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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