Re: [PSUBS-MAILIST] Pressure Hulls

["Captain Nemo" <vulcania@hawaii.rr.com>]

Alec,

I think Ian may be on vacation.  Thanks for the formulae.  I've got some
ideas about the variance; need to reference a table in a book I took to work
last night.  Will get back to you about this later.

Pat


----- Original Message -----
From: "Alec Smyth" <Asmyth@changepoint.com>
To: <personal_submersibles@psubs.org>
Sent: Wednesday, July 03, 2002 9:58 AM
Subject: RE: [PSUBS-MAILIST] Pressure Hulls


> Ian,
>
> I looked up sphere calculations in Allmendiger and found two similar
> equations. Note that both formulas assume Poisson's ration = 0.3, which I
> interpret to mean only use them for calculations with steel.
>
> a) The first equation is derived theoretically:
>
> p = 1.21*E*(t/R)^2
>
> where:
>
> E = modulus of elasticity in compression
> t = shell thickness
> R = radius to midplane of shell
>
> WARNING: It mentions that in practice, due to variations in sphericity and
> shell thickness spheres tend to collapse at much lower pressures than
those
> calculated... as much as one-fourth the pressure.
>
>
> b) The second equation is empirical:
>
> p = 0.84*E*(t/R0)^2
>
> where R0 = outside radius
>
>
> Now for comparing results:
>
> The two equations are very similar, with the second one being more
> conservative. But the numbers I get are about an order of magnitude above
> what Pat got using your ABS formula. I used
>
> E = 29,000,000 lbs/in2
> t = 1"
> R0 = 54"
> R = 53.5"
>
> Which gives a result of 12,259 psi for (a) and 8,353 psi for (b), while
Pat
> got a more reasonable 1,111 psi. I think my results are obviously
incorrect,
> yet the only obvious way I can see to bring down the values is that I
might
> be using an incorrect value for E. Yet I checked that in several sources.
E
> = 2,900,000 would give a nice result, but the units don't jibe.
>
> I'm at a bit of a loss on this discrepancy. Any ideas?
>
>
>
> thanks,
>
> Alec
>
>
>
> -----Original Message-----
> From: Ian Roxborough [mailto:irox@ix.netcom.com]
> Sent: Monday, July 01, 2002 1:54 AM
> To: personal_submersibles@psubs.org
> Subject: Re: [PSUBS-MAILIST] Pressure Hulls
>
>
> Hi Dewey,
>
> On Sun, 30 Jun 2002 21:20:44 -0700 (PDT)
> Dewey Mason <drmason2001@yahoo.com> wrote:
>
> > Hey All,
> >    Today I found 3 stainless steel spheres about 9
> > feet in diameter+/-. They are about 1" minimum
> > thickness with a small increase near the inlet head.
> > They are used, having been commercial CO2 tanks for a
> > local gas producer. While I do realize the potential
> > problems of using used stuff, I think the maintenance
> > was good enough to consider it. Does anyone have a
> > means to calculate the unreinforced crush depth on
> > such a piece?
>
> P = 2Ut/R
> Where:
>  U = min yield strength
>  t = hull thinkness
>  R = outside radius
>
> Beware, I could have gotten this wrong...
>
> I copy this out of  The American Bureau of Shipping's
> "Rules for Building and classing: Underwater vehicles, systems
> and hyberbaric facilities."  But I've seen it many other places
> as well (so no worries on copyrights and such).
>
> I'd recomend getting the above book, I've hardly started
> it and it's proved very useful.
>
> I'd also recommend check the crush depth several different
> ways, such as using different equations and checking them
> against a Finite Element Method.  I guess using a real high
> safety factor since it's "used" would help too.
>
> >  Hopefully the much smaller diameter will leave enough strength
> > difference to put the crush depths very nearly the same. At least to
> > where I can P-test the cabins at 75%, and run them at
> > 50% of calculated crush, and P-test the mechanical
> > compartment at 90%, running it at maybe 80% or so.
>
> Do you have access to a pressure testing facility/chamber?
>
> >    Maybe someone will think I am one of the ones
> > needing banishment after this, but I hope to hear the
> > thoughts and ideas of those who think I might can
> > stay.
>
> Let's save the banishment for people who post multilevel
> marketing schemes and adverts not relating to submarines...
>
> Ian.
> --
> The moon ate the sun on Monday and it was an awsome sight to see.