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How to set thickness of the wall with thread? - confetti launcher Answered

We are working with my kids on CAD project - precisely on a simple pressure valve for DIY confetti launcher. Yesterday we decided to design a thread, which assembles nipple of the valve to the wall . We prefer 1.6 mm wall (0.064 inch) because that saves volume and is enough for 1450psi air in case of stainless stell grade 310 (my son found a piece of SS at home, so we decided to use it).

However, I'm afraid that in case of turning a thread there would be a need of redesigning wall thickness. 

Question: How much thicker should be a wall for 1450psi (working pressure, pumped air or co2) if we add a thread reinforced with green Loctite (hard glue for threads)? What is optimal metric thread for that pressure? 

I would be very grateful for standard, formula or table.

7 Replies

bwrussell (author)2012-08-13

Any sort of modifications to the sheet lower it's strength. The hole and threads are a weak point and will blow out well before you reach your calculated max load. Also Steve brings up a very important point about having a safety factor. Personally I would have a safety factor of at least 2 since there are children involved, meaning your max operational pressure should be 500 psi. This is still several hundred psi more than you need.

The important variable in for pushing the confetti further is volume, not pressure. Confetti is loose, and has low mass and high drag. This means it decelerates very quickly once the force of the air blast has subsided. This means you will get very similar results with 1500 psi and 200 psi at the same volume. If you set the pressure lower(100-200 psi) but increase the volume the blast of air will be much longer which means the force will continue to act on the confetti particles even after they have left the barrel.

Just to be clear, no one here recommends or condones using such high pressure. You are putting the lives of those children and your self in very grave danger for no reason. Besides the calculator you used was not for pressure vessels but for heat absorbing tubes in steam generation plants. Those calculations are likely not valid at low temps and with low levels of heat exchange.

As far as threads go, I would make sure to have at least 2 if not three threads fully engaged, more if you insist on such a ludicrous pressure. I would also recommend using a tapered pipe fitting like NPT or BSP which are designed for this sort of thing.

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mdt2170m (author)bwrussell2012-08-14

We are designing launcher for pressure of 1500 psi because co2 in room temperature reaches 1200 psi and can be used without regulator. I have seen DIY dust generator using liquid co2 without reg. and tried to import this idea to our project, but after your comment we will consider using regulator for long-term money savings.

I have calculated tube formula because it is very similar to Pressure Vessel formula from Pressure Vessel Handbook by Eugene F. Megyesy. Adding water corossion allowance of 1/6 just "eats" that small difference.

Calculator is also useful for determining NPT pipes thickness (code PG 27.2.2) - which is larger than tube thickness for the same pressure. However, we decided to use oring on opening cover rather than NPT thread.

But after your suggestion of using NPT in case of wall presented on the picture we consider turning it. Previously mentioned calculator give us formula we need. This solution won't be as cheap as oring / metric thread in our coutry, but we already have PG 27.2.2 formula from calculator. Thank you for advice.

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mpilchfamily (author)2012-08-11

You want to launch confetti not dust. That much pressure in such a small space will destroy your payload of confetti. Unless you looking to put sheets of paper in it and have it launched as confetti.

Have you ever heard of Pumpkin Chunkin? Its a competition where competitors build machines to try and launch a pumpkin as far as they can. The Air Cannon division only uses a few hundred PSI to get those pumpkins to go just short of a mile. If they dial it up too high the pumpkin goes splat in the barrel. So you shouldn't need 1400 PSI to launch a bit of paper out of a small barrel.

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mdt2170m (author)mpilchfamily2012-08-11

We will use round piece of cardboard and large diameter SS pipe to prevent damage of confetti.

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steveastrouk (author)2012-08-11

Any device, filled with 1400 PSI gas becomes a bomb, and requires serious, professional, specialist design to be considered safe. An item built for that pressure REQUIRES hydraulic pressure testing before use, and periodic testing afterwards.

The damage potential rises with the square of the pressure and is proportional to the volume.

If you seek out a member here "Maewarts", who describes in vivid detail being very badly injured by only a 100PSI cannon that explodes - leaving him with a damaged eye socket, and no sensation in part of his jaw, you will see what I mean.

There are videos of tanks exploding on youTube

Please reconsider attempting any high pressure design like this.

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mdt2170m (author)steveastrouk2012-08-11

Of course we well perform stress test using remote control. Wall thickness has been calculated using HRSG design pressure calculator and then I made "manual" calculation on the table to be sure.


Howewer, I'm still searching for more info about pressure design for safety reasons.

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steveastrouk (author)mdt2170m2012-08-11

You have to over-pressure by 100% whatever you make, as a safety margin for proofing. That put the internal pressure close to 3000 PSI - beyond the pressure in an industrial gas bottle.

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