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Re: [PSUBS-MAILIST] Calculations
Hi Michael
I read your explanation and i agree with most of it but i find it still does
not account for the Full Vacuum scenario.
No offence meant, but your arguement seems to revolve around the same old
'pop bottle experiment' but doesn't address the main question about the
vacuum.
eg, if the pop bottle were rated to withstand a full vacuum, then we could
expect to suck all the air out of the bottle and expect it to retain its
cylindrical shape rather than look like a flat plastic pancake.
My research has shown that pressure cylinders designed to accept full vacuum
pressure generally have some sort of internal bulkhead / stiffener built
into the design to prevent the collapse of the shape.
Lets say, for arguement purposes, the pressure on the cylinder at full
pressure is 210psi and the pressure on the cylinder at full vacuum is also
210psi, that would equate to roughly 15ATMs in each example. would it not
be reasonable to expect the cylinder to withstand 15ATMs external pressure
if the internal pressure is only 1ATM? If so would it then be unreasonable
to assume that if the hull is capable of 15 ATMs / 140meters pressure then
it could be (tentatively) assumed that a design depth of 30 meters would be
OK
Although these bulkheads etc.would undoubtedly be unsuitable for use in the
design of the sub, it still leaves the original (re-worded) question:
Assuming a pressure cylinder is rated for full vacuum (and has an internal
structure to prevent the deformation at said pressures), can the vacuum
pressures be compared to the equivalent external pressures?
Regards
Steve Bosworth
UK
p.s. i am not using this as a basis for building my own psub, so it's not
like i'm going to rely on this data but i'm trying to get my head around the
physics side of things
----- Original Message -----
From: "Michael Wright" <mwright@smallip.com>
To: <personal_submersibles@psubs.org>
Sent: Wednesday, July 09, 2003 1:34 AM
Subject: Re: [PSUBS-MAILIST] Calculations
> First, since air at mean sea level is roughly 14 PSI, a tank at full
> vaccume is the same as a tank with 1 atmosphere inside at 28 PSI outside
> (or 32 feet of water).
>
> Steel is about the same stregnth and modulus of elasticity in
> compression as in tension.
>
> Here's the thought model I use to envision the difference between
> internal and external pressure. Take an empty one liter plastic pepsi
> bottle. It can hold upwards of 55 PSI internal pressure. However you
> can collapse it with your lungs by suckin on it (far less than 2 PSI).
>
> This should easily illustrate that a tank rated for X psi internal
> pressure is not likely safe to use for X psi external pressure. A tank
> designed for internal pressure needs no structure to keep it' shape.
>
> The physics of the situation are relatively straightforward. If a tank
> deforms due to internal pressure it becomes a shape that is loaded
> entirely in tension. The force from the internal pressure keeps the
> round shape.
>
> If a tank deforms due to external pressure it gets less rather than more
> round, becoming an ellipse. As this deformation occurs the tank
> structure is loaded more and more in bending rather than compression.
> This leads quickly to a posetive feedback loop that leaves the tank
> looking like the soda bottle with the air sucked out.
>
> The posetive feedback nature of this process is what should scare anyone
> thinking of building an atmospheric submarine. If you exceed the
> capability of your internal structure to maintain the round shape, the
> hull will begin to oval, as it ovals it will be loaded in bending rather
> than compression, and quite rapidly the hull will collapse on it's self.
>
> So if you wanted to use a tank you'd have to build in all the structure
> to keep it from ovaling under external pressure. In doing so you'd have
> to make sure not to create stress concentrations in the skin.
>
> It seems to me that you'd rapidly spend more effort and more mass
> stiffening the inside of the hull than would be required to build
> Thijs's double end cap flyin saucer shaped craft (a shape that looks
> quite useful to provide space for a crew of two with a minimum of excess
> air space).
>
> I supose if one had access to a plasma or lazer cutter you could build a
> bolt together, laminated steel internal structure for the tube section
> that could be installed via the main hatch opening. All this would
> require 3d solid modeling to get the geometry and placement right but
> would be doable. The modeling will probably show that the end caps
> being spherical makes them less susceptable to the posetive feedback
> loop that would affect the tube section.
>
>
> At any rate, assuming that because a tank is rated to a certain internal
> pressure it will handle that external pressure is incorrect and likely a
> fatal mistake.
>
>
> michael
>
>
>
> Chris Jackson wrote:
>
> > Very good point. Logically, if it thick enough steel to be
> > able withstand internal pressure, it should mean that it is thick enough
> > to withstand an equal amount of external pressure.
