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> Supercritical Co2 Chamber., with sight glass.
johansen
Posted: May 04, 2012 01:30 am
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Here's the numbers: The bolts are on a 5.5 inch diameter circle, and assuming the oring seal has a 5 inch outside diameter* then that makes 2400 pounds per bolt for a hydro test of 3000 psi, which is about 3 times what it would normally be run at.
The bolts theoretically snap at 4700 pounds.

*the actual Oring diameter will be as small as i can fit, probably 4.5 inch outside diameter and 4 inch inside diameter, this reduces the bolt stress down to 1700 pounds per bolt at 3 Kpsi.

I will not be machining the hole for the sight glass until the deflection is measured under 3000 psi. if there's too much deflection then there won't be a sight glass installed, or perhaps i might try and find a smaller magnifying glass lens to use as a sight glass. (the glass i'm intending to use came out of a projection tv, and it really is as thick as shown in the diagram.) For the 1.6 inch diameter hole shown the pressure exerted on the glass by the steel is 6000 pounds at the 3 K psi hydro test.
this doesn't seem to me to be excessive.

Btw:
I came across a 5 inch diameter pyrex viewport on ebay (.75 inches thick) and it was rated to 300 psi (Turns out the company derates them 8 fold, and in fact individually tests them to 8 times the rating.) So compared to known pyrex viewports, i'm running the glass at around 1/10th the stress approximately, assuming equal conditions. i would like to think there's less stress in the glass lens i have than there would be for a flat disk.

user posted image
user posted image

So anyways i guess my question is should i go ahead and drill and tap the holes for 24, 1/4th inch diameter bolts or use 12 x 3/8th inch diameter bolts?

i'm thinking if it turns out that equalizing the stress between 24 bolts turns out to be too difficult then i could just replace them with 12, 3/8th inch bolts using the existing holes, leaving the other holes empty. (i don't think they will create any excessive stress risers)

my plan right now is to drill the 1/4 inch hole in the plate, then drill the .201 inch diameter hole in the chamber and thread that for 1/4-20.
if i need to upgrade to 3/8th diameter bolts then i can just redrill the holes out and start over.

I would rather not go for a 1/2 inch diameter bolt, they will fit but the preload stress becomes so much more critical because the bolt is so short. if i were to drill the hole through completely and use nuts instead of threading the chamber that would probably be the best thing to do?

btw, it doesn't look like it but when the 1/4th inch bolts are counter sunk .050" and the plate is machined flat by taking off another .05 inches there is sufficient thread engagement.


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CWB
Posted: May 04, 2012 02:08 am
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through bolting will work as long as the tensile strength the nuts and length of the threads meets or exceeds the "strip out" of the tapped steel collar .

for *some* reason i am thinking that the the lower number of larger bolts would work better that the larger number of small bolts .
in the back of my mind , there is something about the amount of material removed that will weaken the cap/collar/flange face . .



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Nothing40
Posted: May 04, 2012 02:38 am
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If you haven't seen this yet,This guy's done some stuff with supercritical CO2.. (and lots of other crazy stuff)
https://www.youtube.com/watch?v=-gCTKteN5Y4


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AwesomeMatt
Posted: May 04, 2012 03:19 am
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I never know what all your high pressure projects are for.

Same thing as the last two? Did those get scrapped or is this entirely different?
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MacFromOK
Posted: May 04, 2012 04:15 am
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QUOTE (CWB @ May 03, 2012 08:08 pm)
for *some* reason i am thinking that the the lower number of larger bolts would work better that the larger number of small bolts .
in the back of my mind , there is something about the amount of material removed that will weaken the cap/collar/flange face . .

A few large bolts require more material removal than several small ones. For example, one 1/2" bolt requires a hole area that would equal several 1/4" bolt holes. I'll let some of you math geniuses (genuii?) do the calculations on that though... tongue.gif

I'd also use bolts and nuts (whenever possible) rather than threading the flange/plate/whatever, as you can always use a higher bolt/nut grade if/as required, plus it eliminates the headaches of stripped threads (just replace the offending bolt and nut).

