Re: [PSUBS-MAILIST] Pressure Hulls

[Ian Roxborough <irox@ix.netcom.com>]

Hi Alec,

On Wed, 3 Jul 2002 15:58:33 -0400 
Alec Smyth <Asmyth@changepoint.com> wrote:

> 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
>
> b) The second equation is empirical:
> 
> p = 0.84*E*(t/R0)^2
> 
> where R0 = outside radius

The ABS book has a formula similar to this, but the constant,
0.84, is replaced with:

0.84 = (2 / SQR(3 * ( 1 - v^2)))

where v = Poisson's ratio.
It is refered to as "linear classical buckling pressure".

> I'm at a bit of a loss on this discrepancy. Any ideas?

They are calculating different type/modes (I think) of
sphere failure.


The ABS books goes on to calculate the pressure limit of
a sphere buy the following method:

When Pes/Pys > 1

  Pcs = 0.7391 * SQR(1 + (Pys / (0.3 * Pes))^2)

When Pes/Pys =< 1

  Pcs = 0.2124 * Pes / Pys

Where Pes = Classical linear buckling pressure (Given above)
      Pys = Yield pressure (The one I quoted orginially)
      Pcs = limit pressure

The ABS book then goes on to calculate the maximum
allowable working pressure as

  P = Pcs * 0.67

P being the maximum working pressure.

Warning: I've distilled down what the ABS book said on the
topic of spherical pressure vessels for the purpose of this
discusion.  Besides the typos I might have introduced, this
is my interpretation and is no substitution ABS book. 

I like the idea of calculating the working and/or crush depth
of the subs in the Busby book to check a formula's validity.

Ian

> -----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.


-- 
The moon ate the sun on Monday and it was an awsome sight to see.