Marvin J. Cohn

Asset Integrity Management of High Pressure Piping Systems Subject to Creep

Safety and reliability are the preeminent concerns in the design, operation, and maintenance of power piping. Recent additions to the ASME B31.1 Power Piping Code (Code) have addressed condition assessment of covered piping systems (CPS). Mandatory requirements for the condition assessment of CPS are discussed in Chapter VII of the Code and nonmandatory guidelines are discussed in Appendix V of the Code. These documents discuss design, fabrication, construction, operation, and maintenance issues, and do not provide detailed guidance in the evaluation of high pressure piping systems subject to creep.An asset integrity management (AIM) program should integrate and consider all attributes that influence the intended function of the original design. In addition to the evaluation of specified design, fabrication, construction, operation, and maintenance issues, an asset integrity management program should also consider and evaluate significant time-dependent anomalies, such as flexible operation modes, malfunctioning supports, and creep redistributed stresses. An AIM program includes the identification of governing drivers that accelerate piping system damage and then develops countermeasures to mitigate or reduce the driving mechanisms.This paper discusses the large range of piping system stresses and the sensitivity of stress increase to 50% creep life reduction, indicating the need for a robust stress ranking methodology. The process results in an accurate selection of the most critical creep damage weldments for nondestructive examinations.Copyright

Main Steam Piping Girth Weldment Stresses and Life Consumption Considering Malfunctioning Supports

A high energy piping (HEP) program is important for the safety of plant personnel and reliability of the generating units. HEP weldment failures have resulted in serious injuries, fatalities, extensive damage of components, and significant lost generation. Since creep/fatigue is a typical failure mechanism, the probability of HEP failures increases with unit age. The main steam (MS) piping system is one of the most critical HEP systems. Weldment failures are typically due to a combination of high temperature creep and fatigue. Industry best practices include (1) the evaluation of historical operating conditions, (2) examinations of critical weldments to reveal NDE indications, microstructural material damage, and detailed geometry data, (3) hot and cold walkdowns to document the field piping system behavior and anomalies, (4) simulation of as-found piping displacements to estimate actual stresses, (5) ranking of critical weldments, (6) recommendations for support repairs and adjustments, (7) recommendations for future examinations, and (8) remaining life estimates at critical weldments. Appropriate examinations, condition assessments, and recommendations for corrective actions are provided as a cost-effective life management process to maintain the piping system integrity. This paper provides examples demonstrating that the girth welds ranked below the top five to six welds are subject to significantly less applied stress and have substantially more creep/fatigue life than the top ranked welds. Hanger adjustments, along with selective identification, NDE, and possible repairs of top ranked welds provide substantially greater life to MS piping systems. Some fitness-for-service and risk-based programs for MS piping system girth weldments recommend a stress evaluation using typical pressure vessel or boiler tube calculations, in which the hoop stress is the principal stress. In some cases, the effective weldment stresses can be more than 50 percent above the hoop stress, resulting in the estimated remaining lives less than 15 percent of the life estimates using the hoop stress methodology. Some HEP life management programs may vaguely discuss using the principal stress based on a finite element analysis of the piping system. These principal stress values may be based on a conventional as-designed piping stress analysis. In the majority of the as-found piping stress analyses performed by the author, the maximum as-found stresses are substantially greater than the maximum conventional as-designed piping stresses. Consequently, an as-designed piping stress analysis will typically underestimate the life of an HEP system and typically not predict the locations of maximum creep/fatigue damage.Copyright

Ranking of Creep Damage in Main Steam Piping System Girth Welds Considering Multiaxial Stress Ranges

