MIL-PRF-27720G
3.5.1 Tank assembly. The tank assembly shall be a self-contained liquid oxygen or nitrogen
transport and storage unit, consisting of a storage tank with inner and outer shells, a control
housing assembly, necessary controls and accessories, and a skid base (see 4.5.1). The tank
assembly shall be capable of:
a. Being filled with liquid oxygen or nitrogen at a supply location (see 4.5.1).
b. Being transported (see MIL-HDBK-1791 for guidance) while filled with liquid oxygen or
nitrogen and pressurized to 50 pounds per square inch gage (psig) (see 4.5.1).
c. Storing the liquid with low evaporation loss rates during transport and at destination until
needed, and at that time transferring the liquid into smaller servicing containers (see
4.5.1).
3.5.2 Reliability. The tank assembly shall have a mean time between failures (MTBF) of not
less than 21 servicing cycles at a 0.9 confidence level (see 4.5.1, 4.5.17, 4.5.19).
3.5.3 Maintainability. Parts and assemblies shall be located and mounted to provide adequate
clearance for repair and other maintenance and, where practicable, to permit removal and
replacement of any part or assembly by removing or disconnecting only mounting bolts or
fasteners, panel doors, tubing, or control cables to the part or assembly being removed. All
removable lines shall include special means for gripping the lines for attachment and
detachment. The tank assembly, where practicable, shall include readily detachable and
attachable type fittings in the liquid and vapor lines to permit rapid component removal and
replacement. Disconnect points shall be clearly indicated and identified. The design shall ensure
clearance for operation, servicing and maintenance at low ambient temperatures by personnel
wearing heavy gloves or mittens, bulky clothing and footgear. The design shall ensure
maintenance and servicing can be performed using only common handtools. Intricate locking
devices, controls, or fasteners that can be easily overtorqued by personnel lacking feel through
thick gloves or numbness shall be avoided. Component installation design shall be such that the
items can only be installed in their proper operating position to avoid reverse installation (see
4.5.1, 4.5.20).
3.5.4 Pressure vessels. The liquid storage tank assembly, piping, and connections thereto shall
meet the requirements of section VIII of the ASME Boiler and Pressure Vessel Code (see 4.5.1,
4.5.2). Also, the inner shell shall be certified under the code for the specified working pressure
(see 6.2).
3.5.5 Inner shell.
3.5.5.1 Design pressure. The inner shell shall have a maximum working pressure the same as
the inner shell pressure relief device (see 3.5.5.2). The normal working pressure shall be not less
than 50 psig for transferring liquid oxygen or nitrogen. The inner shell shall withstand an
absolute pressure of 100 microns of mercury (Hg) or less with the annular space at atmospheric
pressure. The shell shall not sustain damage when the annular space is evacuated to a high
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