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.

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.

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.

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.

Comparison of a Life Consumption Analysis to Section III, Subsection NH Design Rules for a Main Steam Girth Weld Creep Crack

2003 ◽  
Author(s):  
Marvin J. Cohn
Keyword(s):  
Power Plants ◽  
Creep Damage ◽  
High Energy ◽  
Piping System ◽  
Design Rules ◽  
Class 1 ◽  
American Society ◽  
Girth Welds ◽  
Main Steam ◽  
Life Consumption

Creep damage of high energy piping (HEP) systems in fossil fuel power plants results from operation at creep range temperatures and high stresses over many years. Typically, the operating stresses in an HEP piping system are substantially below the yield stress. They tend to be load controlled and time dependent. In spring 1999, Arizona Public Service Company performed an examination of several girth welds of a main steam piping system at Cholla Power Station, Unit 2. A significant creep-related crack was found in a weld after 158,000 operating hours. The American Society of Mechanical Engineers (ASME) Subsection NH methodology was used to evaluate the load controlled stress design rules for nuclear Class 1 components in elevated temperature service as applied to this piping system. A high energy piping life consumption (HEPLC) analysis was performed prior to the examination to select and rank the most critical welds. After obtaining critical information during the outage, the software was also used to estimate the life exhaustion at the most critical weld. A discussion of results for the two approaches is provided in this paper.

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.

The TransAlta High-Energy Piping Program—A Five-Year History

2000 ◽  
Vol 123 (1) ◽  
pp. 65-69 ◽  
Author(s):  
Marvin J. Cohn,
Keyword(s):  
Creep Damage ◽  
Analytical Techniques ◽  
High Energy ◽  
Management Plan ◽  
Piping System ◽  
Engineering Consulting ◽  
Piping Systems ◽  
Creep Fatigue ◽  
Sustained Loads ◽  
Selection Of

In 1995, the High-Energy Piping Strategic Management Plan (HEPSMP) was initiated at TransAlta Utilities Corporation (TAU) for the three generating facilities. At that time, it was recognized that several of the piping systems were exhibiting signs of creep relaxation, with some hangers bottomed or topped out online and/or offline. Previous hanger adjustment attempts were not always adequate. The program workscope included: 1) hot and cold piping system walkdowns, 2) selection of high-priority girth weld inspection locations, 3) examination of critical weldments, 4) weld repairs where necessary, 5) adjustments or modifications of malfunctioning steam line hangers, and 6) recommended work for future scheduled outages. Prior to 1996, examination locations were limited to the traditional locations of the terminal points at the boiler and turbine, with reexaminations occurring at arbitrary intervals. Since the terminal points are not necessarily the most highly stressed welds causing service-related creep damage, service damage may not occur first at the pre-1996 examined locations. There was a need to maximize the safety and integrity of these lines by ensuring that the highest risk welds were identified and given the highest priority for examination. An engineering consulting company was selected to prioritize the highest risk weldments for each piping system. This risk-based methodology included the prediction and evaluation of actual sustained loads, thermal expansion loads, operating loads, multiaxial stresses, creep relaxation, and cumulative creep life exhaustion. The technical process included detailed piping system walkdowns and application of advanced analytical techniques to predict and rank creep/fatigue damage for each piping system. TAU has concluded that the program has met its objective of successfully prioritizing inspection locations. The approach has also resulted in reducing the scope and cost of reexaminations. Phases 1 and 2 evaluations and examinations have been completed for all units. Results of some of the important aspects of this program are provided as case history studies.

UNCONVENTIONAL BOREHOLE BREAKOUT ROTATION ANALYSIS PROVIDES A QC TOOL FOR STRESS MODELS

2006 ◽  
Vol 46 (1) ◽  
pp. 307
Author(s):  
B.A. Camac ◽  
S.P. Hunt ◽  
P.J. Boult ◽  
M. Dillon
Keyword(s):  
Principal Stress ◽  
Input Data ◽  
Distinct Element ◽  
Maximum Principal Stress ◽  
Principal Stresses ◽  
Stress Regime ◽  
Seal Integrity ◽  
Borehole Breakout ◽  
Stress Models ◽  
Kupe Field