A high-energy piping (HEP) asset integrity management program is important for the safety of plant personnel and reliability of the fossil plant generating unit. HEP weldment failures have resulted in serious injuries, fatalities, extensive damage of components, and significant lost generation. The main steam (MS) piping system is one of the most critical HEP systems. Creep damage assessment in MS piping systems should include the evaluation of multiaxial stresses associated with field conditions and significant anomalies, such as malfunctioning supports and significant displacement interferences. This paper presents empirical data illustrating that the most critical girth welds of MS piping systems have creep failures which can be successfully ranked by a multiaxial stress parameter, such as maximum principal stress. Inelastic (redistributed) stresses at the piping outside diameter (OD) surface were evaluated for the base metal of three MS piping systems at the piping analysis model nodes. The range of piping system stresses at the piping nodes for each piping system was determined for the redistributed creep stress condition. The range of piping stresses was subsequently included on a Larson–Miller parameter (LMP) plot for the grade P22 material, revealing the few critical (lead-the-fleet) girth welds selected for nondestructive examination (NDE). In the three MS piping systems, the stress ranges varied from 55% to 80%, with only a few locations at stresses beyond the 65 percentile of the range. By including evaluations of significant field anomalies and the redistributed multiaxial stresses on the outside surface, it was shown that there is a good correlation of the ranked redistributed multiaxial stresses to the observed creep damage. This process also revealed that a large number of MS piping girth welds have insufficient applied stresses to develop substantial creep damage within the expected unit lifetime (assuming no major fabrication defects). This study also provided a comparison of the results of a conventional American Society of Mechanical Engineers (ASME) B31.1 Code as-designed sustained stress analysis versus the redistributed maximum principal stresses in the as-found (current) condition for a complete set of MS piping system nodes. The evaluations of redistributed maximum principal stresses in the as-found condition were useful in selecting high priority ranked girth weldment creep damage locations. The evaluations of B31.1 Code as-designed sustained load stresses were not useful in selecting high priority creep damage locations.

Optimization of NDE Reexamination Locations and Intervals for Grade 91 Piping System Girth Welds

It has become apparent with the development of creep strength enhanced ferritic steels, the mandatory ASME B31.1 Chapter VII and the non-mandatory ASME B31.1 Appendix V guidelines require a more rigorous method to manage the Grade 91 piping integrity at Genesee Unit 3. Given the relatively young age of Genesee Unit 3, three questions have been asked: 1) when do the examinations start, 2) what locations should be examined first, and 3) how often should the same location be reexamined? To ensure that the best value is obtained from the reexamination budget, a five-step process can be effectively used to define and categorize the scope of each set of reexaminations in the girth weld integrity management program. The five processes are performing the following analyses: 1) an evaluation of the historical information, 2) piping system hot and cold walkdowns, 3) as-designed and as-found piping stress analyses, 4) creep life consumption evaluations, including elastic and inelastic axial and radial stress redistributions, and 5) creep crack growth curve analyses. Reexaminations of the few critical lead-the-fleet weldments are performed with lower examination costs and higher confidence.Evaluations of the Genesee Unit 3 main steam (MS) piping system revealed that the applicable weldment stress is probably the most significant parameter in determining the Grade 91 girth weld critical reexamination locations and intervals. ASME B31.1 piping stress analyses of the MS piping system have sustained load stress variations of more than 100% among the girth welds. The lower bound American Petroleum Institute (API) 579 creep rupture equation for Grade 91 operating at 1,060°F (571°C) indicates that the creep life is a function of stress to the power of 8.9; consequently, a 15% stress increase results in about 2/3 reduction of creep rupture life. Creep crack growth analyses of several of the MS piping system weldments revealed that the creep crack growth time to grow from 1/8 inch to through-wall is a function of stress to the power of 8.8; consequently, a 15% stress increase results in about 2/3 reduction of time for a 1/8-inch crack to grow through-wall.This evaluation reveals that a few critical lead-the-fleet locations should be reexamined most frequently and justification can be provided for much longer reexamination intervals of the remaining girth welds with much lower applied stresses.Copyright

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

The ASME B31.1-2012 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 an evaluation of the permitted increased life consumption associated with the above Code operating allowances for piping materials operating in the creep range. Three base metal materials are considered in this study, ASTM A335 Grades P11, P22, and P91.Two case studies 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 damage life consumption than Allowance B.Main steam and hot reheat 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.Copyright

