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cutting and welding of old propane tanks

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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cutting and welding of old propane tanks

Unread postAuthor: MrDEB » Sat Nov 21, 2009 8:42 am

I found two large (about 4ft tall) propane tanks that are for FREE
Question concerns safety in cutting and welding of tanks.
I understand that removing the valve then dropping dry ice into tank will rid tank of any flammable gas or is there a better way.
leaving out in weather upside down(valve end down) for say a month. Seeing how propane is heavier than air?
recommended method to clean out propane tanks???
Thinking of using one for high tech cannon (plan on adding electronics. have post here somewhere).
this method would be cheaper than buying 10 ft. of 3inch pvc for air chambers. thinking of two back to back but maybe one will give enough punch?Accuracy and distance contest in August of 2010.
have several spent small propane bottles as well (marshmallow guns before building larger gun. These can be brazed instead of welded??
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Unread postAuthor: inonickname » Sat Nov 21, 2009 9:01 am

A method similar to welding petrol tanks is best. Remove the valve, and completely fill with water when drilling etc. After several washes basically all risk should be gone. I haven't welded mine yet, though I drilled two 1 3/4" holes in one and didn't have a problem. It will still smell due to the extremely strong "perfume" they use. I rinsed it several times, completely filling it with water. A passover the inside with a blowtorch showed it wouldn't be a problem.

It's up to you, but if you're making large holes it may be an idea to paint or coat the inside of the cylinder in epoxy to prevent rusting. (propane does not contain water, but air will).

Off topic: Anyone know how to get rid of the perfume they add that seems to remain as a residue in the tanks?
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Unread postAuthor: Heimo » Sat Nov 21, 2009 9:15 am

I have cut several propane tanks in my lifetime there are a few methods going round to do this safely
the first thing you would have to do is remove the valve
then you can fill it with water completely then there are no flammable vapors left. unlike popular believe propane does not soak into the metal the smell that they add to the propane to aid leak detection does. it is called methanethiol if you want to clean your tank of that smell you are going to have to wash the tank out with straight bleach and after that coat the metal with a light coat of oil to prevent corrosion (I have not tried to clean the smell out of a tank this way but someone else that has done this told me this is the way to do it)


here is a link to a thread on a other forum that this has been discussed on briefly
http://www.backyardmetalcasting.com/forums/viewtopic.php?t=3840
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Unread postAuthor: Technician1002 » Sat Nov 21, 2009 11:47 am

Even after washing, expect the welding process to release flammable oil vapor from oil or grease in the tank. To prevent an explosion, use either water, dry ice, shield gas (argon etc) to purge the tank of OXYGEN. You will still get flammable vapor when welding, but not have an explosive fuel oxygen mixture inside the tank. See my 2.5 QDV build. I had a flame about 4 inches tall dancing on the end of the pipe while I welded my tank. It burnt as it contacted air outside the tank. Being oxygen poor, it was a thin dark flame like you expect to see dancing in BBQ grill coals. I hope this helps. Do not permit an explosive mixture of fuel and air to build up in the tank.
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The thin dark flame danced at the contact with air like this. Edited photo, not the real flame.
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Unread postAuthor: MrDEB » Sat Nov 21, 2009 1:03 pm

I like the idea of dry ice.
I wonder if I filled tank with water, droped some weighted dry ice (so it won't float) into tank then proceeded to cut and weld
On the subject of welding, contemplating on using exhaust pipe.
cheaper that 2inc galvanized and smoother inside.
this is for the valve cylinder?
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Unread postAuthor: D_Hall » Sat Nov 21, 2009 1:13 pm

MrDEB wrote:I like the idea of dry ice.

Ummm... Why?

I wonder if I filled tank with water, droped some weighted dry ice (so it won't float) into tank then proceeded to cut and weld

Dry ice sinks all by itself. No weight required. But just what do you expect to accomplish with this method that water alone wouldn't?
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Unread postAuthor: Technician1002 » Sat Nov 21, 2009 1:14 pm

MrDEB wrote:I like the idea of dry ice.
I wonder if I filled tank with water, droped some weighted dry ice (so it won't float) into tank then proceeded to cut and weld


Dry Ice is denser than water. It sinks without needing weights. Use enough dry ice to fully purge the tank of oxygen several times over. You may need to plan on feeding more dry ice before you are finished.

But just what do you expect to accomplish with this method that water alone wouldn't?


Ever try to turn over and empty a four foot tall propane tank full of water? That's a lot of water.

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Unread postAuthor: D_Hall » Sat Nov 21, 2009 1:58 pm

Technician1002 wrote:Ever try to turn over and empty a four foot tall propane tank full of water? That's a lot of water.


And adding dry ice to that container of water makes it lighter?

