Help! Worried about Dad's homemade pressure chamber

Two points:

1. Considering the heft of the steel, Failure would probably either be at the lid seal, or at one of the fittings attachments.

1. This is compressed air, not STEAM!

I doubt you will get shrapnel. On the other hand, if it does blow, the effect on your Dad's disposition may be more dangerous to others than to him. :-)

Bill Shuey

I would think leaving the emergency relief valve would be a good idea. You should probably contact Presto or whoever made your pot and ask them. Campbell-Hausfield makes pressure pots for painting that are more suitable for this application. hth

The Keeper (of too much crap!)

You have every reason to be concerned.

The amount of energy that can be stored in a compressed air tank can be enormous. Exploding tanks can be very dangerous. Big ones can level houses. I doubt your dad's rig is in that class, but having that lid blow off and fly across the room could do some serious damage to bystanders.

It's impossible to give you an accurate answer about a "safe" pressure limit. There are too many variables. The quality of the steel, the presence of corrosion and cracks, the geometry of the latches, etc. A pressuse cooker just isn't designed for that kind of service. I'd say that the 20 psi you saw on the original gage was a good indicator.

However, I CAN recommend a safer way to test it. Fill it with as much WATER as you can. This is how SCUBA tanks and industrial air compressor tanks are tested and certified. Since water is incompressible, it doesn't store energy as you pressurize the chamber. If something does blow, you get wet, not dead.

If you sucessfully test it to 100 psi, it's probably safe to operate routinely at 50.

I would also recommend that you build a blast shield for the thing. Not a closed box, but more like a 2 x 4 cage with narrowly spaced bars. Or put it under the workbench and behind something heavy when you use it. For God's sake don't put it on top of the workbench and stare at it while you pump it up. It really is a bomb and should be treated with all possible caution.

Greg Reynolds mechanical eng>I need some advice about the pressure limits of my Dad's homemade

Run fast, run far....that simple enough?

I would think he has his logic reversed. At work I use a lot of different potting compounds and epoxy resin two part mixtures and any of them to remove the bubbles that form within the mixture during the mixing process we put them in a vacuum chamber. By lowering the pressure it causes the trapped gas to boil out simular to what a diver goes through when they have the bends.

Well, wouldn't it be better to make a vacuum chamber? To lower pressure in the chamber, and that would suck out bubbles from resin...

Ciao di

Tomislav Martan 9A4Tc

True.

Also true, but... do you mean that it doesn't take as strong a vessel to hold compressed air as it does to hold steam under pressure... or the other way around?

That we can deal with. I just don't want him getting fragged - or one of my kids getting fragged, when they go to see Grandpa's new gadget!

Thanks, Bill.

Dave

Yeah, me, too. I'm not sure if he can restore it now that it has been removed, but maybe he can drill a new hold and add a new emergency valve of some kind.

I doubt that I could get a helpful answer. Either I'd get "Sure, no problem" from somebody with no more knowledge than I have myself, or else if I got somebody knowledgeable they'd be reluctant to stick their neck out to approve of anything beyond the purpose for which it was sold.

His book on resin casting advised something like that, but we couldn't find one at Lowe's and he didn't want to spend a lot of money on it, so decided to make his own.

Thanks, Keeper.

Dave

I will pass this along to him, in hopes that he will take it a little more seriously than he does my concerns on the subject. As for the lid, it is fastened with some strong-looking flanges, where you set the lid into place and then rotate it about 3" clockwise to lock it in place. I'd expect one of his fittings to fail before the lid would come off.

Well, it is brand new and shiny, no corrosion or cracks, but I'm concerned about the quality of the steel - as noted, it just wasn';t designed for this.

I hope I can talk him into this. Last night he was talking about running it up to 150 psi air pressure, and if it didn't blow he would consider it safe. I asked him not to do that, and I didn't hear any sirens during the night (his house is not too far from mine), so I hope all is OK. I like the idea of a water test to 100 psi much better!

I'll make these suggestions to him.

Three guesses where it sits right now.

Thanks, Greg.

Dave

I think he tried something like that first. He already has a vacuum chamber, also homemade, but it seems like there was some problem of the resin setting too fast, while it was still all foamy. He showed me one attempted casting that looked like a Coke had been poured too fast and foamed over the cup, except that the "foam" was hard resin.

He was talking about giving the resin a brief shot in the vacuum chamber, either before of after pouring into the mold, then putting the resin-filled mold under pressure to force any remaining gasses into solution. I think this was also in his "how-to" book, although I haven't read that part of it myself.

Dave

Yah, but see my reply to Spaceman.

Dave

Actually, the better resin casters use both. A vacuum to initially "suck out" any air bubbles...followed by an *increase* pressure to force the resin back into any spaces that were occupied by air bubbles.

He's got it correct for polyurethane resin. 1)it has a lower vapor pressure than epoxies and potting compounds and vacuum makes the bubbling worse. 2)it sets too fast for vacuum so rather than remove the bubbles, you pressurize it at 65psi and collapse them.

Getting lots of foam sounds like he's shaking the two parts of resin, rather than stirring. Or, perhaps he's being too vigorous when stirring.

The more careful you are during stirring and mixing, the less bubbles you get.

Another possibility would be to use a resin with a longer "pot" time. This would allow him to thoroughly mix without shaking. Which should produce very little bubbles. Resin mixes with a longer pot time generally have a longer set time too. Allowing you time to remove the bubbles after the pour but before the mix hardens.

No, it's a vapor pressure thing when placed in too high a vacuum.

Neither. What he meant is that steam is very hot, enough to cause serious burns. Ever hold your hand over the spout of a boiling tea kettle? The steam in a pressure cooker is even hotter.

I think William is too optimistic. While the body and lid of the pot are probably strong enough that they won't break up into shrapnel, a small part such as a latch, rivet, or fitting could break loose and be thrown across the room. And it doesn't have to fly very fast or far to damage an eye.

Your father's proposed 150 psi test won't prove anything. In fact, I wouldn't even trust it at 20 psi after that. The pressure might not break anything the first time, but it could leave microscopic stress fractures, weakening the metal. Every time he uses it beyond its safe limit, it'll get even weaker, lowering its safe limit, until one day it breaks.

Steam SCALDS! I have been unfortunate enough to see the results of steam explosions, it ain't pretty. Compressed air is simply kinetic energy, no heat! As I said, the biggest problem I can see is either the fittings blowing off where they are threaded into the metal vessel or the seal around the lid or lid latches failing. If you are standing close bye when this happens, better duck fast. Bill Shuey

At those pressures, I repeat, the thing will eventually fail at the flanges. Maybe not today or tomorrow but it will. I personally know of someone who has had that experience and lived to tell about it. He's bought a paint pressure pot. I'm trying to get in contact with him to see if he'll releate his 'holy s***' experience for you.

In any event keep your self and your loved ones out of the casting shop when pressure is being applied to that rig.

I had made one very similar to your dad's. After my friends experience I now only use it as a vaccuum chamber.

Cheers and good luck - Jim.

It doesn't sound like the safest of set-ups. Is his insirance paid up?

I don't know about the steel pot but a hose that comes loose still charged with considerable pressure can whip around with considerable force for a couple seconds until the pressure bleeds (pun indended) off.

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