MIL-PRF-27720G
vacuum or is pressurized to the outer shell pressure relief device maximum opening pressure (see
4.5.1, 4.5.2).
3.5.5.2 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
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, 4.5.2.1).
3.5.5.3 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. 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 4.5.1,
4.5.16.4.1).
3.5.5.4 Sump and drain provisions. The inner shell shall have a sump with a drain line, and a
smooth inside bottom surface throughout an arc of not less than 3 inches length on each side of
the vertical centerline for the entire length of the shell for proper drainage under gravity, without
inner shell pressurization. The drain line shall include an accessible, manual valve for draining
the sump and obtaining liquid samples for analysis. The sump shall be not less than 3.0 inches in
diameter and the drain line shall have an inside diameter of not less than 0.5 inch (see 4.5.1).
3.5.5.5 Baffles. The inner shell shall include lateral and longitudinal baffles to prevent liquid
sloshing and surging during tank transportation. The baffles shall not restrict flow during liquid
transfer operations or drainage, and shall permit pressure equalization between compartments
(see 4.5.1).
3.5.6 Outer shell.
3.5.6.1 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).
3.5.6.2 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 the
outer shell safety relief device. The seal-off valve components and assembly shall conform to
USAF Drawings 68A39469, 68A39470, 68A39471, 68A39472, and 68A39473 (see 4.5.1).
3.5.7 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 (see 3.6.2). 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.
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