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35-3 CNG tank size

treilley

Sustaining Partner
Can anyone tell me what size the CNG tank is on a 35-3? I always forget which one I have and I need to swap it for a fresh one this week.

TIA
 

u079721

Contributing Partner
In my experience there was only one CNG tank size, at one pressure. About the only thing to worry about is whether the test date stamp is over 5 years old, meaning you may have to pay additional for testing of your old tank.

I mentioned this before in another thread, but we bought an adaptor that allowed us to fill up our CNG tanks at a local CNG alternative fuel filling station. The first year it cost us just $0.46/tank for fill ups, instead of the $50 we had been paying to send them away for refills.
 

Chris Miller

Sustaining Member
we don't get a choice of sizes either. Just a big, old, steel scuba tank full of cng. I'm going to be sad to leave the realm of cng for LP- heavier than air explosive gas kind of scares me.
 

u079721

Contributing Partner
Hi, Steve-
Where did you get this adaptor? Thanks.



We bought ours from Corp Brothers ( http://www.corpbrothers.com/catalog/cng.htm ). BUT, their lawyers got involved, and not only did they stop selling the units, they actually went to the expense of sending me a registered letter informing me that the use of the adaptor was no longer considered safe, and that they strongly suggested I stop using the unit and dispose of it.

Why? Well the tanks are rated for 2250 psig. Much of the time I swapped for a full tank it would read way over that though, as much as 2800 psig.

If you go to a CNG filling station pump you have your choice of two settings: 3000 psi and 3600 psi. We would use the 3000 psi setting, which would result in a gauge reading of about 2700. This semed safe enough, and made me figure the professionals who refilled the CNG tanks for swap at marinas were using the 3000 psi setting too. But then what does that 2250 figure mean on the side of the tank?

But there was no mechanical lock out to keep one from using the adaptor and pump to fill the tank at the 4500 psi setting. And while I don't know the safety factor built into the tank, I'm guessing that would way exceed it. Which is probably why Corp Brothers wanted us to stop, and why they quit selling the adaptor. We didn't quit using the adaptor, but when we sold the boat, we chose to NOT include the adaptor with the rest of the gear.
 
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Cory B

Sustaining Member
Back when we had CNG our local Natural Gas company would fill up our CNG tanks for $20 each. No adapter required.
 

Shadowfax

Member III
Is there an adapter now available? They want like $70 to fill an exchange tank around here. I don't know how much they are if available but it wouldn't take too many fill ups to make it pay.
 

u079721

Contributing Partner
I don't know of any. At the time our adaptor cost $120, and it more than paid for itself the very first time we used it.

But the adaptor shouldn't be that hard to make yourself. I'd be surprised if you couldn't buy the parts from McMaster Carr ( http://www.mcmaster.com/ ). To begin with go to the alternative fuel web site:

http://www.eere.energy.gov/afdc/stations/find_station.php

and find a CNG station near you. Then go to the station to be sure what type of fitting they use, and take a picture. If you then took this photo and your CNG tank to a welding supply type store I bet they could sell you the parts. IIRC the adaptor was just a reverse thread brass fitting on one end attached to a brass T fitting, a gauge in the middle (which you don't really need, but makes you feel safe) and a quick release fitting on the other end of the T that matched the pump at the filling station. I think this fitting is standard, but there may be others out there.

Sounds like a bit of work, but I would sure give it a try if I still had CNG tanks.
 

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u079721

Contributing Partner
Here's how to do it

I did a little searching, and found a DIY page at a Catalina web site with instructions on how to make your own adaptor. There's a bit more than I thought to the challenge, and it appears that you can't buy everything you need off the rack. I have to admit that I am not comfortable with idea of trusting something I altered the threads on to such a high pressure.

http://www.c34.org/projects/projects-cng-refill-adapter.html
 

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u079721

Contributing Partner
Here's a better one?

Here is a set of instructions that doesn't seem to need you to drill and tap your fitting. Not sure about getting all the parts, but it sounds safer.

http://morgan38.org/discus/messages/1/13490.html?1116597305





Click on the photo to enlarge to see detail.

From the article:

"Parts list for CNG Adapter

1) brass nut (LH thread) – CGA 350 *
2) Brass nipple – CGA 350 *
3) “hydraulic Tee” high pressure** 1/4 inch
4) gage 0-4000 psi (Note: must be “no oil” type. Oil filled gages may explode!
5) Sherex/OPW LE-36 (3600 psi) receptacle ***
6) Adapter – joins Sherex /OPW to “Tee” ****. The “thread” on the Sherex end must be 9/16” straight thread.

Note: all fittings must be rated at 5000 (preferably 10,000) psi working pressure or higher. More is better.



* “CGA 350” denotes “CNG” fittings. Oxygen and other gases have a different CGA number. The fittings for different gases use different threads on their connections. Purchase these fittings and the gage from a welding supply shop. Total cost = $20.00. Most shops don’t stock CGA 350 fittings and will have to order them for you.

** High pressure Tee can be purchased at a hydraulic marine supply or repair shop. Cost = $8.00.

*** Sherex adapter and receptacle assembly is purchased from the OPW Corp. Total cost = $90. Order by calling 513-870-3100. It is a quick disconnect, ball lock type fitting.

**** Adapter can also be purchased from a Marine hydraulic. The problem of refilling these tanks using any CNG vehicle refueling “pump” is the fact that CNG powered vehicles are designed with SAE straight thread fittings. Our tanks are designed with NPT pipe thread fittings. This adapter (actually, the Tee) joins these two different thread styles together."
 
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jkm

Member III
We still can get the tanks refilled in the Marina-cost is $30.

Where and how they get refilled is kind of a mystery. The dock workers know very little about the where and how. Fortunately the is a good supply.

I have checked and the tanks are all current of their dates.

A dive master told me that the tanks are tested at the same lab that tests scuba tanks. All they really do is clean out the insides and retest their pressure ratings, stamp them and put them back into service.

I've heard of a few exploding but never in a boat. I wonder if it's usually operator error while refilling them.

John
 

ted_reshetiloff

Contributing Partner
Saying I was to build/buy an adapter anyone know where to buy CNG near Annapolis? I just swap out the tank for a full one at Jabins every other summer. We grill out mostly as the stove heats up the cabin too much and I figure $60 for 2 summers worth aint too bad. Guess if I was using more I would go another route though. There was a time where I was going swap to propane but it was going be expensive and dangerous and I figure as long as CNG is available I'll spend my money on more important stuff like rum!
 
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