Available online at www.sciencedirect.com Procedia Engineering 55 (2013 ) 421 427 6 th International Conference on Creep, Fatigue and Creep-Fatigue Interaction [CF-6] Real-Time Monitoring of High Temperature Components Rajesh Daga a, Mahendra Kumar Samal b* a NTPC Energy Technology Research Alliance, NTPC Ltd., Greater Noida, India b Reactor Safety Division, Bhabha Atomic Research Centre, Trombay, Mumbai-85, India Abstract The remaining life assessment of components in a thermal power plant operating in the creep regime has drawn considerable attention. The structural integrity of such critical components is essential for operational safety, reliability and economic plant operation. Boiler headers and high energy pipings operating at high temperatures and pressure are subjected to different degradation mechanisms synergistically. A finite-element analysis based real-time monitoring system for four 210 MW units of a power station of NTPC Ltd. is evaluating creep and fatigue usage factors being accrued in the components. The candidate components under 24 x 7 assessments are super-heater-outlet-header, re-heater outlet and inlet headers, and the piping bends before the control valves of intermediate pressure turbine of each unit. The cyclic stresses with a mean stress at an elevated temperature lead to a damage mechanism due to creep, fatigue and creep fatigue interaction. The process data i.e., steam pressure, steam temperature and steam flows of the components are made available through a local plant information server on a real-time basis. The system evaluates the material temperature, stress and usage factors due to creep, fatigue and creep-fatigue interaction and computes the remaining life fraction in all the components. The application of a real-time monitoring system thus enables the on-line damage assessments followed by surveillance inspection of critical components of a station comprehensively. 2013 The The Authors. Published Published by Elsevier by Ltd. Elsevier Ltd. Selection and/or peer-review under responsibility of the Indira Selection Gandhi and Centre peer-review for Atomic under responsibility Research. of the Indira Gandhi Centre for Atomic Research. Keywords: Creep; fatigue; creep and fatigue usage factor; real time monitoring; finite element analysis; remaining life assessment 1. Introduction There has been global concern and sustained interest in economically operating the components in the creep regime with safety and reliability. The structural components of the fossil power plants operating at elevated temperature under the influence of process transients are subjected to cyclic stresses. For the operational safety of such critical components it becomes essential to estimate the usage factors being accrued due to these transients [1-6]. The components of boiler and turbine of the thermal power plants under cyclic stresses with high mean stress at elevated temperature are subjected to combined creep and fatigue damages. Thus *Corresponding Author: E-mail address: mksamal@barc.gov.in 1877-7058 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the Indira Gandhi Centre for Atomic Research. doi:10.1016/j.proeng.2013.03.274
422 Rajesh Daga and Mahendra Kumar Samal / Procedia Engineering 55 ( 2013 ) 421 427 economics of reliable and safe plant operation has been driving the power generation industry towards remaining life prediction. The plant is conservatively designed on the basis of assumed process transients whilst their actual lives exceed the designed life estimates. The ageing effects of fatigue, creep, creep-fatigue interaction and creep-fatigue crack growth are the most common causes of component failures. The paper discusses the finite element (FE)-based real-time monitoring system which is in use at one of the thermal power stations of NTPC Ltd. The details of the operation of the monitoring system are described in [7-9]. This system is also being in operation in several other heavy water and thermal power plants and the details are discussed in Ref. [10-12]. The candidate components which are continuously being monitored round the clock are superheater outlet header, reheater outlet header, reheater inlet header and the hot reheat pipe bends (left and right) before the control valves of the intermediate pressure turbine cylinder. The system acquires the thermal-hydraulic process parameters, computes the material response using the finite element analysis and estimates the creep and fatigue usage factors being accrued by the components. 