Pressure question


by 72Zorad
Tags: pressure
72Zorad
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#1
Jul26-10, 11:37 PM
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How much pressure can I safely put in the following container.

Two round flat aluminum discs with an acrylic cylinder.
The discs are 6061 1/8" thick aluminum.
The cylinder is acrylic .25" thick, 1 inch high, 6" OD.
The discs are just larger than the cylinder and have 18 holes just outside the 6" diameter of the cylinder 20 degrees apart. The 18 holes are for 1 1/2" 8-32 bolts/nuts.

I'd like to potentially pressurize this container but need to ensure it is a safe pressure.

Thanks,

Mark
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stewartcs
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#2
Jul27-10, 10:32 AM
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Quote Quote by 72Zorad View Post
How much pressure can I safely put in the following container.

Two round flat aluminum discs with an acrylic cylinder.
The discs are 6061 1/8" thick aluminum.
The cylinder is acrylic .25" thick, 1 inch high, 6" OD.
The discs are just larger than the cylinder and have 18 holes just outside the 6" diameter of the cylinder 20 degrees apart. The 18 holes are for 1 1/2" 8-32 bolts/nuts.

I'd like to potentially pressurize this container but need to ensure it is a safe pressure.

Thanks,

Mark
I'd suggest consulting (i.e. hiring) a Professional Engineer since this is safety related. Alternatively, the OEM should have the answer to your question since they designed and built the pressure vessel.

It would involve quite a bit of work for the volunteers here (presuming you provided all of the design and material specs). Plus we don't generally give this type of device for safety reasons.

CS
72Zorad
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#3
Jul27-10, 10:50 AM
P: 17
I understand and appreciate the response. I built the container. It is for a low temperature Stirling engine. I'm not looking at putting too much pressure into it but considering PSI means every square inch I'm aware that a low pressure could be quite a bit of pressure on the plates. I figured I'd halve whatever number I got just for safety sake.

Thanks again,

Mark

Q_Goest
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#4
Jul27-10, 05:18 PM
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Pressure question


Hi Mark,
Engineers don't like providing the kind of information you're asking for without a full understanding of the design and how it's being used. There are a number of things that would go into the analysis, such as operating temperature, cyclic stresses, how the unit is sealed, actual dimensions of flat heads including bolt holes, material spec on the acrylic, etc... things that you've not provided.

Regardless, doing a quicky analysis on this, it seems the rated pressure is going to be very low, around 10 psi or less depending on many factors you haven't provided. The heads are very thin, and the acrylic is a big unknown. My biggest concern is operating temperature since you've suggested this is for a Stirling engine. If temperature is much above ambient, the acrilic is going to loose strength very quickly. The stress allowables then have to be located for the specific type of acrylic you have. That might not be possible though, I don't know about acrylic.

Have you also considered doing a hydrostatic test on this? That hydrostatic test needs to be adjusted for according to what temperature the container is operating at, so even doing that much isn't something that can be nailed down very easily.
72Zorad
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#5
Jul27-10, 05:32 PM
P: 17
I figured I'd end up here

A whole bunch of questions I don't have the answer to. This is basically a design much like those you see on the internet that you hold on your hand and they run off the heat of your hand. Upper temp would be sunlight on a black piece of painted aluminum, lower temp would be 50 degree water if I'm lucky. I'm only looking to put 1 or maybe 2 atmospheres of pressure in it and just wanted to ensure I'm not building a bomb since PSI can really build up a lot of pressure on larger surface areas.

I understand to do this right engineers would need to have tons of data to assess. What I was looking for was a 'you will be fine as long as you don't go over X'. I have no real idea if 1 atmosphere (somewhere around 417 pounds on the plates if my math is correct) is not an issue or something I should be concerned about. Maybe a couple atmospheres is nothing to be concerned about and maybe it could be a really big deal.

If there isn't a comfort level with lower pressures without a thorough analysis I'll just stick with no pressure.

Thanks,

Mark
Q_Goest
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Jul27-10, 08:25 PM
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The bolts and acrylic aren't the limiting factors as near as I can tell from your description. The problem is with your flat aluminum heads. They limit you to less than 15 psig, more like 10 psig. The bolts are good and assuming the temperature is no more than about 150 F, the acrylic should be fine too. To get to 2 barg, you need heads about 1/4" thick. That and do a hydrostat at around 50 psig and you should be good.
nvn
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Jul27-10, 11:40 PM
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Q_Goest: In case this helps for your estimate, to prevent acrylic plexiglas (also called PMMA) crazing (which turns plexiglas a cloudy white color), it is my understanding that the allowable tensile stress (presumably anywhere in the ballpark of room temperature) should not exceed a sustained value of Sb = 10.4 MPa. (This value already includes a built-in factor of safety.) I am referring to Plexiglas G, Plexiglas GS, and Altuglas CN, not Plexiglas MC, Plexiglas XT, nor Altuglas EX, if I recall correctly.
Q_Goest
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Jul28-10, 03:30 AM
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Thanks nvn. You've confirmed the acrylic isn't the limiting factor.


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