Life Management of Main Steam and Hot Reheat Piping Systems: Part 1

2006 ◽  
Author(s):  
Marvin J. Cohn
Keyword(s):  
Data Collection ◽  
Terminal Point ◽  
Rational Approach ◽  
Critical Examination ◽  
Piping System ◽  
Material Damage ◽  
Life Management ◽  
Piping Systems ◽  
Girth Welds ◽  
Main Steam

Many utilities choose critical girth welds in their main steam (MS) and hot reheat (HRH) steam piping by consideration of some combination of the ASME B31.1 Code [1] (Code) highest sustained load and thermal expansion stresses, terminal point locations, and fitting weldments subject to stress intensification. As an alternative, a life management methodology is used to prioritize material damage locations based on realistic stresses and applicable damage mechanisms. This methodology is customized to each piping system, considering applicable affects, such as operating history, measured weldment wall thicknesses, observed support anomalies, actual piping thermal displacements, and more realistic time-dependent multiaxial stresses. The life management methodology for MS and HRH critical girth welds may be considered as a rational approach to determine critical weldment locations for examinations and to determine appropriate reexamination intervals as a risk-based evaluation technique. This methodology has been implemented over the past 15 years to provide more realistic estimates of actual displacements, strains, stresses, and material damage based on the evaluation of field conditions. This high energy piping life consumption (HEPLC) methodology can be described as having three basic phases: data collection, evaluation, and recommendations. The data collection phase includes obtaining design and post construction piping and supports information. The effects of current anomalies are evaluated to prioritize critical examination locations. Results of the examinations at the most critical locations are used to determine the degree of material damage at lead-the-fleet locations. The author has performed many HEPLC studies of MS and HRH piping systems. This paper will provide examples of data collection results and documentation of observed piping system anomalies.

Life Management of Main Steam and Hot Reheat Piping Systems: Part 2

2006 ◽  
Author(s):  
Marvin J. Cohn
Keyword(s):  
Data Collection ◽  
Fatigue Damage ◽  
Terminal Point ◽  
Piping System ◽  
Material Damage ◽  
Life Management ◽  
Piping Systems ◽  
Creep Fatigue ◽  
Girth Welds ◽  
Main Steam

Since there have been several instances of weldment failures in main steam (MS) and hot reheat (HRH) piping systems, most utilities have developed programs to examine their most critical welds. Many utilities select their MS and HRH critical girth welds for examination by consideration of some combination of the ASME B31.1 Code [1] (Code) highest sustained stresses, highest thermal expansion stresses, terminal point locations, and fitting weldments. This paper suggests the use of an alternative life management methodology to prioritize material damage locations based on realistic stresses and applicable damage mechanisms. This methodology is customized to each piping system, considering applicable affects, such as operating history, measured weldment wall thicknesses, observed support anomalies, actual piping thermal displacements, and more realistic time-dependent multiaxial stresses. The high energy piping life consumption (HEPLC) methodology for MS and HRH critical girth welds may be considered as a rational approach to determine critical weldment locations for examinations and to determine appropriate reexamination intervals as a risk-based evaluation technique. The HEPLC methodology has been implemented over the past 15 years to provide more realistic estimates of actual displacements, stresses, and material damage based on the evaluation of field conditions. This HEPLC methodology can be described as having three basic phases: data collection, evaluation, and recommendations. The data collection phase includes obtaining design and post construction piping and supports information. The effects of current piping loads and anomalies are evaluated for potential creep/fatigue damage at the most critical weldments. The top ranked weldments of the HEPLC study are than selected as the highest priority examination locations. The author has completed many HEPLC studies of MS and HRH piping systems. The previous paper (Part 1) provided examples of data collection results and documentation of observed piping system anomalies. This paper will provide examples of evaluation results and recommendations, including a few case histories that have correctly ranked and predicted locations of significant creep/fatigue damage.

