MIL-STD-1411B
5.3.2.2.4 Interpretation of visual inspection. The interpretation of the visual internal inspection must be left to experience. Surface irregularities may appear greater in height or depth than they really are because of the shadows they cast. Harmless mill scale and metal discoloration can be misinterpreted as heavy contamination. When in doubt, however, the safe and desirable procedure is to clean by a procedure appropriate to the nature of the apparent impurity. When significant corrosion or possible flaws are suspected, the cylinder shall be further inspected in accordance with the requirements of CGA C-5. A desirable inspection device that can be of great assistance in inspecting questionable cylinders is a borescope.
5.3.2.3 Cylinder structural maintenance. Cylinder structural deficiencies shall be grouped as follows: neck, flange and threads, collar and foot ring, sidewalls, shoulder and bottom heads.
5.3.2.3.1 Government cylinders DOT 3A and DOT 3AA. Cylinders greater than 625 cubic inches in water capacity are supplied with a neck flange in accordance with MIL-DTL-17376/3. The flange is installed by press fit or by peening into place. Heat cannot be applied to the cylinder, either in removal of the flange or in replacement, as the temper of the steel in the DOT 3A or
DOT 3AA cylinder may be changed. Flanges shall be peened tight when found to be loose. Flanges stretched by abuse shall be replaced in accordance with MIL-DTL-17376/3. Any permanent markings on a replaced flange shall be permanently marked in the steel of the shoulder of the same cylinder. DOT 3A and DOT 3AA cylinders with service pressures over 500 psig are seamless in construction, and welding or heating the cylinder is prohibited. DOT 3A and DOT
3AA cylinders with service pressures up to 500 psig are often equipped with welded foot rings to protect the bottom areas and to provide a flat surface for upright stability. Repair of DOT 3A or DOT 3AA cylinders shall be restricted to similar cylinder fabricators.
5.3.2.3.2 Government cylinders DOT 4 and DOT 8. Cylinders requiring maintenance shall be grouped into those requiring repair maintenance and those requiring rebuild maintenance.
These services shall be performed only by repair facilities that have been certified by the Associate Administrator for Hazardous Materials Safety of the Office of Hazardous Materials Safety, and shall function in accordance with the procedures and limits specified in 49 CFR and as specified herein. Repairing or rebuilding of welded DOT 4 or DOT 8 cylinders shall be in accordance with
49 CFR 173.34.
5.3.2.3.3 Heat treatment of cylinders. Whenever reheat treatment of a cylinder is proposed, it shall be in accordance with the 49 CFR specification covering the manufacture of the cylinder in question. Data from the original manufacturing reports for the cylinder shall be available. DOT 3A and DOT 3AA specifications require quenching, annealing, and tempering of the drawn cylinder. The temper of the steel is relative to the strength of the high-pressure cylinder and any change by heat will render the vessel unfit for high pressure service. DOT 4 and DOT 8 specifications require heat treatment and annealing of the major components to relieve the stresses of drawing the shells and shaping the sidewalls. These steels are selected for welding properties and therefore, repair and rebuild can be provided by repair facilities certified by the Associate Administrator for Hazardous Materials Safety of the Office of Hazardous Materials Safety to restore the life of a damaged cylinder, as applicable.
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