Assessment of the primary loads in pipes with D/t larger 50 according to the nuclear codes

The German code KTA 3211.2 for the evaluation of primary stresses in piping systems includes stress indices B2 that are only valid for S/t smaller or equal 50. According to KTA 3211.2 under certain conditions the primary stresses for pipes (test group A 2/3) can also be calculated using stress factors i, that are valid for D/t smaller or equal 100. The ASME Code revision 2007 contains stress indices for the total range 50-100. The contribution compares the allowable moment values based on B2 and on the i-factors for thin-walled pipes under low inner pressure. It is shown that for pipes with D/t about 50 the i-factors give significantly higher moments. Using the B2 stress indices as found in the ASME code even higher differences of the allowable moments are found. The authors' FE calculations performed for straight pipes with D/t = 100 taking into account imperfections revealed the necessity of corrections for the B2 values (D/t larger 50) given in KTA. This fact is confirmed by an evaluation of experimental data in the published literature. The reason is among others the failure behaviour of thin-walled pipes that do not fail in the elastic-plastic range by plastic strains but by buckling. It is recommended to surrender in KTA 3211.2 the requirement of an evaluation based on i-factors and to include the corrections for stress indices from the ASME code