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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
>>
>>