Propane Tank to Vacuum Tank conversion

I only need about 30 inches of vacuum. I will be using this as a vacuum thermoformer because my shop-vac isn't quite strong enough.

I'm not sure about sticking a torch on this thing.. that would make me a bit nervous! Maybe i'll attempt to remove it in a vise with a long prybar first... :o)

Thanks for your feedback! Mike

I would be concerned about the tank sides imploding, just a guess, but under pressure as in propane service the hoop strength will be quite high, for vacuum service you may need to add rings to avoid implosion.

Jack

I would think that it would have greater strength being used in a vacuum than for outward pressure, being submarine shaped already... but I didn't do all that well in physics either! :o) Thanks for the feedback!

Mike

I removed a valve on mine with a big monkey wrench, its in there but not to bad. I made a bird feeder out of mine. As to the vacuum tank I would think it would work fine. We used an old compressor tank in my dads shop for his vacuum former. You need that large volume to pull the plastic down. We had a

30 gallon tank.

It should be no problem. When a new tank is filled for the first time the propane dealer uses a vacuum pump to remove all air before filling.

Scp

A torch on a propane tank. Even if the tank is empty that sounds like a good way to make it onto the Darwin awards list!

When I converted a propane tank to an air tank I stuck a piece of half-inch pipe into the outlet of the valve. Since I was going to throw away the valve I didn't care if I messed up the threads. It turned with no problem. I had tried wrenches with no luck. Propane dealers have a special wrench to remove the valves.

The new rule that all tanks had to be converted to ones having an OPD seemed like a smart scheme to sell lots of tanks and valves. I wonder how many times companies get the government to implement mandatory rules as a way to increase sales? Sounds like a good racket to me.

Scp

"Mike Gercevich" wrote: I only need about 30 inches of vacuum. I will be using this as a vacuum thermoformer because my shop-vac isn't quite strong enough.(clip) ^^^^^^^^^^^^^

*Only* 30 inches? That's a perfect vacuum. I am not sure what a thermoformer is, but it sounds like you are pulling heated plastic down around something. Am I close? Well, your propane tank will have the *30 inches* only until you crack the valve, and air starts to flow in.

If your shop vac is not quite strong enough, where are you going to get the vacuum connection to pump down the propane tank?

As faras imploding the tank, not to worry. it is shaped roughly like a Dewar flask, which is made of very thin glass, and holds 30" of vacuum very nicely.

Remember that even if you could get a "perfect" vacuum on the tank, the maximum pressure differential would be 14 psi. Even if the tank did fail, it would just collapse. No danger of pieces flying around. OTOH, the smaller the tank, the better. Big tanks have much less resistance to crushing:

-- Bob (Chief Pilot, White Knuckle Airways)

I don't have to like Bush and Cheney (Or Kerry, for that matter) to love America

They do things like that as a result of stupid people demonstrating their diabilities and sometimes sending innocents to heaven when in the process.

Consider asking a non techie son or daughter to go out and get the barbeque tank filled. And they get it done by some ill trained kid who overfills it so much that the safety pops after that tank sits in the kid's car trunk for a few hours on the fourth of July.

That could lead to a some kind of tragedy, couldn't it?

Just my .02,

Jeff (Who wonders how many of you noticed him demonstrating untypical PC sensitivity by listing "son" before "daughter" in his narrative? )

So how dangerous could a vacuum collapse of a ductile steel tank be anyway?

Probably no worse that the high school physics demo of callapsing a one gallon tin can with condensing steam.

Jeff (Who still remembers the time he bumped the neck of a 19 inch B&W CRT he was carrying and watched the gun assembly fly right through the faceplate and make a pretty good dent in a plasterboard wall.)

I've never done any thermoforming so I gotta ask, izzat that 30 inches of water or 30 inches of mercury?

I'll guess that it's water, given your shop-vac comment.

Jeff

Smaller tanks are stronger but to do vaccuum forming you need a large volume nearly all at once. We would draw down on our 30 gallon tankfor a good time before opening the valve and then the plastic just sucks into place. Now the bigger the mold the more volume you need

Ah yes, 29.8+ is about the closest to a perfect vacuum that can be made on Earth (duh! at me). Actually the Vacuum pump is rated at 28.8 inches.. so I'm sure I'll get less than that!

I plan on connecting the tank between the vacuum pump and the dump valve. I need the initial volume to quickly pull the soft plastic around the form, I will leave the vacuum pump running as it will be need to continue to pull until the plastic hardens. Depending on the size of the base, I may need to use a couple of tanks in series to force the initial strong pull.

A shop-vac has high CFM but will only pull about 4 inches and that will not create enough vacuum for higher detailed forms.

Thanks for your input, Mike

Your correct that a shop vac has high volume but low draw power, the molds use very small holes so both volume and a strong vacuum is needed. I used to do some real neat stuff with my dads former. Most of the Gast pumps are very easy to rebuild if worn. Let me know where you live my dad my have some forming stuff left over.

Just remove it, that's what I had to do when I converted a propane tank to air storage It took two people, a huge monkey wrench and a propane torch to get it off though.

Not sure about us> Has anyone ever attempted to use an old propane tank as a vacuum tank

These tanks are hydrostatically proofed at 300 psi internal pressure. They can handle -14.7 psi ( full vacuum) without any problem. My guess is it would take about 1000 psi external pressure to collapse one of these tanks. In any case, having a tank collapse is not something likely to cause injury.

Jack,

I've been using an old propane tank for a high vacuum chamber for several years, no problems so far.

On one occasion I did have a failure of a plate glass window I was using for an electrical pass thru. The glass cracked as I was really pushing the filament temperature up, causing the bolt I was using for an electrical conductor to heat up. The bolt went thru the glass window, and the heat caused the glass to crack rather suddenly while at full vacuum (2x10-5 torr)

Asides from ruining a nearly finished coating, nothing exiting happened other than air leaking into the chamber.. (I will admit it caused me to take several steps back!)

Take Care, James Lerch

(My telescope construction, Testing, and Coating site)

Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge

When I converted my extra propane tank to an air tank, I put a vacumn on it several times using my Harbor Freight vac pump. Vacumn in the mid 20's mercury and no problems. Getting the valve out required a 6" piece of pipe, burned out and ground to go over the valve, and a hole through the pipe to take the longest prybar I own. Tank was strapped to a post using ratchet tie downs.

John H.