10.
WINDOW SEALS.
a.
As primary seals for standard window geometries shown in Figure
3-11, a soft elastomer compressed between the high pressure face of the
window and retainer ring will be acceptable. The soft elastomeric seal may
take the form of a flat gasket, or a seal ring with O, U or X cross section.
The gasket or seal ring must be of sufficient thickness to permit adequate
compression without permanent set. Double bevelled disc windows must utilize
as primary seal a seal ring radially compressed between the cylindrical
surface of the window facing the pressure and the cylindrical window seat in
the flange.
b.
Retainer rings must provide the following minimum initial
compression of the gasket in millimeters (for values of N, see Tables 3-3 and
3-4).
(0.015 Di)
(1)
N x -------------
- conical Frustrum Windows
Tan [alpha]/2
-
(2)
(0.01t + 0.25) - flat disc windows
c.
The minimum compression of seal rings must, be governed by
specifications of seal ring manufacturers for the given seal ring size and
service.
d.
A secondary seal is required between the window and the steel
cavity seat for flat discs, spherical sectors with square edge and
hemispheres with equatorial flange. The secondary seal also serves as a
bearing gasket for the window. This gasket must be bonded with contact
cement to the metal flange seat. Thickness of the gasket must not exceed 1/8
in (3.0 mm). Neoprene impregnated nylon cloth, neoprene of 90 durometer
hardness, and cork gaskets have been found to be acceptable for such
application.
e.
Seal ring grooves are not permitted in the bearing surface of any
window shape, or the bearing surface of the seat in metal flange unless
experimental data, showing the cyclic fatigue life of such a viewport
assembly at design pressure and temperature to be in excess of 104 standard
pressure cycles (four hours under pressure followed by four hours of
relaxation), is submitted to and accepted by the Committee on PVHO. Seal
ring grooves are not allowed in any window shape or its seat in the metal
flange.
11. DIMENSIONAL TOLERANCES AND SURFACE FINISH.
a.
Thickness of the window must be everywhere equal to or greater than
the nominal value determined by procedure of paragraph 7d.
b.
The major diameter of the conical bearing surface on a window, must
be machined within +0.000/-0.002 in. of nominal value.
c.
The included conical angle of the window must be within
+0.25/-0.000 degree of nominal value.
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