A visual and UT inspection of a 27'-0" diameter, exposed, steel penstock at a hydropower station has revealed numerous pits due to corrosion, both iside and out. The original plate thickness was 1". Some of the corrosion pits, which reduce the plate thickness down to .25" in some areas, are isolated but many are in clusters of 1 foot square or more.
My question is: knowing the internal pressure which causes the hoop stresses and the loading which causes the beam stresses in the penstock, is it reasonable or too conservative to use the reduced plate section in the combined stress analysis equation? Should isolated corrosion pits be assessed as if the entire penstock were of that thickness? And, if not, at what point, as the pitting forms larger areas of reduced thickness would one base the analysis on a total penstock thickness of that reduced area thickness? ASCE manual No. 79 "Steel Penstocks," Section 16.6.4 only states, "From this data (information obtained through testing) the associated stress level can be determined and compared against the maximum allowable stress." It doesn't elaborate further.
Thanks.
My question is: knowing the internal pressure which causes the hoop stresses and the loading which causes the beam stresses in the penstock, is it reasonable or too conservative to use the reduced plate section in the combined stress analysis equation? Should isolated corrosion pits be assessed as if the entire penstock were of that thickness? And, if not, at what point, as the pitting forms larger areas of reduced thickness would one base the analysis on a total penstock thickness of that reduced area thickness? ASCE manual No. 79 "Steel Penstocks," Section 16.6.4 only states, "From this data (information obtained through testing) the associated stress level can be determined and compared against the maximum allowable stress." It doesn't elaborate further.
Thanks.