This standard replaces DLT709-1999, Technical code for safetyinspection of steel penstocks. Besides editorial changes, the maintechnical revisions are as follows: —Adding the content of corrosion degree assessment (see Clause7). —Supplementing the content of time of flight diffraction(TOFD) inspection, and revising the detection ratio of welds (see 8.4 and 8.6). —Supplementing and improving the content of stress detection(see Clause 9, which is Clause 8 inDLT 709-1999). —Supplementing and improving the content of vibrationdetection (see Clause 10, which is Clause 9 in DLT 709-1999). —Deleting the content of water quality and sediment inspection(see Clause 10 in DLT 709-1999). —Adding the content of check calculation (see Clause 11).