| Now, pressure piping is extensively used in petrochemical,metallurgy,electric power,medicine and so on. It is also developing with large-scale,diversification,and complication. But how to deal with the pressure pipings with defects is an important question in project area.Penstock is a kind of large pressure piping and is an important constitute of the building of power plant with water-transportation. It takes on bigger inner hydraulic pressure, the weight of itself and the watery weight. During the manufacture and fixing of penstock, it is easily to get welding defects and can be repaired by repair welding, but some shallow defects are usually neglected and don't bring the recognition. Where the defects are, the size of received section could be changed. It results in bigger stress concentration, mostly reduces the bearing power of the welded joint and has an important impact on the static strength, the fatigue strength, the brittle fracture and the resistance to stress erode cracking. So it is necessary to finish the mechanics analysis and safety assessment of the penstocks with welding defects.Undercut and unfitness are common welding defects in the producing and fixing of penstocks. They are hard to be analyzed by using general methods. In this paper, based on the fact of hydroelectric power station, three-dimensional finite element models of undercut and unfitness are built. And the finite element software ANSYS 9.0 would be used to have a stress account and analysis on the undercut and unfitness with different depth, sharpness, and place. Secondly the affection of the defects in different conditions at the welded joint with stress concentration is discussed.By using CVDA-84 and "Safety assessment for in-service pressure vessels containing defects", this paper makes safety assessment about the penstocks with longitudinal undercut of different depth and works out the largest depth of undercut allowed. It has large significance for engineering.
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