Bull Run Fossil Plant Main Steam Piping Creep Evaluation

2007 ◽  
Author(s):  
Darryl A. Rosario ◽  
Blaine W. Roberts ◽  
M. Scott Turnbow ◽  
Salah E. Azzazy
Keyword(s):  
Power Plants ◽  
Structural Integrity ◽  
Piping System ◽  
Fossil Plant ◽  
Contributing Factors ◽  
Engineering Consulting ◽  
Commercial Operation ◽  
Burning Coal ◽  
Girth Welds ◽  
Main Steam

Bull Run Unit 1, rated at 950 MW, is the first of four fossil supercritical power plants at TVA; the unit went into commercial operation in 1967. The boiler, built by Combustion Engineering (CE), has a radiant reheat twin divided furnace with tangential-fired burners for burning coal. The unit’s maximum continuous rating (MCR) is 6,400,000 lbs/hr of main steam flow, with a design temperature of 1003°F and pressure of 3840 psig. Through the end of November 2003, the unit had a total of 589 cumulative starts and 253,343 operating hours. In 1986 TVA located and repaired extensive cracking in the mixing link headers (27 of 32 saddle welds cracked) downstream of the superheater outlet headers. Visible sag was also noted at the mid-span of the mixing headers. Since that time through 2003, additional cracking of girth welds in the mixing link headers was discovered, followed by cracking in the main piping girth welds at the connections to the mixing headers and at one of the connections to the turbine. From 1988 through 2003 several elastic analyses which were performed were unable to explain the observed girth weld cracking and sagging in the piping. In October 2003, TVA contracted with Structural Integrity Associates (SI) and BW Roberts Engineering Consulting to perform elastic and creep analyses of the Bull Run main steam piping system to determine the most likely contributing factors to noticeable creep sagging and cracking problems in the mixing header link piping and main steam piping girth welds, and, to develop recommendations to mitigate additional cracking and creep/sagging. The evaluations concluded that improper hanger sizing along with longer-term hanger operational problems (non-ideal loads/travel, topped/bottomed out hangers) contributed to the observable creep sagging and girth weld cracking. The elastic and creep piping analyses performed to address these issues are described in this paper.

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.

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.

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.

Bull Run Fossil Plant: Technical Design Methods for Superheat Pendant Outlet Headers Replacement

2010 ◽  
Author(s):  
Salah E. Azzazy ◽  
Russell D. Cochran ◽  
Larry Sam Cox
Keyword(s):  
Power Plants ◽  
Point Of View ◽  
Piping System ◽  
Main Challenge ◽  
Tennessee Valley ◽  
Pipe Segment ◽  
Technical Evaluation ◽  
Engineering Mechanics ◽  
Main Steam ◽  
Unconventional Method