Now, if they were using the dry ice as a purge AFTER the water, it would make some sense. But WHILE the water is in the tank? That accomplishes exactly nothing.
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Unread postAuthor: jrrdw » Sat Nov 21, 2009 2:13 pm

D_Hall wrote:Now, if they were using the dry ice as a purge AFTER the water, it would make some sense. But WHILE the water is in the tank? That accomplishes exactly nothing.


Except a nice fog 6 inch's deep. :P
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Unread postAuthor: MrDEB » Sat Nov 21, 2009 4:22 pm

yea it could get hevy for sure.
maybe just a little water and lots of dry ice = lots of fog??
going to have local welding shop do the welding anyway. I am going to do the small propane cylinder by brazing.
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Unread postAuthor: Technician1002 » Sat Nov 21, 2009 5:13 pm

MrDEB wrote:yea it could get hevy for sure.
maybe just a little water and lots of dry ice = lots of fog??
going to have local welding shop do the welding anyway. I am going to do the small propane cylinder by brazing.


Commercial industrial dry ice fog machines use a 55 gallon drum with a basket for the dry ice, a blower, and a 1500 watt heater. Hot water is used to generate lots of water vapor. The basket of dry ice is lowered into the top of about 1.5 feet of water. This produces a thick ground hugging dry ice fog that rolls over the floor and quickly dissipates. Used one for some stage work I did. I did the lighting and special effects.

Used one somewhat like this;
Image
This one uses a water pump to spray hot water onto the basket of dry ice.

Do to the short life of the fog, this style is fairly rare now. For a ground hugging fog, often a traditional juice fogger is vented through a basket of dry ice to super cool the fog to get it to hug the ground.

Back on topic;

have several spent small propane bottles as well (marshmallow guns before building larger gun. These can be brazed instead of welded??


It is highly advised to braze the small tanks. It is too easy to melt through them when trying to get a weld on them. I brazed mine. In some places you can see the brass.
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Piston retention showing some brass in the threads from brazing.
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Unread postAuthor: Major Collins » Mon Nov 23, 2009 6:59 am

ok my uncle did for me and i witnessed it .. what he did was put the blow torch adapter onto it and kept burning and burning till there was only residue of gas left in the bottom of the bottle after he used a screw driver and depressurized most of the bottle through a shrader in the middle where the neck of the bottle is and then he drilled a hole through the shrader in the bottle and then lit up the top then let the gas burn itself out. After he welded a steel fitting on it for me and now i can use it as my air tank :D but make sure you burn out or just release the gas till its empty as it will go . just a note while he was welding the was a little gas that leaked out and was burning like slowly like a candle and he let it burn itself out to rid more of it. AND Once you pressurize it a few times and fire it rids the gas anyway with the air that is shot . :D hope that helps
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Unread postAuthor: Major Collins » Mon Nov 23, 2009 7:18 am

sorry for the double post but hope th pics of the bottle helps ;)
The bottle now
Image
top of the bottle (i know its messy weld but it holds :) and i grind ed the weird bits off)
Image
Steel bit that was welded on (this was the off cut, with a thread on the opposite side and my steel bit was obviously shorter)
Image
the top look of it bottle (you can see where the hole was drilled through and the little thingie on the side is a inbuilt shrader used for refilling but i can use it cos it needs an adapter)
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hope these helps
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Re: cutting and welding of old propane tanks

Unread postAuthor: jimmy101 » Mon Nov 23, 2009 2:53 pm

MrDEB wrote:leaving out in weather upside down(valve end down) for say a month. Seeing how propane is heavier than air?

Leaving it upside down won't do anything. Even though propane is denser than air, once two gases are mixed there is no significant "unimixing" regardless of the relative densities.

Leaving the cylinder open for a few months will work, but you need a fairly large orifice.

Water really is the easiest way to do it. CO2 is OK when you are actually welding since welding a water filled container is kind of tricky.
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Re: cutting and welding of old propane tanks

Unread postAuthor: Technician1002 » Mon Nov 23, 2009 7:25 pm

jimmy101 wrote:
MrDEB wrote:leaving out in weather upside down(valve end down) for say a month. Seeing how propane is heavier than air?

Leaving it upside down won't do anything. Even though propane is denser than air, once two gases are mixed there is no significant "unimixing" regardless of the relative densities.

Leaving the cylinder open for a few months will work, but you need a fairly large orifice.

Water really is the easiest way to do it. CO2 is OK when you are actually welding since welding a water filled container is kind of tricky.


When his uncle did the tank, the tank was full of propane vapor but had no oxygen. Using a tank that had been purged with air and then heated has killed several people. Most of the statics are those who weld larger tanks such as a car gas tank. You will get flammable vapors when it is heated. The trick is to either keep a purging flow running to keep the fuel vapor down, or purge with an oxygen free gas, such as propane like his uncle did.

http://www2.worksafebc.com/i/posters/2002/ha0201.html
http://www.odt.co.nz/the-regions/west-coast/72579/man-killed-greymouth-explosion-named
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