MIL-PRF-38170H
3.5.4.2 Inner shell safety head. A bursting-disc type safety head using a readily replaceable disc
in a union-type fixture shall be provided to relieve the inner shell vapor phase pressure to the
atmosphere should this pressure become excessive and the pressure relief valve fail to open. The
safety head relief pressure shall be not greater than 1-1/2 times the inner shell maximum working
pressure (see 4.5.1, 4.5.2, 4.5.2.1).
3.5.4.3 Inner shell vapor space pressure relief device. An automatic pressure relief device shall
be connected into the vapor vent line between the outer shell and the vent line shutoff valve to
relieve excess pressure (see 4.5.1, 4.5.3). The relief device shall open at a pressure sufficiently
below the inner shell bursting-disc type safety head breaking pressure to ensure against rupture
of the safety head unless the relief device fails, but not less than 50 psig, and shall be sized to
insure against the inner shell vapor space pressure exceeding 65 psig under the conditions
specified (see 4.5.11.5).
3.5.5 Outer shell.
3.5.5.1 Design pressure. The outer shell shall be designed for a working pressure that will
ensure sufficient strength and rigidity to prevent distortion or damage when the annular space is
evacuated to a high vacuum or pressurized to the outer shell pressure relief device maximum
opening pressure (see 4.5.1, 4.5.2).
3.5.5.2 Vacuum seal-off valve. The outer shell shall have a vacuum seal-off valve that will
serve as the vacuum pump-out for evacuation of the annular space, an isolation valve, and outer
shell safety relief device (see 4.5.1, 4.5.3). The seal-off valve shall conform to USAF drawings
68A39469, 68A39470, 68A39471, 68A39472, and 68A39473.
3.5.6 Annular space insulation. The annular insulation space shall contain an insulation that
will, together with evacuation of the annular space, ensure compliance with the tank evaporation
rate requirement. The insulating material shall be of a composition that will not ignite or burn
when heated to 400 °F in 99.5% pure oxygen gas at a pressure of not less than 10 psig. The
insulation shall not settle or be displaced in service. If a getter (impurity absorbing) material is
used in the annular space, it shall be secured in direct thermal contact with the inner shell (see
4.5.10).
3.5.7 Liquid lines. A single internal fill-drain and service line shall be provided. Externally, the
line shall be connected to provide separate liquid fill-drain and liquid service lines, valves,
filters, and disconnects (see 4.5.1).
3.5.7.1 Liquid fill-drain line. The tank assembly shall have a liquid fill-drain line for liquid
oxygen flow or transfer into or out of the inner shell. The line shall be connected to the bottom
of the inner shell and shall ensure complete drainage of the inner shell into the line. The
installation shall ensure against liquid entering the line from the inner shell when all liquid line
valves are closed and the tank is tilted 10 degrees along either the longitudinal or lateral axis.
The fill-drain line shall have a manual shutoff valve as near as possible to the point where it
enters the outer shell. The line shall terminate outside the control housing in a 1-inch external
pipe thread for installation of the disconnect coupling. The fitting mounting shall withstand a
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