My two cents. beer.gif


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johansen
Posted: May 04, 2012 05:37 am
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QUOTE (Nothing40 @ May 03, 2012 09:38 pm)
If you haven't seen this yet,This guy's done some stuff with supercritical CO2.. (and lots of other crazy stuff)
https://www.youtube.com/watch?v=-gCTKteN5Y4

i won't mention him by name because my motivation is to one up him. (and he could run the test the moment he sees this thread, should i mention it.)

i should have this bolted together tonight or tomorrow so within a couple days i should have results assuming the bolts hold.

i'm going to go ahead and drill the bolt holes, then upgrade to 12 x 3/8th inch diameter bolts (which are stronger than 24x 1/4") if the bolts start acting weird and don't take the stress evenly.

now to figure out how to generate 3 kpsi.. i'm thinking i can just sacrifice a 20$ walmart bottle jack, or one alternative is using my grease gun, (but there's no one way valve if i don't use a zerk fitting).. hmm.

i wonder if i can just find a steel rod and a sleeve, thread the sleeve with 1/4 npt pipe fittings and push the steel rod into the water via a known pressure.


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johansen
Posted: May 04, 2012 09:11 am
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update: grease gun works for up to 1300 psi, the limit of my 0-1000 psi gauge.

i have to rework my 3000 psi gauge because i accidentally bought a left handed threaded model doh.gif So i've got to mill the threads down and then cut threads for 1/8 npt.


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MacFromOK
Posted: May 04, 2012 09:15 am
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Be careful about getting oil/grease and air up to 3k psi. Diesels fire on less. beer.gif


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johansen
Posted: May 04, 2012 02:39 pm
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so far the grease gun doesn't have a problem pushing 3.5Kpsi.
i ground the threads down on the left handed oxygen gauge by hand and then threaded it for 1/8 npt.
user posted image
user posted image

so far the flange is very stiff. tightening every bolt to a known torque, then tightening all but one to a higher torque, produces no measurable change in the torque required to tighten the last bolt.

anyhow, i went with a 6 inch diameter for the 24 bolts. this will leave room for 12x 3/8 or even 12 x 1/2 if that is warranted.


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AwesomeMatt
Posted: May 04, 2012 06:10 pm
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Looks crazy and fun. Can't wait to see where this is headed.
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johansen
Posted: May 04, 2012 10:19 pm
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update: held 1000 psi before the gasket blew out.

the gasket was a 5 inch diameter strip of viton, about 1/4th inch wide.
so it was not unexpected..

what was unexpected was it only took about 1cc of grease to pressurize it to 1000 psi, and that includes the seal expanding before it let go.


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tekwiz
Posted: May 04, 2012 11:14 pm
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The way you have your gasket setup arranged, you'd need a sheet gasket covering the entire surface, made of lead or tin. True O-ring seals are always arranged so that the O ring is in a groove, & is placed under compression. The mating parts always fit very closely, without more than a couple of thou gap. Reaon being, at those pressures, rubber becomes basically a fluid & will leak through the smallest of gaps. To do direct sealing of flanges like yours requires a gasket material with a compressive strength greater than your maximum pressure... 3000PSI. For this purpose, compressive & tensile strengths can be considered the same for most materials.
With that turntable, & a tiny carbide endmill, you shouldn't have any problems machining an O-ring groove. It doesn't even have to have a flat bottom, so one of those end mill style surplus commercial circuit board drills would do as a cutter. I've used those things to cut grooves in tool steel & they work great. The groove width should be the same as the O ring thickness, with the total volume equal to 90-95% of that of the O ring.

Your engineering at the beginning of the first post looks perfectly adequate to me. Plenty of safety factor.
Just bear in mind that ordinary hardware store nuts are shit, not equal to any more than half the diameter of good thread in steel. You're better off with tapped holes, & they reach maximum strength at 1.5 diameters in length, in mild steel. If you must use nuts, then use proper clamping nuts. They are engineered to match the strength of Gr 5 fasteners.


BTW:For the peephole, have a look at some of the deep sea submersibles & see how they do things. They have to deal with similar pressure levels. I haven't looked closely, but they seem to rely on bolted, seriously thick plexiglass, & O ring seals, with simple bolted flanges like what you have.
I would recommend against a magnifying lens, unless what you want to see is all along a single plane. Anywhere else in the chamber would be out of focus. The entry of light into the chamber will be similarly affected. Plastic is also better than glass in that it tends to fail gradually, instead of suddenly, like glass. Especially if you use polycarbonate.