In distinct element (DEM) numerical stress modelling, the principal stress magnitudes and orientations are applied to the boundary of the 3D model. Due to data restrictions and typical depths of investigation, it is possible to have much uncertainty in the conventional methodologies used to constrain the regional principal stress magnitudes and orientations.A case study from the Kupe field in the Taranaki Basin, New Zealand is presented where the uncertainty in the input data made it difficult to determine which stress regime—a transitional normal/strike-slip or reverse/thrust—is active at reservoir depth (approximately 3,000 m). The magnitudes and orientation of the principal stresses were constrained using published techniques. A sensitivity analysis was applied to account for the uncertainty in the input data. A model of the Kupe field incorporating 18 major faults was subsequently loaded under both derived stressed regimes, using the calculated magnitudes.Borehole breakout analysis was used to acquire interpreted orientations of the maximum principal stress (Shmax). The work presented herein describes a different or unconventional approach to the general petroleum geomechanics methodology. Typically, the breakout data is averaged to get one data point per well location. Here, all breakout data is retained and displayed vertically. The data is actively used and the variations with depth can be seen to show how faults can generate local perturbations of the regional stress trajectory. These data are then used to compare the observed or field indications of the breakouts along the borehole with the modelled Shmax predicted by both end point DEM stress models. This comparison has provided additional confidence in the derived stress regime and the derived stress models for the Kupe field. The stress models are used to predict areas of enhanced hydrocarbon pooling and low seal integrity.

Creep Relaxation Behavior of High-Energy Piping

2000 ◽  
Vol 122 (4) ◽  
pp. 488-493 ◽  
Author(s):  
Raymond K. Yee ◽  
Marvin J. Cohn
Keyword(s):  
Stress Relaxation ◽  
Thermal Loading ◽  
Elevated Temperatures ◽  
Three Dimensional ◽  
Creep Damage ◽  
High Energy ◽  
External Loading ◽  
Piping System ◽  
Dead Weight ◽  
Piping Systems

The analysis of the elastic stresses in high-energy piping systems is a routine calculation in the power and petrochemical industries. The American Society of Mechanical Engineers (ASME) B31.1 Power Piping Code was developed for safe design and construction of pressure piping. Postconstruction issues, such as stress relaxation effects and selection of maximum expected creep damage locations, are not addressed in the Code. It has been expensive and time consuming to evaluate creep relaxation stresses in high energy piping systems, such as main steam and hot reheat piping. After prolonged operation of high-energy piping systems at elevated temperatures, it is very difficult to evaluate the redistribution of stresses due to dead weight, pressure, external loading, and thermal loading. The evaluation of stress relaxation and redistribution is especially important when nonideal conditions, such as bottomed-out or topped-out hangers, exist in piping systems. This paper uses three-dimensional four-node quadrilateral shell elements in the ABAQUS finite element code to evaluate the time for relaxation and the nominal relaxation stress values for a portion of a typical high-energy piping system subject to an ideally loaded hanger or to an overloaded hanger. The stress relaxation results are evaluated to suggest an approximation using elastic stress analysis results. [S0094-9930(00)01304-4]

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.

On-Line Health Monitoring Research on T-Joint of Main Steam Piping

2013 ◽  
Vol 330 ◽  
pp. 549-552 ◽  
Author(s):  
J.H. Jia ◽  
H.C. Zhang ◽  
X.Y. Hu ◽  
L.P. Cai ◽  
S.T. Tu
Keyword(s):  
High Temperature ◽  
Power Plant ◽  
Residual Life ◽  
Thermal Power ◽  
Creep Damage ◽  
Piping System ◽  
Main Challenge ◽  
On Line ◽  
Main Steam ◽  
Creep Monitoring

The main challenge of long-time creep monitoring on site is a reliable sensor. In this paper, a sensing device is developed specifically for high temperature creep monitoring. And it is applied to on-line monitor the strain of material on T-joint of main steam piping. Its reliability is verified theoretically using the finite element method and experimentally by high temperature on site test. The creep damage of the T joint is evaluated basing on the creep rate sensed by the sensing device. And the residual life is predicted for the piping system using the Monkman-Grant equation. This system is useful for safety assessment procedures in thermal power plant, nuclear power plant and petrochemical industries.

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