3.
MATERIAL SELECTION.
a.
Glass in the form of flat discs is suitable for use in hyperbaric
pressure vessels, The material should be tempered soda lime glass for maximum
strength and/or Pyrex for corrosion resistance. To avoid catastrophic
failure modes, glass windows are usually laminated of two or more discs.
Only glass windows of known performance should be used in hyperbaric pressure
vessels. Windows of this type are available from commercial suppliers for
pressures to 600 psi and temperatures up to 500 deg. F. Typical glass
windows are shown at Figure 3-7.
b.
Acrylic plastics are alternatives to glass in pressure-resisting
windows (See Reference 2, Stachiw, Critical Pressures of Conical Acrylic
Windows 1967). The material properties of acrylics are well defined.
Acrylics do not exhibit catastrophic failure modes and prior to reaching the
failure point there are obvious indications of the impending failure. In
this subsection, design information is given on the selection and fabrication
of acrylics for hyperbaric chamber viewports.
c.
Other materials such as Lexan may be used if their physical
properties have been well established and their performance has been verified
by tests.
4.
SEALING METHODS. Sealing a viewport against leakage involves
consideration of both the window and the supporting flange. With clean,
properly fitted mating surfaces of moderate smoothness, an acrylic viewport
will develop a pressure-energized seal simply by the application of silicone
grease to the mating surfaces. However, grease can channel with time, and
conservative design requires the addition of deformable seals for increased
reliability and more evenly distributed sealing loads.
a.
Seal Types. Both O-ring and gasket seals have been successfully
applied in viewport designs (Figures 3-1 through 3-6). Neoprene is usually
used for gaskets and seals. In general, gasket configurations are not
critical; however, reasonably thick sections prevent unloading due to window
deflection, and compliant materials distribute loads more uniformly.
Conventional gaskets are usually 1/8 inch thick but thickness should be
varied to suit the application. Cork may also be used for gaskets.
b.
Sealing Techniques. O-rings or gaskets can be used to seal the
windows depending upon the configuration of the window.
(1) Flat disc viewports (Figure 3-1) may be sealed by gaskets or a
combination of O-rings and gaskets. If the O-rings are used, the groove
should be sized to produce about 20 percent compression.
(2) Conical frustum windows (Figures 3-2, 3-3, and 3-6) do not
lend themselves as readily as flat plates to the use of gaskets for primary
sealing. However, an O-ring in a groove located as close as possible to the
high pressure side can provide a satisfactory seal. A gasket placed under
the retainer ring on the high-pressure (large diameter) face will provide
O-ring preload.
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