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Re: [PSUBS-MAILIST] ASME - say what??
Jon - assuming you have PVHO-1-2002, look at figure 2-2.20 (c), on page
36. This gives you basic dimensional constraints on the seat cavity
geometry. I am assuming that you have already designed the window
itself (i.e. required diameter and thickness), as this will dictate the
cavity geometry. D_o is the outer diameter of your acrylic window. D_s
is set by section 2-2.12.9, D_f is constrained by the relation in the
figure (1.250 <= D_o/D_f). You will note in that figure that the
dimension "K" in that figure is selected on the basis of structural
analysis - this must be so, as there is no way for the standard to
anticipate the shell loads that must be accommodated by the viewport
seat in the absence of the shell material in the hole (since the acrylic
viewport is specifically prohibited from bearing loads in this manner).
The necessary analysis is not that difficult. Perhaps the simplest one
to start with is the direct shear, as the shear area will be simply the
circumference of the viewport (pi multiplied by D_o in the figure)
multiplied by the minimum thickness K. Calculate the total load
(pressure x area of the viewport) and then divide by the shear area
above to arrive at the shear stress, and then adjust K to get your
desired safety factor.
Next, examine the bending stress. Simplify the problem by analyzing it
as a beam in 2D, just as it is presented (in section view) in the
figure. To put that another way, imagine cutting the ring radially at
one location, and then straightening it so you end up with a bar with a
ledge on it - i.e. nothing more than a cantilevered rectangular beam,
with width (conservatively assuming the smallest diameter) b = pi * D_f,
height K, and section modulus Z = (b*K^2)/6. Conservatively assuming
that the entire load will occur at the inner diameter of the cavity
(D_f), the maximum bending stress at the corner will be the load W
(pressure * window area), multiplied by the length of the beam l ((D_s -
D_f)/2), divided by the section modulus. Obviously, the fact that it is
not a beam but a circular object will change the assumed geometry, and
the fact that the load is distributed and not a point load will change
the actual stress, but since all of the assumptions made above are in
the direction of increased conservatism, that gives the worst-case
scenario. Again, check this stress against the material maximum, and if
not sufficient, adjust K to obtain your desired safety factor.
Torsional stress doesn't really need to be considered in this case,
since there is no torsional loading. The only other stresses
experienced by the viewport cavity are those imposed upon it from the
shell. If you have not already included reinforcement in the adjacent
shell, then the viewport cavity insert could be considered as the
necessary reinforcement as long as it meets the requirement for
sufficient material replaced. I would suggest incorporating this seat
into the shell in a manner which avoids abrupt geometry changes (i.e.
taper the outer diameter of the seat into the shell through some
combination of machining and/or weld deposition), just as with any
transition between plates of differing thickness.
-Sean
Jon Wallace wrote:
I've just purchased ($179 US) and downloaded the ASME PVHO-1 standard.
ASME uses the following wording to market their PVHO-1
standard..."This Standard provides requirements for the design,
fabrication, inspection, testing, marking, and stamping of pressure
vessels for human occupancy, having an internal or external pressure
differential exceeding 2 psi."
As far as I can tell so far, except for some narrow conditions, PVHO-1
points you to BPVC Section VIII for hull and penetration design which
is what...about $675?? I'm trying to figure out the dimensions for
the seat (not the walls) of a flat disc viewport thru-hull. Am I
missing something in PVHO-1?
Jon
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