(1) According to SY/T6194-96 regulations, the length of domestic casing is not fixed, and its range is 8-13m. However, sleeves no shorter than 6m can be provided, and their quantity should not exceed 20%.

(2) The inner and outer surfaces of the casing shall not have folds, hairlines, delamination, cracks, folds, or scars. These defects should be completely removed, and the removal depth should not exceed 12.5% of the nominal wall thickness.

(3) The outer surface of the coupling should not have defects such as folds, hairlines, delamination, cracks, folds, scars, etc.

(4) The thread surface of sleeve and coupling shall be smooth, and no burr, tear or other defects that can interrupt the thread and affect the strength and Tight junction are allowed.