Remaining Life Assessment and Extension (RLA/RLE) of critical piping of thermal power plants
ProSIM is a engineering services and consultancy services company providing engineering outsourcing and engineering design services.??Focus of ProSIM is on computer aided engineering (CAE) services including CAE outsourcing.?Using competencies in CAE, FEA, and multi-physics simulation, ?ProSIM provides finite element analysis services and finite element analysis consultancy.?Application of FEA services of ProSIM include structural integrity assessment service of power plant equipment.?As remaining life assessment (RLA) consultants, ProSIM adds value to operators and EPCs to determine remnant life of power plant and propose remaining life extension (RLE). ProSIM has competence for onsite inspection of plants, interface with plant operators and EPC contractors, and NDT personnel, to provide structural integrity assessment and structural integrity evaluation.
?In this technical note we describe summary of some of the projects carried out for remaining life assessment and extension (RLA/RLE) of critical piping of thermal power plants. Critical piping include MST, HRH and CRH piping. ?ProSIM make site visits to plant to study the health and status of piping, supports, hangars, insulation, and other input data collection. Modeling and analysis work will be carried out at ProSIM offices.?Such modeling and analysis includes support and hangar design verification, analysis, code check and qualification, pipe modeling, pipe stress analysis, finite element analysis (FEA) of defects, mapping of stress and strain, determination of allowable stresses as per relevant codes, determination of fatigue and creep damage by FEA, and estimation of life in different scenarios such as design condition; as-is-where-is. Typically, the life as per as-is-where-is condition will be lower than the design condition.?ProSIM competence includes conducting dynamic pipe stress analysis (for eg, for seismic load cases)..
?ProSIM using their expertise, will suggest various recommendations to the operator to increase / extend life of piping and allied equipment. This could include change of supports, hangars or provision of scrubbers, or carrying out repair as permissible.?Once the operator is convinced of the feasibility of such measures to extend the life, an analysis of the ‘as recommended’ condition will be carried out (using finite element analysis), and life will be assessed.?A typical results summary for the main steam line (MST) is indicated in table below.
?NOTE: In this case, original design was carried out several decades back.?In that period, consideration of occasional loads such as seismic and wind loads was not a part of standard design procedure, and hence is absent.?In one particular piping, due to failure of supports and hangars, in the as-is-where-is condition (as observed during start of RLA work), the system was not qualifying (and hence failing) in occasional load cases, in both seismic and wind loads.?Based on structural integrity analysis done, ProSIM proposed some recommendations to extend the life of the piping.?As seen from the table below, ?‘as recommended case’ qualifies for all types of load cases including sustained load, expansion load, and occasional load cases.
?MS system stress summary
Simulation condition Loading condition & Maximum stress ratio [Acceptable <82%]
Sustained Expansion Occasional
As Designed 58.8% 38.9% --
As‐is‐Where‐is 67.2% 46.6% 277.4% (Seismic & Wind)
As Recommended 67.2% 48.8% 79.4%
METHODOLOGY for Remaining life assessment and structural integrity assessment
?As remaining life assessment (RLA) consultants, ProSIM undertakes studies which have to be carried out mandatorily, as per regulatory norms.?Any risk of failure has to be assessed and mitigated.?Remnant / residual/ remaining life depends on the usage / service pattern, operating cycles of pressure and temperature, materials of construction of equipment, design considerations, site location, etc.
?ProSIM has been supporting energy asset operators for structural integrity evaluation, RLA/RLE studies, and make the assets available for longer time, reduce the down time, and increase the throughput / productivity of assets. This article deals with studies related to critical piping of thermal power plants including Main Steam (MS), Hot Re-Heat (HRH), and Cold Re-Heat (CRH) piping.?ProSIM has carried out such studies for a variety of plant capacities from 60 MW to 500 MW.
?SITE VISIT, visual inspection, Data Collection:
RLA work starts with a site visit for data collection and preparation which results in a piping hangar and support inspection report. During site visit engineers have to take care of hangar inspections, and readings; visual inspection of supports and collection of input data such as GA drawings, P&IDs, Iso, design basis reports etc.??Some photographs taken during such visits are shown below.?Figures show damaged piping and supports.
NDT Evaluation:
Based on visual inspection of site, NDT will be performed to assess the extent of damage.?Critical damage sites will be short listed for deeper evaluation using finite element analysis and pipe stress analysis.
?CODES and Standards used in this study:
Codes used are in conjunction with requirements as per technical specifications in the tender documents from the operator.
2. ASME B 31.1 (Power piping System)
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3. ASME Sec II Part D
4. RCC MR
5.?IS:1893 (Part 1):2002 and IS:1893 (Part 4):2005
6.?IS:875(Part-3)
Using the design data, GA drawings, ISO drawings, PiD, and other inputs, a pipe modeling has to be done.?Supports and hangars and boundary conditions have to be defined in the piping model.?Pipe stress analysis is then carried out.?Pipe stress analysis is a numerical analysis of flexibility of pipes using 1-D finite element analysis (FEA) method.?This is done using software such as CEASAR or PipeStress (Peps). ?A key issue in pipe stress analysis is to use the actual stiffness of the hangars and supports as observed / recorded in hot / cold walks.?Figures below show the piping models developed and pipe stress analysis results.
Figure: (left) pipe modeling of a power plant piping showing supports. (right) pipe stress analysis (pipe flexibility analysis) results showing pipe stress.
3D Finite Element Analysis (FEA) of defects:
In the piping system there can be a variety of defects such as corrosion, dents, micro / macro cracks,?creep-fatigue damage etc.?Effect of these defects on stress distribution and life of piping has to be studied using 3D FEA.?These 3D FEA models have to be connected to 1D pipe stress models using sub-modeling techniques.?Figure below shows the extraction of a segment of piping for such detailed 3D FEA.?Figures also show the stress distribution patterns determined using 3D FEA
?Figure: ?(left) ?pipe stress analysis results showing regions where critical regions with defects.?(right) detailed 3D finite element analysis result indicating critical stress regions.
?Remaining Life Assessment:
Using the above method, remaining life of the piping is assessed.?Figure below shows the life in different segments of piping.?Critical regions are indicated in red.
?Figure: pipe stress analysis results showing log life.?Regions having no residual life are indicated in RED.
?To calculate the remaining life a creep-fatigue interaction curve as given below is used. ?Creep-fatigue interaction is estimated using a FEA results in conjunction with ASME codes.
?EPILOGUE
?ProSIM works with operators and EPCs involved in remaining life extension (RLA/RLE) activities of power plants.?ProSIM competence to provide engineering judgment based on complex finite element analysis and correlation with codes of practice.
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