Re: [PSUBS-MAILIST] TANK / PSI QUESTION

["Al Secor" <wreckdiver@usadatanet.net>]

Hmmmm, I believe this is what I was trying to say in very simplistic
terms....  I guess next time I should let my engineering background show
through
 :-)

BSEE working for IBM......

Al Secor  ARS: WA3PWX  Scuba Instructor SSI PDIC TDI
http://www.geocities.com/SubDiverI
----- Original Message ----- 
From: "Sean T. Stevenson" <cast55@telus.net>
To: <personal_submersibles@psubs.org>
Sent: Sunday, February 12, 2006 5:19 PM
Subject: Re: [PSUBS-MAILIST] TANK / PSI QUESTION


> These are very dissimilar cases.  In the case of internal pressure, the
> pressure actually acts to deform the vessel in a manner which increases
> roundness.  Imagine looking at a cross section of the cylinder in
> question (i.e. a circle, with a wall thickness).  Now mentally divide
> that circle in half.  The tensile stress in the cylinder wall will need
> to be the value that holds those halves together in the presence of the
> internal pressure - i.e. the pressure multiplied by the projected area.
> In this case, that internal pressure divided by the inner diameter times
> the length of the cylinder (giving an area) gives you the force, which
> is countered by the tensile stress in the area of the cylinder walls -
> i.e. (OD-ID) multiplied by that same length - the length cancels, which
> is why we can examine this as a 2-D problem.  Now imagine perturbing the
> geometry of the circle - i.e. making a dent in the cylinder wall, either
> to the inside or the outside - the direction doesn't really matter.  You
> will notice that the tensile force in the cylinder wall acts to
> straighten out the dent.  As with a pressurized can or pop bottle, when
> you squeeze it, provided you don't do it so hard as to cause plastic
> (non-recoverable) deformation, the internal pressure will return it to
> its former shape.
>
> This is not the case with an externally pressurized vessel.  A vessel
> subjected to external pressure acts similarly in that the compressive
> stress in the cylinder wall will ideally be equal to the external
> pressure multiplied by the projected area (which is now the OD, not the
> ID of the cylinder), giving the compressive force, and we divide this by
> the area of the cylinder walls to get the compressive stress in the
> cylinder wall.  The difference in this case is that, if you then dent
> the cylinder wall in either direction, putting it out of round, it is
> not self-correcting, and in fact will cause a stress concentration which
> tends to cause the cylinder to buckle and crush.  Any deviation from the
> perfectly round case needs to be accounted for by calculating based on
> the largest perfectly round circle (largest OD, smallest ID) than can be
> inscribed within the circle that actually exists (subject to
> manufacturing and measurement accuracy).  This is complicated even
> further when extrapolated to the 3-D case, since our largest circle
> becomes a largest cylinder, which must exist in an actual cylinder that
> needs not only be round, but also of concentric cross-section as you
> travel along its length.  Anything outside of that is extranneous
> material, as far as the calculations are concerned.  A good
> demonstration of this is standing on an empty pop can.  A 200 lb person
> can, if careful, balance themselves on an intact aluminum soda can.
> Now, if someone bumps it, even slightly, the resulting deformation is
> unrecoverable and the can crushes.  Since it is not possible to
> fabricate perfectly round cylinders which do not deform, we must do the
> next best thing - a combination of thicker than theoretically required
> wall thickness, in conjunction with bracing (i.e. ring stiffeners) to
> prevent any deformation from excessively reducing the theoretically
> perfect cylinder that exists within the fabricated one.
>
> Clear as mud?
>
> -Sean
>
>
> Norm Parmley wrote:
>
> > Here's a question to the more mechanical types.  If I have a normal
> > rounded metal tank rated for an internal pressure of 200 psi then, I
> > could assume it would be capable of 200 psi external pressure?
> >
> > Norm P.
> >
> >
>
>
>
>
>
>
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