CNG Cylinders

[kmacdonald]

Does anyone use dive cylinders for CNG storage? Aluminum or steel?
The tank outlet is different but an adapter could be used. Easy enough to make up an adapter to fill at CNG refueling stations. Pressure regulators are readily available.

[john61ct]

The refilling places for LPG are usually very strict about only allowing standard tanks.

Maybe CNG things are looser, dunno.

[Jammer]

The valve outlets used on various compressed gas cylinders are deliberately incompatible to avoid accidents where the wrong gas is used. CNG cylinders use the CGA 350 connector. Both styles of SCUBA valve in widespread use (DIN and yoke) are incompatible, by design, with any industrial or medical gas valve, so that SCUBA regulators cannot be connected to industrial or medical gas cylinders.


Filling a SCUBA cylinder with a non-breathable gas, such as CNG, can easily result in a fatality if the cylinder is then used for SCUBA diving. Such fatalities have actually occurred. There have also been fatalities in medical and industrial settings where "adapters" have been used to allow cylinders to connect to a non-matching regulator or gas fill source.



I would urge you to rethink your plan.

[kmacdonald]

Yes, I'm familiar with the different tank fittings. There are adapters to convert one to the other. For example, the SCUBA tank valve would not be used for CNG service. A CGA 350 valve would be used and an adapter between that and the tank. To use the tank for SCUBA would require the valve be changed making a mistake nearly impossible. The only issue I can think of is not having a guard to protect the valve on tank but I have never heard of it being an issue with divers. The industrial cylinders are available but more expensive.
It only cost a couple of dollars to fill a 80 cubic foot cylinder with CNG at a refueling station.

[a64pilot]

Your aware I assume that most Scuba tank valves are machine thread and have an O ring for a seal, where I believe all other pressure vessels I have experience with are a a type of interference tapered thread, may be pipe thread, not sure.

There were a few "real HP" Scuba tanks that were pipe thread valves, 3500 PSI steel tanks I think. They are rare though, almost all what are referred to as HP scuba tanks are actually exemption 3442 pressure tanks, and they all use the same machine screw threaded valves as other scuba tanks.

In other words, just buy the proper thanks and save yourself a lot of hassle, surely if you don't you will run into the occasional I only fill proper tanks, and that isn't

[puffcard]

You can find use CNG tanks all the time, there's a chap selling four in Annapolis.

[Jammer]

Originally Posted by kmacdonald

Yes, I'm familiar with the different tank fittings. There are adapters to convert one to the other. For example, the SCUBA tank valve would not be used for CNG service. A CGA 350 valve would be used and an adapter between that and the tank. To use the tank for SCUBA would require the valve be changed making a mistake nearly impossible.

I see. SCUBA cylinders use 3/4" NPSM threads with an o-ring seal. I do not believe adapters/bushings are available to adapt this thread form to any of the several thread forms used by the packaged gas industry.

Originally Posted by a64pilot

Your aware I assume that most Scuba tank valves are machine thread and have an O ring for a seal, where I believe all other pressure vessels I have experience with are a a type of interference tapered thread, may be pipe thread, not sure.


There were a few "real HP" Scuba tanks that were pipe thread valves, 3500 PSI steel tanks I think. They are rare though, almost all what are referred to as HP scuba tanks are actually exemption 3442 pressure tanks, and they all use the same machine screw threaded valves as other scuba tanks.

Substantially all SCUBA cylinders sold for the North American markets use 3/4" NPSM threads with an o-ring. This thread form was arrived at more or less by accident and isn't used by the packaged gas industry. The 3500 PSI cylinders to which you refer use a 7/8" UNF thread form, also with an o-ring; these cylinders have been out of production for around 20 years. European and Asian market cylinders generally use an M18 thread; a few of these were sold in the USA in the early 1970s also. There's other oddball stuff out there. Some much older (1950s/1960s) cylinders used 3/4" NGT, which is one of the threads used by the packaged gas industry, but there are relatively few of these left.


The medical and packaged gas industry in the USA pretty much uses 1/2, 3/4, 1, and 1 1/4" NGT for steel cylinders, and 5/8" UNF for small aluminum cylinders (with an O-ring). I believe they use some of the larger UNF threads for larger aluminum cylinders. The whole industry is moving to metric threads as part of the change to UN cylinders that can be used internationally.

In other words, just buy the proper thanks and save yourself a lot of hassle

This.


You're not going to save any money fiddling around.

[kmacdonald]

It's not the money but the size of the tanks. Methane tanks are 2000 psi tanks and common SCUBA tanks are 3000 psi. HP steel are 3442 psi. More gas, less space. A lot of the tanks I see at marina swaps have never been retested and are close to 40 years old. Do you think those are safer than new SCUBA tanks?

[john61ct]

Those details, and our & your assessment of safety are not relevant to the question as to whether or not you're going to get anyone to fill your non-standard tanks.

[kmacdonald]

Originally Posted by john61ct

Those details, and our & your assessment of safety are not relevant to the question as to whether or not you're going to get anyone to fill your non-standard tanks.

I'll stick my credit card in the CNG pump and fill em up. They are unattended stations open 24 hours a day. There's nothing unsafe about it.

[Jammer]

Originally Posted by kmacdonald

It's not the money but the size of the tanks. Methane tanks are 2000 psi tanks and common SCUBA tanks are 3000 psi. HP steel are 3442 psi. More gas, less space.

HP steel SCUBA cylinders are not authorized for use with flammable gases under the terms of the special permits that authorize their manufacture and use.

I would suggest giving https://allsafe.net a call and telling them what you want. They sell new and used cylinders. They can get things that are not on their web site, and will set you up with the correct valve and a current hydro test. I have found them willing to work in small quantities.


High-pressure steels that are authorized for CNG do exist however I am not sure if they are made in the sizes you want. I have some 3960 PSI cylinders in my garage, but they're big. I don't use them for CNG -- they're for air -- but they are 3AA cylinders and could be used for CNG by changing out the valve.


Finding high-pressure CNG fills may be a problem, not sure. Something you may want to check.

A lot of the tanks I see at marina swaps have never been retested and are close to 40 years old. Do you think those are safer than new SCUBA tanks?

No, but that's a red herring. If you run older cylinders, get them hydro tested. It's not expensive and it keeps you and everyone else safe.

[john61ct]

Then your only challenge is getting the adapters, may need to get them made, CNC outfit?

[kmacdonald]

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.

[kmacdonald]

Is no one using SCUBA cylinders or is it that they don't want to publicly say they are? I haven't found a place that will sell CNG tanks to the public so if there are some that will the price will be outrageous and the tanks are huge 2000 psi dinosaurs. The places that swap tanks for full ones don't seem to test the tanks. Has government over reach created a potentially dangerous situation? 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.

[john61ct]

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