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johansen
Posted: May 04, 2012 11:51 pm
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good point about the lens.
when its filled with water or some other liquid the magnification is reduced sufficient that you can see inside.
I do have a chunk of 1 inch thick plate glass i could use, but that would require cutting it..

i should be able to get the mating surface flush to within .002, in fact i think it already is, if you can see in the first photo, the inside edge is shiny, i polished it with flat with a sheet of sandpaper on top of glass.

I am using tapped holes btw, there's about 7 threads worth of engagement on all the bolts.

oh, and i did forget to mention, i cut the hole in that steel plate with a 1/8 inch carbide endmill. using just the mill, not the rotary table. it took a long time because it was so deep and getting chips out was a challenge, i only took off .01 inches per cut.

right now the biggest problem with the mill is the spindle is not solidly fixed to the mounting plate, in fact by design it can't be.. but that's for another thread.


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tekwiz
Posted: May 05, 2012 12:14 am
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That method of lapping works very well for flatness, as long as your glass is sitting on a flat surface strong enough not to deflect under the weight of the workpiece. I like to turn my workpieces 90 periodically when lapping, in an attempt to keep things as even as possible. If you use wet/dry paper & keep it wet, it cuts at least 10X faster. I do all my lapping wet now.
Those lapped surfaces are perfect for both O ring grooves, or sheet gasket. The only problem with sheet gasket is that the necessary sealing pressure detracts from your strength. Sealing pressure being that amount necessary to make the gasket conform to surface irregularities well enough to make a good seal. The amount is quite high for most sheet gasket materials, because of the large surface area that must be compressed. This is not the case with O-rings in grooves, because there is no clamping pressure beyond the little bit to deform & compress the O ring itself into it's groove. Once the plates are in contact, no further pressure is required, & this means bolts need only be tightened enough to ensure contact & to keep from coming loose.


BTW: There's a trick you can use to drastically lower the amount of sealing pressure for sheet gasket flange setups. What you do is weld a circular bead around the flange & machine the top flat, sort of the opposite of an O ring groove. This reduces the area that must be compressed & lowers the overall force required. The narrower the bead, the better. The bead need not be any more than .025-.030" high for most gasket materials, including tin or lead.


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For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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CWB
Posted: May 05, 2012 01:02 am
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"I like to turn my workpieces 90 periodically when lapping ..." .
blink.gif
oh , never mind .
laugh.gif



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AwesomeMatt
Posted: May 05, 2012 01:04 am
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Wooh, when'd you get a mill?
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tekwiz
Posted: May 05, 2012 05:56 pm
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Just a FYI. Your average grease gun will develope close to 10,000PSI, so you should have no problem pressure testing your project. Suitable guages are also available & you should be able to score one for less than $20.
A grease gun has been my favored source of pressure for hydrostatic testing for decades now. wink.gif



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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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CWB
Posted: May 06, 2012 05:14 am
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this reminded me of a technique for tenderizing meat .
the meat is placed in a vacuum packed hermetically sealed plastic bag and then subjected to a high hydrostatic pressure .
i read about this many years ago and then it just dropped off the radar .


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johansen
Posted: May 07, 2012 11:44 pm
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well i cut a groove for a soft rubber washer, and it failed at 1500 psi or so.. turns out it was extruded through what now appears to be a .010" thick gap between the lid and the pot.
i'll need to find my feeler gauges to confirm for sure.
This morning I ordered an assortment of 4 inch diameter O rings.

according to this: http://www.scribd.com/doc/18244344/ORing-Seal-Application I need to get the gap down to .002 or better.


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AwesomeMatt
Posted: May 07, 2012 11:51 pm
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Why not go with the tin/lead gasket idea over rubber?
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johansen
Posted: May 08, 2012 12:04 am
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Because it needs to be food safe.


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CWB
Posted: May 08, 2012 12:34 am
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isn't tin "food safe" ?


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johansen
Posted: May 08, 2012 02:16 am
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yeah tin is food safe..
I suppose i could cast a tin washer in plaster.


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AwesomeMatt
Posted: May 08, 2012 02:35 am
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QUOTE
I suppose i could cast a tin washer in plaster.


Or a cast iron frying pan. Or a caseroll dish. Or a cookie sheet.

445'F.

Get it flat, cut it with sheers, let the squishiness pressure take care of the rest.
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johansen
Posted: May 08, 2012 04:47 am
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uh.. Houston we have a problem.


user posted image

@2200 psi.

it used to be flat..

i used a 3/16th solid lead solder ring as a seal. i can get above 2200 psi for a couple seconds but the increased deflection makes the seal leak.


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