Help! Worried about Dad's homemade pressure chamber

I need some advice about the pressure limits of my Dad's homemade
pressure chamber, and the possible severity of a failure of it.
Dad's hobby is model aircraft, and earlier this year he decided to try
casting some of his own details in some kind of two-part resin (I
don't know the exact chemicals, but you mix two liquids together and
then it hardens within one or two minutes - exothermic reaction, it
gets hot as it hardens). He was having problems with bubbles in the
resin and decided that he needed a pressure chamber. Being the handy
guy that he is, of course he decided to build his own.
I was over at his house today and we went down to the basement so he
could show me his newly completed pressure chamber. It is made out of
a store-bought pressure cooker (the kind for canning fruits and
vegetables and so forth), and he had added a new pressure guage and
some valves and fittings to hook it up to his big air compressor, and
had plugged and sealed both the steam escape valve (for maintaining
constant pressure on a stovetop) and the pressure relief valve (a
rubber plug that would have popped out at too-high pressures).
As he started it up and the pressure was climbing past 50 psi, I
casually asked him what the pressure rating of the cook pot was - and
HE DIDN'T KNOW. He said that it was good thick steel and that he had
pressurized it to 75 last night and left it all night to see if it
would leak and lose pressure. I found the box that it had come in, and
the old pressure guage that he had removed and replaced - the guage
was marked only up to 20 psi, with a CAUTION area above that. I got
real nervous then and started asking more questions about how he knew
that this pot would be safe at pressures above the 20 psi it was made
for, and he got kinda ticked off at me for questioning his new
creation, but the bottom line is that he had no reference or factual
basis at all for the assumption that it would be safe, other than it
looked to him like good thick steel and it had already taken 75 psi
without exploding.
So now I ask for someone knowledgeable on the subject of pressure
chambers - Is this thing safe? How can I tell? I examined the pot
after he had shut it down, and the steel appears to me about 3/16"
thick - more than 1/8", less than 1/4". That is thicker than the steel
in an old air compressor tank that I examined later - that was only a
bit less than 1/8" thick, maybe about 3/32", and that tank was rated
to 150 psi - but maybe they are different kinds of steel? How can I
tell? It just worries me that it has no pressure rating on it, and the
original guage only went up to 20 psi, and he is planning to use it at
50-70 psi. And if this thing does fail, is it just going to blowout a
hole in the side (there are no seams or welds), or is there likely to
be shrapnel?
Please advise, as I really don't want him to kill himself in pursuit
of his hobby!
Reply to
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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.
2. 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
Reply to
William H. 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!)
Reply to
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 engineer
Reply to
Greg Reynolds
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.
Reply to
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
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(On-line at last...)
Reply to
Duke Nukem
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.
Reply to
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.
Reply to
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.
Reply to
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.
Reply to
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.
Reply to
Greg Heilers
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.
Reply to
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.
Reply to
John McGrail
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.
Reply to
Wayne C. Morris
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
Reply to
William H. Shuey
******* Convince him to buy, or buy him a paint pressure pot for Christmas. The 'pressure cooker' WILL eventually let go at the flanges and the cover will become a uncontrolled flying object. If luck is with you it will go straight up........ That's ok if you're not standing with your head over it. Also remember that even if it goes up without doing any damage, what goes up must come down.
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.
Reply to
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.
-- -- -- -- -- "We sleep safe in our beds because rough men stand ready in the night to visit violence on those who would do us harm." George Orwell
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Bill Woodier

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