life of the clamping mechanisms. For instance, in the original design, the
lb
11,000 --- load was chosen as to simulate a preload greater than the
in.
separating force caused by the internal pressure acting upon a 40.0 inch
diameter. However, the computer program calculated that the flanges would
actually tend to pivot about the outside contact line of the flanges and at
lb
the point of application of the 11,000 --- ring loads the flanges
in.
separated by 0.8 mils. This indicates that the preload was insufficient and
the clamping mechanism would be subjected to a fatigue loading. Further, the
computer output showed that at the inner seal position the flanges would
separate by approximately 8.0 mils. The increase in flange thickness would
decrease this 8.0 mil separation but would have little effect upon the flange
separation at the line of load application. Thus a greater preload is
required.
(11)
Miscellaneous Considerations. Small penetrations for
instrumentation and other types of small closures not requiring reinforcement
of the vessel wall should be so designed as to prevent any local bending
moments from being applied to the shell. Further, consideration should be
given to the seal configuration employed so as to prevent the formation of
crevices in the vessel wall.
The design of lifting lugs and the effect that their means of attachment to
the shell have on the local stress condition in the shell must be determined
by the designer. Again strong emphasis is placed on the prevention of
crevices and local stress concentration configurations. The same is true
about any types of attachments to the shell, whether outside the shell, as
say the hinging mechanism of the "door" or inside the shell, such as the
attachments for floors or other interior structures. The methods of
attachments for such devices should be governed by the provisions of Article
D-9, "Attachments and Supports." Where there are interior attachments which
will be subjected directly to a seawater environment great care must be taken
to ensure the prevention of crevices and other localized areas of corrosion
attack.
d. Design Finalization. When all of the above procedures have been
carried out, the pressure vessel has been designed to meet the adequacy test
of the stress intensity limits. However, inherent in all the "design by
analysis" procedure described above is the understanding that the basic
design dimensions developed and analyzed were in the "as corroded" condition.
This means that what has been developed is a vessel having components whose
dimensions are as they would be at the end of its design life, in this case,
after 20 years of operation. The designer must, at this time, give some
consideration to material loss and dimension changes due to corrosion.
The outside of the vessel can be painted and such covering can be easily
maintained and inspected. The inside of the chamber may also be painted. In
this case, the designer must show that such paints do not give off noxious
fumes, or enter into a rapid chemical reaction with seawater or oxygen, at
the temperatures and pressures specified for this chamber (this is true after
the drying and curing of the paint).
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