Using an air tank for vacuum?

In general, no. I have personally seen an ASME-rated small air tank (portable, probably 5 or 10 gallon) collapsed by pulling a vacuum in it.

Hi,

Obviously it will depend on the design of the tank, and I'm not sure what these portable air tanks are like, so take Don's word here. The risk is that the walls of the tank will buckle under compression - it's much easier to buckle a metal sheet than it is to pull it apart in tension. You could look at small propane cylinders instead. I am pretty sure the smallest propane cylinders we have in England would be strong enough to take a vacuum. And if you don't mind destroying an air tank, you could try it. The energy storage wouldn't be huge, the steel wouldn't shatter, it would collapse slowly, and any bits would go inwards, so I think it would be a reasonably safe thing to try. Just pump it down slowly with a pump which can pull a good vacuum. It might be that you don't need a really good vacuum in your de-soldering application, anyway.

Best wishes,

Chris

I wouldn't suggest trying it, especially after reading this post in this NG from "Steve" in May of 2000:

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Someone passed the following message to me. I have no idea who originated it.

Good food for thought !!

------------------ Please share this visual with all of your employees in upcoming meetings. It is a great reminder that though many of our vessels are pressure-rated, many will not withstand significant vacuum. A vacuum can be created by simply draining a vessel without venting, or by steam cleaning and not venting --- the subsequent condensing of the steam is an extremely strong vacuum. The hotter or higher pressure of the steam, the stronger vacuum.

Even the condensing of a hot hydrocarbon, with low vapor pressure can have similar results if the system is not gas/N2 blanketed or equipped with a vacuum breaker.

The tank car in the linked photo is not one of ours. It is a general purpose PYGAS car that was being steam cleaned to prepare it for some maintenance work. The job was still in progress at the end of the shift, so the employee decided to block it in. Problem was there was no vacuum relief; the car cooled, the steam condensed and the car imploded."

_____________________

Note: the links have been "adjusted" to point to the current locations of the files.

The danger of tank collapse not withstanding, your idea should give some 'extra' vacuum. After all, don't they use vacuum storage in automobiles to assist wiper motors (old cars) etc.?

Wow. Amazing pictures. Thanks, Ram, Don, and Christopher. I will not use these cheap air tanks. I do indeed have a couple of propane tanks that are sitting here doing nothing (we now have a natural gas grill and electric stove/oven), so I think that I will simply convert one to vacuum. They should be good enuf. You are right that desoldering does not require deep vacuum, but it is good to "pool" vacuum so that it provides plenty of "pull" just at the right moment.

I do actually have a desoldering station, but I am not satisfied with its performance. It does not suck well enough, and yes, I did install a new tip. I have a heated compressed air gun RayChem HT-900B, which does a good job at desoldering, but splatters solder a little bit too much.

I recently bought a vacuum pump DV-142, similar to ebay item

7587026645, and want to hook it up sensibly. (I bought it from the military with a bunch of other stuff, like a box of machine belts. If anyone who participated on this thread has a need for a particular belt, let me know and I will check). i

Absolutely. By the way, here's another link to the story about collapsed rail tank car

i

Yes.... some were standard tin cans. Size matters. I think you would be hard pressed to get a small propane tank to implode. Something the size of a railroad car is another story.

Mark

If you've got the tank, try it. Tie a piece of copper wire around the girth and watch it for looseness as you evacuate it for the first time. It the wire gets significantly loose, then the tank is too weak. I don't think the posts about the railroad tank car are applicable to your situation. If you get to 26" Hg, then most of the air pressure that can be applied already is. If somebody used a rusted out air tank or one that already had a dent, and it "gave in", I wouldn't be surprised. If you are worried, put a box around it while you test it. We used a 1 lb propane bottle as a vacuum reservior for years on a machine sold and none of them ever gave up. If you don't know what implosion looks like in-process, boil some water in a one gallon paint can, take it off the heat, then put the cap on. Pretty dramatic, but look at the wall thickness.