Frequency Distribution Curves for Main Steam Piping Multiaxial Stresses

A high energy piping (HEP) asset integrity management program is important for the safety of plant personnel and reliability of the generating unit. HEP weldment failures have resulted in serious injuries, fatalities, extensive damage of components, and significant lost generation. The main steam (MS) piping system is one of the most critical HEP systems. Creep damage assessment in MS piping systems should include the evaluation of multiaxial stresses associated with field conditions and significant anomalies, such as malfunctioning supports and significant displacement interferences.This paper presents empirical data illustrating that lead-the-fleet girth welds of MS piping systems have creep failures which can be successfully ranked by a multiaxial stress parameter, such as maximum principal stress. Both the as-found elastic (initial) stress and inelastic (redistributed) stress at the piping outside diameter surface are evaluated for the base metal of three MS piping systems. Frequency distribution curves are then developed for the initial and redistributed piping stresses. The frequency distribution curves are subsequently included on a Larson Miller Parameter (LMP) plot for the applicable material, revealing the few critical (lead-the-fleet) girth welds selected for nondestructive examination (NDE).By including an evaluation of significant field anomalies, multiaxial operating stress on the outside surface, and weldment performance, it is shown that there is a good correlation of calculated creep stress versus the operating time of observed creep damage. This process also reveals the large number of MS piping girth welds that have insufficient applied stress to have substantial creep damage within the expected unit life time (assuming no major fabrication defects).API 579 recommends an effective stress to compute the creep rupture life using the LMP. This constitutive stress equation includes a combination of the maximum principal, von Mises, and hydrostatic stresses. Considering the stresses in these three MS piping systems, this paper reveals that when the axial and hoop stresses are nearly the same values, the API 579 effective stress may be 10% greater than the maximum principal stress. However, the maximum principal stresses are greater than the API 579 effective stresses at the maximum stress locations in the three MS piping systems, because the axial stresses are significantly greater than the hoop stresses.This study also provides a comparison of the results of a conventional American Society of Mechanical Engineers (ASME) B31.1 Code as-designed sustained stress analysis versus the redistributed maximum principal stresses for a complete set of MS piping system nodes. A comparison of Code-sustained load versus redistributed maximum principal stress results are illustrated on frequency distribution curves.Copyright

Fitness for Service of Degraded Grade 91 Pipe

The use of creep strength enhanced ferritic alloys such as Grade 91 in fossil power plants has become popular for high temperature piping applications. Since Grade 91 has higher stress allowables than Grade 22, a designer can specify thinner component wall thicknesses, resulting in lower through-wall thermal stresses during transient events and lower material and piping support costs.During the past two decades, Grade 91 has been used successfully in fossil power plants. However, this alloy has had some incidents of non-optimal weldment microstructure. In this case study, Brinell hardness tests of an ASME A182 Grade F91 (F91) wye block, including upstream and downstream F91 spools, revealed several readings of soft material, as low as 168HB. A study of creep rupture tests of degraded Grade 91 specimens revealed that the lower bound creep rupture curve of the degraded Grade 91 material is above the average creep rupture curve of Grade 22 material for the range of the specific piping operating stresses.Based on the empirical evidence that the average Grade 22 material creep rupture curve is conservative for the creep rupture of degraded Grade 91 material, a life consumption evaluation was performed for the degraded Grade 91 weldments using Grade 22 creep rupture properties. A life fraction analysis was performed considering the redistributed maximum principal stresses, based on simulation of piping displacements obtained from the hot and cold walkdowns. This study also considered the recent history of the specific piping system operating pressures and temperatures.This study also considered dissimilar metal welds, from ASME A182 Grade F91 (F91) to ASME A335 Grade P22 (P22) materials. It was determined that the Grades F91-to-F91 weldments had about 30% life consumption and the remaining lives were at least 7 years. The Grades F91-to-P22 weldments had less than 40% life consumption and the remaining lives were at least 15 years.Copyright

Comparison of ASME B31.1 Sustained Load Stresses to Corresponding Tresca Stresses

