylinders, cylinders taken under water, cylinders containing corrosive gases (including CO2), and a variety of other oddball situations like cylinders permanently mounted in a bank. Propane cylinders follow a different set of rules with expiration times that vary depending on how they are tested.



Once a cylinder is filled, it can continue to be transported and used until it needs to be filled again even if the test has expired.


Quote:



I haven't heard of SCUBA tanks exploding except for a 50+ year old expired tank in Australia a few years ago. The tank doesn't care if it has CNG or air in it. It's not rocket science to put together a safe CNG system.

Worldwide, it happens every year. The accidents with air are usually in countries with weak regulatory oversight. There was one on some remote island this year involving a fellow who salvaged an aluminum cylinder from a recycling pile. It had been discarded because it failed inspection. He filled it and it ruptured while he was carrying it to the boat.


There are fires and explosions with oxygen and nitrox cylinders most years in the USA, usually with aluminum cylinders, always as a result of contamination with combustible material. This goes on in medical settings also.


Anyway, as I posted upthread, the problem you're going to have is finding a CNG cylinder valve that will fit the cylinder neck threads of a SCUBA cylinder. There aren't any, and there aren't any adapters.
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Old 26-07-2018, 07:51   #23
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Re: CNG Cylinders

You would have better luck with an oxygen cylinder from a welding setup. They are typically set up for 2640 PSI service pressure (2400 + 10%), and have the threads you need to fit a CNG cylinder valve. You can buy them new on ebay (but be careful to be sure you don't end up with a counterfeit cylinder from China), or you can get used ones on Craigslist etc.
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