MIL-STD-1411B
5.3.2 General cylinder maintenance. These cylinders require valve maintenance, repair or rebuilding, cleaning, or external maintenance as scheduled. Valve service life is extended by a system to preserve the identity of the valve and its mated cylinder. Valves requiring replacement shall be rejected. Repairable valves or acceptable valves shall be forwarded for valve maintenance as applicable.
5.3.2.1 Neck threads cleaning. When a valve is removed from a cylinder for any reason, the mating neck threads of the cylinder shall be cleaned of extraneous debris, such as thread tape remnants, anti-seize compounds, rust and corrosion, prior to any other procedure. Care must be taken that materials removed from the neck threads do not end up in the cylinder. Any material that does go into the cylinder must be removed prior to installation of a valve. Note that rust or corrosion found in the neck threads is indicative of a more serious problem and requires a thorough inspection of the blind area (see 5.3.2.2.3).
5.3.2.2 Droplight testing. Any time that a valve is removed from a cylinder for any reason, the cylinder shall be given an internal droplight examination.
5.3.2.2.1 Light source. A light source of sufficient intensity to clearly illuminate the interior walls should be provided.
NOTE: For safety, this light should be low (3-12) voltage with an isolation transformer (no common ground) and the bulb should be protected with some form of a safety shield. (If the bulb should break, it could ignite flammable liquids or vapors in the cylinder, resulting in possible damage to the eyes and face.) A fiber-optic light with its remote light source removed from the area of potentially flammable liquids or vapors is acceptable.
5.3.2.2.2 Test procedure. Before introducing the light, purge the cylinder thoroughly of all residual vapors with dry nitrogen or dry, oil-free air. Then invert it to remove all loose rust and scale and liquid contaminants. Drop the light into the cylinder slowly, positioning the light and the cylinder so that the entire inside surface of the cylinder, except for the blind area just below the neck, can be examined. Tilt the cylinder several degrees from vertical to get the best view of the cylinder wall. Stop the movement of the light as required to permit prolonged inspection of
suspect areas. Hold the light so that it first shines directly on a subject area and then so that any irregularities will produce shadows to improve the examiner's perception of the suspected area. Continue the examination while the light is being withdrawn from the bottom of the cylinder as some defects show up best at this time.
5.3.2.2.3 Inspection of blind area. The wall thickness in the blind area just below the cylinder neck is heavier than the rest of the cylinder and, because of its location, seldom corrodes as fast as the other interior portions of the cylinder. Contamination in this area can be inferred by an examination of the cylinder neck threads near the bottom of the neck opening and of the visible portions of the upper areas of the cylinder. If the examination discloses substantial internal deposits that make it impossible to inspect the cylinder wall itself, these deposits shall be removed in accordance with mechanical restoration (see 5.3.2.4.4). The cylinder shall be reinspected after the deposits are removed.
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