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Re: [PSUBS-MAILIST] Hatch pressure
I agree that the equatorial ring seems to be more trouble than it's
worth. Stachiw is somewhat non-committal on specifics stating on Page
459 that there is "insufficient experimental data" regarding equatorial
rings on air-blown hemispheres. Given the copious data in other areas,
this leads me to assume that he didn't see a practical need for an
equatorial ring even though he states "...the presence of the flange
provides a simple engineering solution to the attachment of the
hemisphere to the mounting flange".
In reality, I don't see much of an engineering difference (or
complexity) between the suggested mounting in figure 11.86 (no flange)
and 11.90 (with flange) and I think its debatable whether an equatorial
ring offers any true advantage in terms of mounting.
The data he provides for ring fracture and separation was at 8000psi
(took seven hours to fracture at that pressure) which a typical
home-builder will never experience. Even when talking about maximizing
cyclic life stachiw uses 1000psi as a demarcation point, which again our
typical home-builder will never experience. This chapter would have
been much more useful if it addressed pressures at 500psi or less, but I
think Stachiw concentrated on high-pressure applications.
Given that the mounting issues are almost non-relevant, and specific
data for ring cracking at pressures below 1000psi appear to be
non-existent, it would make sense to follow Stachiws lead and machine
off the ring. Not doing so, one would be wise to follow Stachiws advice
and perform comprehensive testing on one or more units (to destruction)
to determine what performance characteristics can be achieved. This
seems like it would be a lot more work than just removing the ring after
forming.
Jon
Smyth, Alec wrote:
Hi Alan,
It's really good you're looking at Stachiw, which is the bible for
viewports. I don't mean to be overly critical, this is probably just
interpretation (just like with the real bible, no?) I looked up page
460, and found a rather dire warning that if you're using a flange you
need to back it up with a bonded acrylic ring. I agree with
your option #1, but I think options #2 and #3 really have to be
combined. Call it option 2.5, which consists of machining down the
flange AND backing it up with a bonded ring.
For those without the book, the dire warning is: "In an actual
application of free-blown hemispheres with flanges, it was found that
when the heel backup ring was eliminated the magnitude of tensile and
compressive meridional stresses increased 1000 and 64 percent,
respectively, although in an attempt to compensate for the expected
increase in stress level the thickness of the wall was doubled.
(Figure 11.90)." Figure 11.90 then shows two ways to use a flange,
labeled "proper" and "improper". The proper one does indeed have a
thinner flange, but crucially it also has a bonded acrylic support
ring inside that flange.
My recommendation would be to do just that, rather than assume the
dome can be backed up with fiberglass instead of bonded acrylic. Or
else just take the proven K250 approach. On the K250 the flange area
is machined off entirely, in other words Kittredge went with option
#1. But for hold-down convenience he added a bonded acrylic ring on
the outer surface of the dome. The external ring is over and above the
structural requirements, it's just to give you a handy hold-down on a
dome that's already strong enough without it.
thanks,
Alec
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