Conventional United States designs of high energy piping (HEP) systems use the American Society of Mechanical Engineers (ASME) B31.1 Power Piping Code. The analytical methodology in this code is based on linear elastic beam theory. The ASME B31.1-2010 Power Piping Code (Code) [1] recommends Equation 15 to calculate the piping stress due to sustained loads. Many practitioners believe that the sustained load stress (SL) results using Equation 15 are not significantly less than using a Tresca methodology for the same set of forces and moments. This paper provides a comparison of the ASME B31.1 SL stresses to the corresponding Tresca stresses in parent material, based on empirical HEP system stress analyses. The results of three piping system evaluations are considered, including examples of longitudinal stress lower than the circumferential stress and examples where the longitudinal stress is greater than the circumferential stress.This study considers the elastic primary stresses on the outside surface of the pipe, prior to any creep redistribution. At locations where the longitudinal stress is greater than the circumferential stress, the SL stress is nearly the same as the elastic Tresca stress. At locations where the longitudinal stress is considerably less than the circumferential stress, the SL stress is considerably less than the elastic Tresca stress. This conclusion is due to the fact that the SL stress is primarily governed by longitudinal loading.The paper also considers inelastic primary stresses, after complete creep redistribution. For piping materials operating in the creep regime, the axial and circumferential pressure stresses are eventually redistributed and are maximum at the outer surface of the pipe. After several years of operation, the Code SL stresses and elastic Tresca stresses are significantly less than the inelastic Tresca stresses. Consequently, the use of SL stresses and elastic Tresca stresses for estimating component inelastic primary stresses would be nonconservative.Copyright

Main Steam Piping Creep Life Consumption in Circumferential Welds

A high energy piping (HEP) asset integrity management program is important for the safety of power plant personnel and reliability of the generating units. HEP weldment failures have resulted in extensive damage of components and significant lost generation. The main steam (MS) piping system is one of the most critical HEP systems. Creep damage assessment in MS piping systems should include the evaluation of multiaxial stresses associated with the field conditions. Typical creep life assessment stress parameters and estimated failure times are evaluated and compared with those of three MS piping system girth weld creep failures. This paper presents empirical data indicating that lead-the-fleet girth welds of MS piping systems have creep failures which can be successfully predicted by a multiaxial stress parameter, such as maximum principal stress. The calibration study indicates that the parent metal maximum principal stress should be increased by more than 20% to predict reasonable circumferential weldment lives in 2-1/4Cr-1Mo material. The correlation of other stress parameters, such as hoop stress, longitudinal membrane stress, and the standard as-designed ASME B31.1 sustained load stress do not provide an adequate ranking of the most critical girth welds subject to creep. In some piping systems, it is possible that spool-to-spool and circumferential variations in pipe wall thicknesses may influence the weldment life consumption estimates. Therefore, field wall thickness measurements should be taken at the most critical stress locations and applied to the life consumption evaluations.Copyright

Fitness-for-Service of Longitudinal Seam Welds

A high energy piping (HEP) asset integrity management program is important for the safety of power plant personnel and reliability of the generating units. Hot reheat (HRH) longitudinal seam weld failures have resulted in serious injuries, fatalities, extensive damage of components, and significant lost generation. The HRH piping system is one of the most critical HEP systems. Since high temperature creep is a typical failure mechanism for longitudinal seam welds, the probability of failure increases with unit operating hours. This paper concludes that some seam welded spools in this specific HRH piping system are more likely to fail earlier than other spools, depending on their actual wall thicknesses and operating temperatures. In this case study, the HRH piping system has operated over 200,000 hours and experienced about 400 starts since commercial operation. There are two separate HRH lines, Lines A and B, for this piping system. The 36-inch OD pipe has a specified minimum wall thickness (MWT) of 1.984 inches. Pipe wall thicknesses were measured in 57 spools. The measured spool MWT values varied from 1.981 to 2.122 inches. On average, Line A operated about 8°F higher than Line B. A comparative risk assessment was performed using the estimated average temperatures and pressures throughout the life of this HRH piping system. Data associated with the reported failures or near failures of seam welded Grade 22 piping systems were plotted as log σHoop versus the Larson Miller Parameter (LMP). The range of log σHoop and LMP values for this unique piping system was also plotted, based on the average operating pressure and the range in the average operating temperatures and the measured spool MWT values. The Line A (with a higher average operating temperature) seamed spool having the lowest measured MWT fell slightly above the threshold line of reported seam weld pipe failures. The Line B (with a lower average operating temperature) seamed spool having the lowest MWT is about 10 operating years from reaching the threshold of reported seam weld pipe failures. The Line A seamed spool having the highest measured MWT is about 8 operating years from reaching the threshold of reported seam weld pipe failures. The Line B seamed spool having the highest measured MWT is more than 18 operating years from reaching the threshold of historical seam weld pipe failures.© 2010 ASME