Ask X3What does 49 CFR 178.45 (Specification 3T seamless steel cylinder) require?
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As Ask X3, your fleet-compliance assistant, here is what 49 CFR 178.45 requires for a DOT 3T seamless steel cylinder:
* Type, Size, and Service Pressure: The cylinder must be a seamless steel cylinder with a minimum water capacity of 1,000 pounds and a minimum service pressure of 1,800 psig. Both ends must have integrally formed heads concave to pressure, shaped either hemispherically, ellipsoidally (major axis two times the minor axis), or in a dished shape between those limits. Permanent closures formed by spinning are prohibited.
* Material & Quality: Only open hearth, basic oxygen, or electric furnace process steel of uniform quality is allowed, meeting specific chemical analysis ranges and tolerances. Material must be free of seams, cracks, laminations, or other injurious defects, and must be suitably identified.
* Manufacturing & Surface Finish: The surface finish must be uniform, reasonably smooth, and free of weakening defects (though surfaces may be machined or treated to eliminate defects if the minimum wall thickness is maintained). Inside surfaces must be clean, dry, and free of loose particles. Welding or brazing on the cylinder is prohibited.
* Wall Thickness: The minimum wall thickness must ensure that wall stress at the minimum specified test pressure does not exceed 67 percent of the minimum tensile strength of the steel (and cannot exceed a wall stress of 90,500 psi). The absolute minimum wall thickness for any cylinder is 0.225 inch. Thickness must also be increased if necessary to meet specific bending and hydrostatic longitudinal stress requirements outlined in 49 CFR 178.45.
* Heat Treatment: Completed cylinders must be uniformly heat treated prior to testing. This involves heating and holding at the proper temperature for at least one hour per inch of maximum thickness, quenching in a suitable liquid medium (cooling rate not exceeding 80 percent of water and quenching temperature no higher than 1750 °F), and then reheating to a temperature below the transformation range (at least 1050 °F) for at least one hour per inch of thickness before cooling under recommended conditions.
* Openings: Openings are permitted on the heads only, must be circular and threaded, and must be centered (except off-center openings are allowed up to 2.625 inches in diameter). Centered openings cannot exceed half the cylinder's outside diameter, and head openings must maintain required ligaments between them. Threads must be clean cut, even, without checks, to gauge, and comply with approved standards (such as NPT, NGT, or NGS from NBS Handbook H-28).
* Pressure Testing: Each cylinder must successfully pass a water-jacket or direct expansion pressure test using calibrated equipment. It must be tested to a minimum of 5/3 times the service pressure, holding the test pressure for at least 30 seconds (or longer to ensure complete expansion). Permanent volumetric expansion cannot exceed 10 percent of the total volumetric expansion at test pressure.
* Ultrasonic Examination: Following the hydrostatic test, the cylindrical section must undergo ultrasonic examination (using ASTM E 213 for shear wave and E 114 for straight beam) calibrated to detect a notch equal to 5 percent of the design minimum wall thickness. Discontinuities greater than that threshold cause rejection unless properly repaired.
* Tension and Charpy Impact Tests: Cylinders must be subjected to tension and Charpy impact testing as prescribed in the regulation.