Re: [PSUBS-MAILIST] Calculations

["Chris Jackson" <trumpetrhapsody@comcast.net>]

Yes I very much see your point now Michael. So in this case how do I find
out what the external pressure rating of a 100psi rated, 150psi rated, and
200psi rated cylinder without taking it down till it pops?


----- Original Message ----- 
From: "Michael Wright" <mwright@smallip.com>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, July 08, 2003 9:55 PM
Subject: Re: [PSUBS-MAILIST] Calculations


> Steve Wrote
>  > ... and the pressure on the cylinder at full vacuum is also
>  > 210psi, that would equate to roughly 15ATMs in each example.  would it
>
> I can see your confusion.  If you take all the air out of a tank (full
> vaccume) there is a pressure imbalance of 1 atmosphere (14ish psi)
> between the inside and outside.  This is because earth's atmosphere at
> sea level is roughly 14 PSI (defined as 1 atmosphere), this is the
> external absolute pressure.  The interior absolute pressure is zero (or
> near enough) so at full vaccume the tank is holding out 1 atmosphere
> (not 15) since the ambient air pressure is more or less 1 atmosphere :)
>
>
>
> steve wrote:
>
> > 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
> >>
> >>
>
>