Risk-Based Inspection Applied to Main Steam and Hot Reheat Piping Systems

2007 ◽  
Author(s):  
Marvin J. Cohn
Keyword(s):  
Cost Effective ◽  
High Energy ◽  
Accurate Estimate ◽  
Terminal Point ◽  
Rational Approach ◽  
Piping System ◽  
Piping Systems ◽  
Risk Areas ◽  
Main Steam ◽  
Life Consumption

Many utilities select critical welds in their main steam (MS) and hot reheat (HRH) piping systems by considering some combination of design-based stresses, terminal point locations, and fitting weldments. The conventional methodology results in frequent inspections of many low risk areas and the neglect of some high risk areas. This paper discusses the use of a risk-based inspection (RBI) strategy to select the most critical inspection locations, determine appropriate reexamination intervals, and recommend the most important corrective actions for the piping systems. The high energy piping life consumption (HEPLC) strategy applies cost effective RBI principles to enhance inspection programs for MS and HRH piping systems. Using a top-down methodology, this strategy is customized to each piping system, considering applicable effects, such as expected damage mechanisms, previous inspection history, operating history, measured weldment wall thicknesses, observed support anomalies, and actual piping thermal displacements. This information can be used to provide more realistic estimates of actual time-dependent multiaxial stresses. Finally, the life consumption estimates are based on realistic weldment performance factors. Risk is defined as the product of probability and consequence. The HEPLC strategy considers a more quantitative probability assessment methodology as compared to most RBI approaches. Piping stress and life consumption evaluations, considering existing field conditions and inspection results, are enhanced to reduce the uncertainty in the quantitative probability of failure value for each particular location and to determine a more accurate estimate for future inspection intervals. Based on the results of many HEPLC projects, the author has determined that most of the risk (regarding failure of the pressure boundary) in MS and HRH piping systems is associated with a few high priority areas that should be examined at appropriate intervals. The author has performed many studies using RBI principles for MS and HRH piping systems over the past 15 years. This life management strategy for MS and HRH critical welds is a rational approach to determine critical weldment locations for examinations and to determine appropriate reexamination intervals as a risk-based evaluation technique. Both consequence of failure (COF) and likelihood of failure (LOF) are considered in this methodology. This paper also provides a few examples of the application of this methodology to MS and HRH piping systems.

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

2016 ◽  
Vol 138 (4) ◽  
Author(s):  
Marvin J. Cohn ◽  
Fatma G. Faham ◽  
Dipak Patel
Keyword(s):  
Creep Damage ◽  
High Energy ◽  
Sustained Load ◽  
Piping System ◽  
Multiaxial Stress ◽  
Analysis Model ◽  
Principal Stresses ◽  
Piping Systems ◽  
Girth Welds ◽  
Main Steam

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.

Power Piping Grade 91 In-Service Cracks

2019 ◽  
Author(s):  
Marvin J. Cohn ◽  
Steve R. Paterson ◽  
Keith Rapkin ◽  
Charles Henley ◽  
Erick Liebl ◽  
...  
Keyword(s):  
Time Dependent ◽  
Piping System ◽  
Material Damage ◽  
Nondestructive Examination ◽  
Commercial Operation ◽  
Piping Systems ◽  
Grade 91 ◽  
Main Steam ◽  
Creep Regime ◽  
Selection Of

Abstract After a power piping system has begun commercial operation, it is recommended that the initial nondestructive examination (NDE) of the welds should be developed and executed to reveal possible fabrication and early service-related material damage. The identified indications and other possible anomalies should be thoroughly documented, evaluated, and dispositioned. Additional early service-related cracks may initiate and propagate several years after the first examination. This paper considers historical early service-related cracks in weldments of power piping systems operating in the creep regime. Factors that dominate these early service-related cracks are discussed in this paper. This paper provides a list of more than 20 historical examples of power piping Grade 91 material macrocracks (partial wall or through-wall) that were not present immediately after construction and propagated substantially in service. Five of the examples are discussed in more detail. The study only includes cases where propagating cracks were confirmed. It does not include examples where NDE reportable indications have not propagated in service to large macrocracks. Due to the time-dependent nature of these girth weld cracks, the results of this study may be used to assist in the selection of Grade 91 reexamination locations after the power piping system (e.g., main steam, hot reheat, high pressure, or intermediate pressure) has been several years in service.

Frequency Distribution Curves for Main Steam Piping Multiaxial Stresses

2014 ◽  
Author(s):  
Marvin J. Cohn ◽  
Fatma G. Faham ◽  
Dipak Patel
Keyword(s):  
Principal Stress ◽  
Frequency Distribution ◽  
Creep Damage ◽  
Maximum Principal Stress ◽  
Piping System ◽  
Principal Stresses ◽  
Piping Systems ◽  
Girth Welds ◽  
Main Steam ◽  
Hoop 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.