> >
> > ----- Original Message -----
> > *From:* steve <mailto:steve@kobol.worldonline.co.uk>
> > *To:* personal_submersibles@psubs.org
> > <mailto:personal_submersibles@psubs.org>
> > *Sent:* Tuesday, July 08, 2003 4:52 PM
> > *Subject:* Re: [PSUBS-MAILIST] Calculations
> >
> > Hi All
> >
> > i noticed a lot of talk recently about propane tanks, the thread was
> > a bit dead but i gathered that the question of 'Is it worth using
> > that old propane tank in the garden', was dumped because the
> > work would probably be more involved to convert a tank than to just
> > buy the tube and end caps.
> >
> > My question though is, how does it compare with cost? If the cost
> > of the tank is say 1/2 the price would it be a viable option?
> >
> > I had the opportunity to check a propane tank up close recently; i
> > keep seeing psubs in all types of pressure tanks now...i think it's
> > called obsessive compulsive disorder.
> > Anyway, thinking as you do, that it would be a great size for a psub
> > i checked out the plate attached to one end.
> >
> > The plate read:
> > /*Manf: Robert Bros*/
> > */BS1500 Class 2/*
> > */210psi and Full Vacuum/*
> > */TP 310psi/*
> > */310 Gals/*
> >
> > A thought came to me, i remember on previous posts, a lot of talk
> > about the pressure vessel only being rated for internal pressure.
> > If it states that it can also withstand a FULL VACUUM, how does that
> > affect its suitability as a psub hull?
> >
> > I may be completely wrong here but, can vacuum stresses be compared
> > with external pressure stresses?
> > It seems to me that if the internal pressure was reduced to 1/2 ATM
> > then wouldn't that be the same as applying 2 ATM pressure to the
> > outside of a hull?
> >
> > i understand that once the pressure vessel used for a purpose it is
> > not intended for (ie. cut / welded etc.) the figures wouldn't be
> > valid anyway, but it makes me think
> >
> > anybody got any ideas???
> >
> > Regards
> >
> > Steve Bosworh
> > UK
> >
> >
> > P.S. I did check out the manufacturer but they apparently don't
> > exist anymore and the BS number is now obsolete and doesn't say
much.
> >
> > ----- Original Message -----
> >
> > *From:* Chris Jackson <mailto:trumpetrhapsody@comcast.net>
> > *To:* personal_submersibles@psubs.org
> > <mailto:personal_submersibles@psubs.org>
> > *Sent:* Tuesday, July 08, 2003 9:10 PM
> > *Subject:* Re: [PSUBS-MAILIST] Calculations
> >
> > After futher research and sudgestions, I have found that at 100
> > meters the water exerts a pressure of about 162psi, but what I
> > am not sure of is whether an air tank (decided on an air
> > tank instead of a propane tank due to price and comments I have
> > read about propane tanks) with a 165psi rating would hold up
> > to that kind of EXTERNAL force, since I assume the rating aplies
> > to the INTNERAL force rating.
> >
> > ----- Original Message -----
> > *From:* Chris Jackson <mailto:trumpetrhapsody@comcast.net>
> > *To:* personal_submersibles@psubs.org
> > <mailto:personal_submersibles@psubs.org>
> > *Sent:* Tuesday, July 08, 2003 1:12 PM
> > *Subject:* Re: [PSUBS-MAILIST] Calculations
> >
> > I have recently aquired more information, however I still
> > uncertain of several things. I found this link:
> > http://hotconnect.com/tank/vertair.htm and I am considering
> > the 36" by 98" tank made with carbon steel, but I am unsure
> > which pressure rating to use, the depths I
> > am considering will be in the 50-100 meter range, but
> > possibly shallower since this will mainly be used in
freshwater
> >
> > ----- Original Message -----
> > *From:* Chris Jackson
<mailto:trumpetrhapsody@comcast.net>
> > *To:* personal_submersibles@psubs.org
> > <mailto:personal_submersibles@psubs.org>
> > *Sent:* Tuesday, July 08, 2003 11:12 AM
> > *Subject:* [PSUBS-MAILIST] Calculations
> >
> > I am trying to do some calculations to figure out such
> > things as crush depth and required hull thickness. The
> > constants are that the pressure hull will be 36 inches
> > in internal diameter and 100 inches long, and what I
> > need to know is the relationship of hull thickness
> > versus crush-depth with these parameters. If anyone can
> > provide me results or information of a simple
> > calculation program, I would be much appreciative.
> >
> > Thank you,
> > Chris Jackson
>
>