Pete Stanaitis

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Ignoramus28190 wrote:

That reminds me of the science experiment we did in junior high school many years ago.

Take a metal 1 gallon can like what acetone comes in. Put a little water in the bottom of it and then heat it up till it is boiling. Take it off the heat and quickly put the cap on it. The can will slowly start to be crushed by the atmospheric pressure when the steam condenses.

It looks like an invisible hand is squeezing it into a ball.

John

Absolutely..... ..

I have used MANY conventional Air Tanks without any issue.

Your worst possible case disaster is it folds up on itself. (NEVER seen it happen)

Propane tanks STINK inside and will smell for a long time... Remember, your pump will evacuate that stink all the time.

Grummy

Yes, and those vacuum storage vessels on later model GM cars are molded in the shape of a sphere for maximum strength, and manifold vacuum from an engine is partial.

If you start getting down into a fairly hard vacuum like that tank car failure with condensed steam, the integrity of the vessel starts being a serious issue. A sphere is the best shape for that job, anything cylindrical or square is going to need serious internal bracing to survive.

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I use several 4" pvc pipes with end caps glued on for vacuum tanks. Been using them for years. Cheap, and they work good for us. They run about 25 to 27 inches of vacuum. Our shop has a central vacuum pump, and each machining center has a vaccum tank for a vacuum chuck system. the tanks allow the colant to seperate from the air, and not get back to the main pump. You could smack them with a hammer, and nothing (bad) will happen. You can drill and tap into the end caps for the fittings.

I will repeat this here too.... Do not use PVC for air pressure!!! Pete

The ASME tank I saw collapsed by vacuum was brand new from Grainger. We saved that crumpled wreck to give to Noel when he retired. He was the engineer that had specified it for purchase. Seemed like a good idea at the time.....

That said, don't be afraid to try it. If the tank fails, all it does is collapse, no big deal other than the cost of the tank. It just goes "crump".

I don't know if a 20 lb propane tank would collapse or not, but it'd be interesting to find out. They're not far from spherical so I'd expect that they'd work OK.

Yes, but not desperately well.

You can't "compress" vacuum. So for a 90psi outlet on a 135psi tank you have nearly twice the energy stored on there, compared to a tank at

90psi. With vacuum you can only store the same (negative) pressure as you're using. Also -15psi of vacuum isn't much energy either (although you probably don't need much).

As you're also working on a pressure difference of a mere 15psi, you need short fat hoses too. Typical airlines will carry vacuum without collapsing, but the flow rate is terrible.

There's also the question of how clean your suction air is, and how you get the crud out of the tank afterwards.

Generally "stored" vacuum is done by storing compressed air and distributing it round the shop, then converting it to vacuum with a venturi device at each bench, where it's needed.

Someone here has some nice vacuum venturi plans on their site. I found them a month or two back. Very simple to make - I was interested in one for desoldering also. I could probably dig them out if anyone wants them and cant find them. IIRC they were based on a Harbour Freight product. rob

Here it is courtesy of Nick Muller:

I saw a webpage from a rec.crafts.metalworking poster where he described making a vacuum accumulator made from a propane tank. The opening in it (after removing the valve) is supposed to be 1" NPT, so it will be easy to step down to 1/4" NPT. I already have some vacuum gauges from two broken vacuum pumps that I had to take apart a while ago.

So, I think, it makes little sense to buy a air tank if I can make something good out of a propane tank.

i

Yes, I do not need much. I do not need "deep" vacuum in millitorr, all I need is "some vacuum", as far as desoldering goes. I do, however, benefit from having a "store" of vacuum.

Good point.

I think that the right thing to do is to have a tube that would catch the crud, between the suction tip and the tank. My desoldering station that I have, has everything, but I think that it could benefit from stronger vacuum. So I will try connecting my tank to the desoldering tool.

That's a good point also. I will give this a thought.

i

How about this? Drill holes in the sides of the tank, and slip steel rods through the holes, so each one follows a diameter. Weld them in. Now the tank is braced from the inside, so it can't collapse.

For future safety, label the tank as "Not a pressure vessel."