Bull Run Unit 1, rated at 950 MW, is the first of four fossil supercritical power plants at Tennessee Valley Authority (TVA). The unit went into commercial operation in 1967. The boiler (consists of two furnaces) built by Combustion Engineering (CE) has a radiant reheat twin divided furnace with tangential-fired coal burners. The unit’s maximum continuous rating (MCR) is 6,400,000 lbs/hr of main steam flow, with a design temperature of 1003°F and pressure of 3840 psig. Through the end of 2008, the unit had a total of approximately 670 cumulative starts and 333,185 operating hours. After years of numerous tube cracks at the Superheat Pendant Outlet Header/Tube Nozzles resulting in repetitive forced plant shutdowns, TVA decided to replace the two Outlet Headers (one for each furnace) in Fall 2008 during a reliability outage. Since the entire Main Steam piping system was installed with cold pull at almost every longitudinal pipe segment, the main challenge from the engineering mechanics point of view was how to restrain the piping system especially at the Crossover Outlet Links inside each furnace Penthouse. Further constructability reviews indicated that there were not enough adjacent steel frames inside each furnace to restrain the four Crossover Outlet Links in the three global directions during the Outlet Headers replacement inside each Penthouse. The only existing steel above the Crossover Outlet Links is embedded in asbestos insulation, and the removal of the insulation to provide access for the temporary restraints was determined to be costly and time consuming. The insulation removal would have also caused the scheduled outage to be extended significantly and unrealistically. After careful assessment, technical evaluation, and several constructability reviews; it was decided to take an unconventional approach for relieving the inherent cold pull in three global directions by cutting the four Mixing Headers outside each furnace. In addition, the concept of installing several temporary restraints was utilized for the vertical and lateral directions inside the furnace Penthouse, as well as several others outside the Boiler to control the piping configuration of the four Mixing Headers. This approach achieved two purposes: 1- relieving the inherent cold pulls in three global directions and 2- controlling the four Outlet Links pipe end positions with respect to the new Superheat Pendant Outlet Header nozzles. This unconventional method used to relieve the piping cold pull from outside the Boilers, to control the Outlet Links movements inside the Boiler Penthouses, and to restrain the entire Main Steam piping system was successfully developed and implemented in the Fall 2008 reliability outage to replace the two Superheat Pendant Outlet Headers. This unconventional method is described in this paper.

Study on Weld Residual Stress and Crack Propagation Evaluations for a Saddle-Shaped Weld Joint

2013 ◽  
Author(s):  
Jinya Katsuyama ◽  
Koichi Masaki ◽  
Kunio Onizawa
Keyword(s):  
Residual Stress ◽  
Finite Element ◽  
Weld Joint ◽  
Power Plants ◽  
Structural Integrity ◽  
Piping System ◽  
Stress Distributions ◽  
Analysis Model ◽  
Element Analysis ◽  
Weld Residual Stress

Stress corrosion cracking (SCC) have been observed in reactor coolant pressure boundary piping system at nuclear power plants. When an SCC is found, the structural integrity of piping should be assessed according to a fitness-for-service rule. However, the rule stipulates the assessment procedures for crack growth and failure only for a simple structure such as cylindrical or plate-wise structure. At the present, the methodology even of an SCC growth evaluation for a geometrically complicated piping such as saddle-shaped weld joints has not been established yet. This may be because analyses on the weld residual stress distribution which affects the SCC growth behavior around such portion are difficult to conduct. In this study, we established a finite element analysis model for a saddle-shaped weld joint of pipes. The residual stress distributions produced by the tungsten inert gas (TIG) welding were calculated based on thermal-elastic-plastic analysis with moving and simultaneous heat source models. Analysis results showed complicated weld residual stress distributions, i.e., residual stresses in both hoop and radial directions were tensile at the inner surface near the nozzle corner in branching pipe. SCC growth simulation based on S-version finite element method (S-FEM) using the weld residual stress distributions in saddle-shaped weld joint was also performed. We confirmed an applicability and the accuracy of S-FEM to saddle-shaped weld joint.

On-Line Monitoring System for Main Steam Piping of Power Plants

2009 ◽  
Author(s):  
Ning Wang ◽  
Zhengdong Wang ◽  
Yingqi Chen
Keyword(s):  
Power Plant ◽  
Monitoring System ◽  
Remote Monitoring ◽  
Power Plants ◽  
Residual Life ◽  
Thermal Power ◽  
Creep Damage ◽  
Piping System ◽  
On Line ◽  
Main Steam

An on-line life prediction system is developed for remote monitoring of material aging in a main steam piping system. The stress analysis of piping system is performed by using the finite element method. A sensor network is established in the monitoring system. The creep damage is evaluated from strain gages and a relationship is given based on a database between the damage and residual life. Web technologies are used for remote monitoring to predict the residual life for every part of the piping system. This system is useful for safety assessment procedures in thermal power plant, nuclear power plant and petrochemical industries.

