Re: [PSUBS-MAILIST] Service Life vs. Design Life of Acrylic Windows

[Jon Wallace <jonw@psubs.org>]

Considering Stachiw was Chairman of the subcommittee on viewports for 
the PVHO standard, I wouldn't take such a black/white view of the 
standards.  Stachiw also wrote in his book:

"It is worth remembering, that since its inception in 1977, the Standard has
maintained a perfect safety record over -14.5 to 20,000 psi pressure
range in hundreds of different manned and unmanned applications.  It would,
therefore, be foolhardy to ignore the guidelines for acrylic window
installations in pressure vessels for human occupancy presented by the
standard."

You have to remember that Stachiw's work was not primarily targeted for 
small personal submersibles but rather, for large research vessels, 
aquariums, and military applications.  His differentiation between 
design life and service life in the text you provided was not, I think, 
intended as a damnation of PVHO regarding that topic but instead, a 
practical look that other factors may be considered.  Specifically, he 
asserted that "in service inspection" and testing was required to make a 
determination one way or the other from the "design" life of viewports.

The conclusion one should draw from this then, is that if you have the 
resources to properly inspect and test your viewports as required by 
PVHO-2, then you may be able to justify deviation from the standard one 
way or the other.  However, for most psubbers this likely will not be 
applicable, or perhaps practical.

To put this in perspective, 10,000 cycles on a psub viewport is one 
dive, every day, for 27 years and 3 months.  I don't know any psubbers 
that spend that much time in their submarine.  That means a psubber is 
more likely to hit the 10 year time limit rather than the 10,000 cycle 
service limit, regarding design life of viewports.  So the only question 
remaining then, is how much do you trust your viewports after a decade 
of time has passed?  Has it been sitting in a temperature controlled 
museum?  Or in a dusty garage with lots of other junk?

Jon


Brent Hartwig wrote:
> *Below Dr. Stachiw describes his professional opinion on what he has 
> calculated and tested verses what the PVHO standard has adopted.  When 
> I see data from experts like this, I have further reason to not wish 
> to follow every thing that ABS and PVHO currently considers to be best 
> practice. Another case in point is when Paul Moorhouse was building 
> Alicia, and ABS required him to use a scaled down tanker prop shaft. 
> He found that to be massive over kill and it added a lot of weight in 
> the stern he didn't need there. Plus the extra cost of the larger parts.*




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