829 15 th International Research/Expert Conference ”Trends in the Development of Machinery and Associated Technology” TMT 2011, Prague, Czech Republic, 12-18 September 2011 CREEP ANALYSIS OF HOT REHEAT LIVE STEAM PIPING SYSTEM Nenad Lorkovic Alstom Power Croatia Mala Svarca 155,Karlovac Croatia ABSTRACT Site inspections of Hot Reheat Piping system have indentifed cracking at a number locations at butt welds between pipe runs and elbows.Furthermore,it was discovered that pipe runs are instaled with wall thickness of SCH60(ANSI/ASME code[3]),and elbows with wall thickness of SCH30 what is not according to design documentation.This wall thickness discrepancy creates a stress enhancement at the connections between elbows and piping runs.Also,because the elbows are more flexible,there could be a propensity for a creep strains to be concentrated in the elbows as the piping system stresses will be relaxed at these lower strength elbows rather than being distributed throughout the main piping runs.So,a detailed finite element analysis,including elastic and creep analysis, is performed to determine if the secondary stresses during relaxation,or if the stress enhacement of the wall thickness discrepancy,play significant roles in the cracking detected in the piping system. Keywords: pipes, crack , creep, stresses 1. FOREWORD During scheduled site inpection of Hot Reheat Live Steam system,lot of cracking have been indentifed at butt welds between pipe runs and elbows[1]. It was also discovered that pipe runs and elbows are not installed according design documentation[2].It could be that wall thickness discrepancy cause stress enhancement at connections between elbows and piping runs.Furthermore, elbows are installed with lower wall thickness and therefore more flexible and weaker.There could be a propensity for creep strains to be concentrated in the elbows with further localization at weld heat affected zones.Also,piping system stresses will be relaxed at these lower strength elbows rather than in main piping runs.So, secondary stresses caused by relaxation in the piping system may act as primary stresses on the elbows [4,5].This phenomenon,of secondary stresses having characteristic of primary stresses resulting in strain localization in a creep weak zone,is commonly referred to as ''elastic follow up''. 2. STRESS ANALYSIS FOR NORMAL OPERATING CONDITIONS Initial stress calculation is performed for normal operating conditions(weight+pressure+thermal expansion) of the system, considering the highest pipe design parameters (e.g. temperature & pressure) [7].Calculation model is created considering ''as built'' design documentation [2,6] for pipe routing,material and pipe supports. Distribution of calculated Von Mises equivalent stresses(given in MPa) is shown on Figure 1. Calculation results are mainly evaluated at elbows.Concentration of stresses at elbows is evident and maximum calculated stresses appear at elbows A & B , signed by arrows on Figure 2. Stresses in main piping runs are predominantly due to pressure.