Creep Life Evaluations of ASME B31.1 Allowance for Variation From Normal Operation

Abstract

The ASME B31.12012 Power Piping Code (Code) paras. 102.2.4, 102.3.3, and 104.8.2 provide an allowance regarding operating above the design temperature and internal pressure for short time periods. This study is a quantitative evaluation of the permitted increased life consumption associated with the 2012 Code operating allowances for piping materials operating in the creep range. Three base metal materials are considered in this paper—ASTM A335 Grades P11, P22, and P91. Results of this evaluation could be used to improve the ASME B31.1 Code, including a technical basis for a recommended revision. The para. 102.2.4 allowables were evaluated: (A) 15% stress increase for 10% of the operating hours and (B) 20% stress increase for 1% of the operating hours. It was determined that these allowances increased the base metal creep rupture life consumption of Grade P11 material up to 8%, Grade P22 material up to 14%, and Grade P91 material up to 25%. Allowance A results in permitting significantly more creep life consumption than Allowance B. An evaluation was performed to realign the increased creep life consumption of Allowance B to be approximately equivalent to the increased creep life consumption of Allowance A. If Allowance B event duration is increased from 80 hrs per year to 400 hrs per year (from 1% to 5% of the operating hours per year), Allowance B increased the creep life consumption which is slightly less than Allowance A life consumption for Grades P11, P22, and P91 materials. Main steam (MS) and hot reheat (HRH) piping system typical operating temperatures and stresses were evaluated for these variation allowances. This study revealed that Grade P22 base metal creep damage is slightly more sensitive to stress than Grade P11 material creep rupture damage, and Grade P91 base metal creep damage is substantially more sensitive to stress than Grade P22 material creep rupture damage.