FSI-Based Assessment of FAC-Caused Wall Thinned Piping

2009 ◽  
Author(s):  
Sun-Hye Kim ◽  
Yoon-Suk Chang ◽  
Young-Jin Kim
Keyword(s):  
Nuclear Power ◽  
Power Plants ◽  
Structural Integrity ◽  
Flow Characteristics ◽  
Limit Load ◽  
Bend Angle ◽  
Piping System ◽  
Integrity Assessment ◽  
Wall Thinning ◽  
Analysis Scheme

Lots of investigations on failures of wall thinned piping have been carried out since the accident of Surry unit 2 in USA. From these preceding efforts, flow accelerated corrosion (FAC) which is a kind of wall thinning phenomenon is revealed main factor of failure of pipes in nuclear power plants. However, there are a few researches which directly take into account of flow characteristics and geometric changes for stress assessment of FAC-caused wall thinned piping. In this paper, structural integrity assessment employing a fluid-structure interaction (FSI) analysis scheme is performed on pipes representing secondary piping system of PWR which consists of straight pipes and elbows of various bend angles. Prior to the assessment, CFD analyses are conducted to predict plausible wall thinning location by considering flow and geometric parameters such as bend angle and radius of elbow. Then, for typical pipe geometry, detailed limit load analyses are performed to calculate maximum stress caused by turbulence and velocity of flow near the wall thinned part. Through these kinds of detailed parametric analyses, effects of FSI were observed, which should be considered for assessment of FAC-caused wall thinned piping.

Experimental Investigation on Failure Estimation Method for Circumferentially Cracked Pipes Subjected to Combined Bending and Torsion Loads

2013 ◽  
Author(s):  
Yinsheng Li ◽  
Kunio Hasegawa ◽  
Michiya Sakai ◽  
Shinichi Matsuura ◽  
Naoki Miura
Keyword(s):  
Experimental Investigation ◽  
Pressure Vessel ◽  
Nuclear Power ◽  
Power Plants ◽  
Structural Integrity ◽  
Estimation Method ◽  
Bending Moment ◽  
Limit Load ◽  
Piping System ◽  
Torsion Moment

When a crack is detected in a nuclear piping system during in-service inspections, the failure estimation method provided in codes such as the ASME Boiler and Pressure Vessel Code Section XI or JSME Rules on Fitness-for-Service for Nuclear Power Plants can be applied to evaluate the structural integrity of the cracked pipe. In the current codes, the failure estimation method for circumferentially cracked pipes includes bending moment and axial force due to pressure. Torsion moment is not considered. The Working Group on Pipe Flaw Evaluation for the ASME Boiler and Pressure Vessel Code Section XI is developing guidance for combining torsion load within the existing solutions provided in Appendix C for bending and pressure loadings on a pipe. A failure estimation method for circumferentially cracked pipes subjected to general loading conditions including bending moment, internal pressure and torsion moment with general magnitude has been proposed based on analytical investigations on the limit load for cracked pipes. In this study, experimental investigation was conducted to confirm the applicability of the proposed failure estimation method. Experiments were carried out on 8-inch diameter Schedule 80 stainless steel pipes containing a circumferential surface crack. Based on the experimental results, the proposed failure estimation method was confirmed to be applicable to cracked pipes subjected to combined bending and torsion moments.

Time History Steam Hammer Analysis for Critical Hot Lines in Thermal Power Plants

2014 ◽  
Author(s):  
Ahmed H. Bayoumy ◽  
Anestis Papadopoulos
Keyword(s):  
Power Plant ◽  
Dynamic Response ◽  
Power Plants ◽  
Thermal Power ◽  
Time History ◽  
Piping System ◽  
Pressure Waves ◽  
Root Cause ◽  
Pipe Line ◽  
Main Steam