Main Steam Piping Creep Life Consumption in Circumferential Welds

2011 ◽  
Author(s):  
Marvin J. Cohn
Keyword(s):  
Principal Stress ◽  
Maximum Principal Stress ◽  
Life Assessment ◽  
Piping System ◽  
Creep Life ◽  
Piping Systems ◽  
Girth Welds ◽  
Main Steam ◽  
Life Consumption ◽  
Stress Parameters

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.

Case History of Solidification Cracks in 2-1/4Cr 1Mo Low Carbon Welds — Cholla Unit 2

2002 ◽  
Author(s):  
Marvin J. Cohn ◽  
Steve R. Paterson ◽  
Dan Nass
Keyword(s):  
Weight Percent ◽  
Piping System ◽  
Low Carbon ◽  
Weld Deposit ◽  
History Of ◽  
Solidification Cracks ◽  
Girth Welds ◽  
Main Steam ◽  
Life Consumption ◽  
Metallurgical Evaluation

An examination of the main steam girth welds at Cholla Unit 2 was performed during a scheduled outage in Spring 1999. The examination revealed two distinct types of cracks. Nine girth welds had extensive arrays of small discontinuous ultrasonic examination indications in the weld deposit near the weld roots. Two girth welds had cracks connected to the outside surface of the pipe. Justifications for removing and replacing 11 of the 35 examined main steam girth welds are presented in this paper. Nondestructive examinations revealed small discontinuous indications near the root of several welds and throughout the weld deposit. In the most severe cases, these weld metal indications extended all the way around the circumference of the pipe. A metallurgical evaluation of both shop and field welds determined that the indications were arrays of small solidification cracks, typically 1/16-inch high by 1/32-inch long. The solidification cracks were attributed to wide weave beads in combination with low carbon content consumables. There was also a concern that those weld deposits with very low carbon (less than 0.05 weight percent) may have significantly shorter creep lives. In addition to the fabrication-induced solidification cracks, two girth welds were identified with service-induced creep cracks. The first of these was detected during ultrasonic and fluorescent magnetic particle examinations of selected welds. The second was detected visually in an auxiliary steam piping weld connection that was identified as a high priority weld resulting from a life consumption evaluation of the piping system.

Paper 10: Steam-Piping Installations

1966 ◽  
Vol 181 (14) ◽  
pp. 34-43
Author(s):  
D. Dimmock
Keyword(s):  
Layout Design ◽  
Piping System ◽  
Power Station ◽  
Pressure Drops ◽  
Plant Expansion ◽  
Piping Systems ◽  
Service Conditions ◽  
Main Steam ◽  
Project Engineer ◽  
Practical Nature

This paper is intended to be of a practical nature and deals with the various aspects and problems which require to be considered and dealt with by a project engineer responsible for the layout, design, and engineering of a main steam piping system. Two types of installation are considered: (a) Main steam piping for power station plants operating within a temperature range of 482°C (900°F) and 568°C (1055°F), and (b) Long steam distribution lines operating up to a temperature of 468°C (875°F). Although many of the problems are common to both types of installation such as pressure drops, terminal reactions on plant, expansion, layout, etc., these in both cases are discussed and procedures proposed for the preliminary and final stages of design, in order to arrive at a reliable and economic installation. The necessity of preliminary and final detailed analysis is shown and the importance of considering the various types of supports, guides, and restraints in conjunction with this work. Other features dealt with are the drainage of steam lines, cleanliness during erection, clean service conditions, lagging, and their contribution to satisfactory operation of the piping systems.

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

2009 ◽  
Author(s):  
Marvin J. Cohn
Keyword(s):  
Stress Analysis ◽  
Principal Stress ◽  
Hoop Stress ◽  
High Energy ◽  
Operating Conditions ◽  
Piping System ◽  
Life Management ◽  
System Integrity ◽  
Creep Fatigue ◽  
Main Steam

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.

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

2010 ◽  
Vol 132 (5) ◽  
Author(s):  
Marvin J. Cohn
Keyword(s):  
Stress Analysis ◽  
Principal Stress ◽  
Hoop Stress ◽  
High Energy ◽  
Operating Conditions ◽  
Piping System ◽  
Life Management ◽  
System Integrity ◽  
Creep Fatigue ◽  
Main Steam

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 nondestructive examination (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% above the hoop stress, resulting in the estimated remaining lives less than 15% 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. In the example case study, the maximum effective weldment stress was more than three times greater than the estimated as-designed piping stress at the same location. This paper illustrates than an as-designed piping stress analysis will typically overestimate the life of an HEP system and typically not predict the locations of maximum creep/fatigue damage.

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