CHAPTER 3. APPURTENANCES
1.
SCOPE. This chapter presents design criteria for appurtenances
applicable to hyperbaric facilities. An appurtenance is defined here as any
accessory structure which directly affects the integrity of a hyperbaric
pressure vessel. Major categories of appurtenances include viewports, doors,
and closures.
Section 1.
VIEWPORTS
1.
VIEWPORTS. This section provides general design guidance for viewports
their configuration, material, fabrication techniques, and certification
requirements. References are provided as further assistance to the designer.
The information in this section conforms to the standards established by
ANSI/ASME PVHO 1, Safety Standards for Pressure Vessels for Human Occupancy
(Reference 1).
2.
BASIC VIEWPORT TYPES. The viewport configurations which have received
the major share of practical development are the flat plate and the conical
frustum. Additional laboratory research has been done on spherical shell
ports, which have advantages in applications requiring a wide field of view.
Viewport parts are the window, seat, supporting flange, bezel and seals.
a.
Flat Plate. The flat plate viewport offers several advantages. It
is easy to fabricate, lends itself to sealing with pressure on either side
(two-way window), and has good structural characteristics. Its primary
disadvantage, by comparison with conical or spherical shell ports, is that
tensile stresses (which increase sensitivity to surface stress risers) can
develop on the low pressure face. An example of the flat plate viewport is
shown in Figure 3-1.
b.
Conical Frustum. The conical window, designed to be pressurized on
its larger face and supported by a conical flange, develops a natural sealing
and wedging action (Figure 3-2). This characteristic is useful in
high-pressure applications, since a predominantly compressive stress field is
generated. A cone angle of 90 deg. (included) has been found to be a good
compromise between the tendency to extrude at small cone angles and the rapid
loss of viewing area with increasing thickness of large cone angles. the
conical window is more difficult to fabricate than a flat plate, but easier
than a spherical shell. When plastic is used, conical windows can be
latheturned.
c.
Other configurations are possible for viewport windows and many
variations have been used for the supporting flanges, retaining rings and
seals. Examples of other viewport geometries are shown in Figures 3-3
through 3-6.
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