For example, there are a total of 50 welds with a total length of 5000mm and a sampling inspection of 20%. Shall we select 20% of the 50 welds for inspection, or shall 20% of the 5000mm (1000mm) be randomly included in the 50 welds, or shall we test all the 50 welds and test 20% of each weld

This mainly depends on what standards you implement.Generally, the pressure vessel according to each weld spot check, the pressure pipeline according to the diameter of different have distinction, the pipe diameter is big, according to each weld spot check, the pipe diameter is small, according to the number of welding joints spot check.