MIL-STD-1411B
f. Low-pressure cylinders with expired test periods may be retested by UE under DOT special permit, or hydrostatic test in accordance with the fabrication specification, or by modified hydrostatic test when authorized by specification and service listed in 49 CFR and identified by the symbol "S" after the test date.
5.2.6.2 External visual inspection. DOT 3A, DOT 3AA, DOT 4AA480, DOT 4B, DOT 4BA, DOT 4BW, and DOT 4E cylinders in low-pressure noncorrosive gas services as listed in 49 CFR with expired test periods may be evaluated by external visual inspection in lieu
of the applicable hydrostatic or UE test method and are identified by the symbol "E" after the test date.
5.2.7 General internal cylinder evaluation. At general internal evaluation, cylinders accepted at receiving inspection not requiring valve removal, except for sulfur hexafluoride and chlorine (see 5.2.8 and 5.2.9), shall be inspected for serviceability by the following procedures. Acetylene cylinders shall be evaluated in accordance with 5.3.3 for general service.
5.2.7.1 Hammer test. Each cylinder shall be subjected to a hammer test to assure viable structural integrity and that internally the surface is free of heavy corrosion residue. The clear ringing sound of acceptable cylinders will readily separate out the cylinder with a dull or dead response. Dull or dead sounding cylinders or a cylinder with a peculiar sound shall be scheduled for devalving, droplight inspection, and maintenance, as applicable. The hammer test is a recommended pre-filling inspection procedure for DOT 3A, DOT 3AA, DOT 3AX, and DOT
3AAX gas cylinders in accordance with the DOT regulations. Cylinders with heavy corrosion deposits will have a muted or "dead" ring when struck a light blow with a wrench or light hammer. Cylinders to be inspected must be standing vertically without touching other cylinders or objects that might deaden the sound. A pigtail connecting the cylinder to a manifold is allowable. Strike each cylinder a light blow with a one-half pound machinists ball peen hammer, wrench, or similar tool on the side wall about one-half or two-thirds down the cylinder. The
blow must be light, as all cylinders will ring if hit hard enough. If there is a thud instead of a ring or if the ring dies out quickly (in one or two seconds), the cylinder shall be set aside for closer examination. A "dead" cylinder can best be simulated by filling an empty cylinder without valve with several inches of dry sand. This cylinder can be used as a periodic check for the sound of a "dead" cylinder. If the ring of any cylinder is substantially different in tone or quality from the ring of other cylinders of the same size being hammer tested, the cylinder shall
be rejected. Hammer testing must be performed on the internally inspected cylinder since severe intergranular and transgranular corrosion can also deaden the ring. This very hazardous form of corrosion is invisible to the naked eye and can be verified only by a microscopic inspection of
the affected portion of cylinder wall.
5.2.7.2 Odor test. Any cylinder except those in poisonous or toxic gas service shall be subject to an odor or sniff test. If a residual pressure is not present, then an odorless inert gas must be introduced into the cylinder being inspected in order to carry out an odor or sniff test. An experienced operator should be able to separate acceptable cylinders with odorless effluent gas from rejected cylinders with odor bearing gas with contamination levels as low as 50 parts per million (ppm). Odor bearing cylinders shall be scheduled for devalving, cleaning, and drying at maintenance, as applicable. This test shall be limited to cylinders for compressed air,
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