Pressure surges and fluid transients, such as steam and water hammer, are events that can occur unexpectedly in operating power plants causing significant damages. When these transients occur the power plant can be out of service for long time, until the root cause is found and the appropriate solution is implemented. In searching for root cause of transients, engineers must investigate in depth the fluid conditions in the pipe line and the mechanism that initiated the transients. The steam hammer normally occurs when one or more valves suddenly close or open. In a power plant, the steam hammer could be an inevitable phenomenon during turbine trip, since valves (e.g., main steam valves) must be closed very quickly to protect the turbine from further damage. When a valve suddenly stops at a very short time, the flow pressure builds up at the valve, starting to create pressure waves along the pipe runs which travel between elbows. Furthermore, these pressure waves may cause large dynamic response on the pipeline and large loads on the pipe restraints. The response and vibrations on the pipeline depend on the pressure waves amplitudes, frequencies, the natural frequencies and the dynamic characteristics of the pipeline itself. The piping flexibility or rigidity of the pipe line, determine how the pipes will respond to these waves and the magnitude of loads on the pipe supports. Consequently, the design of the piping system must consider the pipeline response to the steam hammer loads. In this paper, a design and analysis method is proposed to analyze the steam hammer in the critical hot lines due to the turbine trip using both PIPENET transient module and CAESAR II programs. The method offered in this paper aims to assist the design engineer in the power plant industry to perform dynamic analysis of the piping system considering the dynamic response of the system using the PIPENET and CAESAR II programs. Furthermore, the dynamic approach is validated with a static method by considering the appropriate dynamic load and transmissibility factors. A case study is analyzed for a typical hot reheat line in a power plant and the results of the transient analysis are validated using the theoretical static approach.

An Assessment of Margin in Design Steam Hammer Forces for Combined Cycle Power Plants

2002 ◽  
Author(s):  
D. Zheng ◽  
A. T. Vieira ◽  
J. M. Jarvis
Keyword(s):  
Power Plants ◽  
Classical Method ◽  
Combined Cycle ◽  
Nuclear Industry ◽  
Piping System ◽  
Valve Closure ◽  
Velocity Correlation ◽  
Two Phase ◽  
Best Estimate ◽  
Main Steam

All combined cycle steam plants have rapid-closing stop valves in steam lines to protect the turbine. The rapid valve closure produces a steam hammer in the piping resulting in large forces for which the piping system and supporting structures need to be designed. These forces are typically calculated using the classical Method Of Characteristics (MOC) solution. An evaluation has been conducted which compares the forces computed using the classical methods with a best-estimate approach. This comparison has been done to define margin, and to benchmark and identify potential refinements in the techniques used for evaluating steam hammer loads. The best-estimate approach involves the use of the RELAP5 computer program. RELAP5 is used extensively in the Nuclear Industry to evaluate fast thermal hydraulic transients. It has the capability to analyze subcooled liquid, two-phase and saturated or superheated steam piping system. The models used in RELAP5 are best estimate results in comparison to the MOC solution which are mathematically derived from theory. The compressible flow program GAFT is used to obtain the MOC solution. The main steam line of a single Heat Recovery Steam Generator combined cycle plant is modeled with both the GAFT program and with a PC version of RELAP5. Identical piping lengths, mass flow rates, pressures are used in each model. Also, a stop valve closure time of 100 milliseconds is modeled. As RELAP5 output results are pressure, flow rate, velocity, and density, the resultant forces are generated using the R5FORCE program, a post-processor to compute associated transient forces on straight piping links. The GAFT program, which is specifically designed to compute steam hammer forces, computes the force history internally on straight piping lengths. A comparison of the peak force from GAFT and from RELAP for every piping link has been generated. Through the comparison, both RELAP5 and GAFT have been verified for the evaluation of rapid valve closure reaction loads. The comparison also shows that the classical method typically over-predicts the best-estimate solution by 15% to 20% for straight piping links. Although not confirmed, a better agreement between the two methods would be expected if a more accurate steam sonic velocity correlation and valve closure model are incorporated into the classical solution. Theis study helps to quantify the degree of conservatism inherent in the classical approach.

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