[Date Prev][Date Next]
[Chronological]
[Thread]
[Top]
Re: [PSUBS-MAILIST] Calculations
Ah yes I wish I could figure out how to use that spreadsheet, but yea i plan
on useing ribs about every 10 inches or so.
----- Original Message -----
From: "Ray Keefer" <Ray.Keefer@Sun.COM>
To: <personal_submersibles@psubs.org>
Sent: Wednesday, July 09, 2003 3:46 PM
Subject: Re: [PSUBS-MAILIST] Calculations
> Hi Chris,
>
> There are only two ways. Test it till it pops, then build a few more to
> make sure the first one wasn't unusually strong.
>
> Or calculate. Try the tool at
> http://www.psubs.org/designguide/designguide.html. Click on the link
> Hull Stress Spread Sheet 2 (Download Excel Spreadsheet). You will
> have to guess at the properties of the metal. Things like:
>
> Young's Modulus
> Poisson's Ratio
> Yield Stress
> Weight per in3 (lbs)
>
> That is one problem about using a propane tank. You really don't
> know what material you have.
>
> You will find that without adding ribs your hull isn't going very
> deep.
>
> The calculations are for crush depth.
> Make sure you derate the theoritical numbers you get from the
> calculations for added safety. For instance if you get 100 feet.
> Don't operate in water over 50 feet or some such. Depends on how
> safe you want to be.
>
> Regards,
> Ray
>
>
> > From: "Chris Jackson" <trumpetrhapsody@comcast.net>
> > To: <personal_submersibles@psubs.org>
> > Subject: Re: [PSUBS-MAILIST] Calculations
> > Date: Wed, 9 Jul 2003 13:23:16 -0500
> > Content-Transfer-Encoding: 7bit
> > X-Priority: 3
> > X-MSMail-Priority: Normal
> > X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1165
> >
> > 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
> > > >>
> > > >>
> > >
> > >
> >
>