2. Real-time damage monitoring system The critical components of boiler and high-energy piping operating at an elevated temperature are being monitored on a real-time basis by a FE-based on-line centralised creep-fatigue damage assessment system coined as BOSSES (BARC On-line Structural Safety Evaluation System) developed by Reactor Safety Division of Bhabha Atomic Research Centre, Trombay, Mumbai. The monitoring system is installed at NETRA (NTPC Energy Technology Research Alliance), NTPC Ltd., Greater Noida, U.P., India. The system monitors 20 components of four units of 210 MW of NTPC Ltd. The component specific thermal-hydraulic process parameters (i.e., steam pressure, steam temperature and steam flows) are being acquired on a predefined interval by a centralised plant-information (PI) server of the power station. The monitoring system has been implemented for selected critical components of the four units. Some of the components under monitoring are superheater outlet header (SHOH), reheater inlet and outlet header (RHIH and RHOH) and the hot reheat pipe bends (HRHL and HRHR) etc. SHOH is one of the most critical components operating in the creep regime with a hydraulic pressure of 155 kg/cm 2 and 540 C. The gross dimensions of SHOH are 406.4 mm (OD) x 75 mm (thickness) x 14650 mm (length). The main steam on exit from SHOH is transported to the high pressure turbine. RHOH is the outlet header of the reheater which delivers the hot reheat steam to Intermediate pressure turbine with gross dimensions as 558.8 mm (OD) x 45 mm (thickness) x 14650 mm (length). It operates under pressure of 49 kg/cm 2 and 540 C. RHIH, the inlet header of the re-heater, receives the cold reheat steam exhausted from high pressure turbine with its gross dimensions as 406.4 mm (OD) x 15.2 mm (thickness) x 14650 mm (length). It operates under a pressure of 52 kg/cm 2 and 355 C. The hot reheat pipe bends are in the hot reheat piping before intermediate pressure control valves with a design temperature of 540 C of 46.3 kg/cm 2 conveying steam to its intermediate pressure turbine. Its dimensions are 508 mm (OD) x 30 mm (thickness) with a bend radius of 1200 mm. The material of these headers and pipe bends is ASME SA 335 P22 (low alloy ferritic steel). The importance of these components in an operating plant emerges from the concern of its critical function as a transporter of high-energy steam. To compute the temperature, stress intensities etc., the FE method is used for thermal and stress analysis of the components. The finite element model uses 3D iso-parametric 20-noded brick elements. Both transient thermal analysis and stress analysis are carried out in 3D domain. Elastic material constitutive model is used in the FE analysis. 3. Piping load evaluation The system acquires component specific plant transients namely steam temperature, pressure and flow in the hot reheat piping to compute the creep and fatigue usage factors. The piping loads, namely, forces and moments on the component have been evaluated at design internal pressure, design fluid temperature and dead weights by stress analysis of the piping loop including the low pressure bypass line. They are used to compute fluctuating piping loads for its real-time computation. Fig. 1 shows the components of piping loads (at the two ends of the hot reheat pipe bend) separately for design temperature of 540 o C and dead weight.
Rajesh Daga and Mahendra Kumar Samal / Procedia Engineering 55 ( 2013 ) 421 427 423 Fig. 1. Piping loads on hot reheat pipe bend used in the monitoring system. 4. Creep and fatigue damage assessment computation The real-time monitoring system acquires fluid process parameters and computes heat transfer coefficients for the boundary of the component. The temperature distribution in the whole volume of the model at each time step is obtained by performing transient thermal analysis. Subsequently the stress analysis module computes thermal stresses in the component using the temperature profile derived from the transient thermal analysis and calculates stress distribution in the component considering the fluid internal pressure, temperature gradient and the piping loads at each time step. The stress-time history is converted to stress frequency spectrum using rainflow cycle algorithm. The fatigue usage factor is evaluated from the computed cycles and the material fatigue data using Miner s life fraction rule. The usage factor due to creep using Robinson s life fraction rule is evaluated from the computed temperature, stress histories and the material creep curve, as per procedure given in API code [3]. The total usage factor is then calculated by linearly superimposing the usage factor due to creep and fatigue as shown in Fig. 2. 5. Creep-fatigue usage factor and remaining life assessment The damage assessment and remaining life computations are performed using their service period and the monitoring duration. The monitoring system has been installed after some periods of operation instead of implementing during the commissioning of the new plant. The logged-in data is extrapolated to compute the usage factor for the service life of the components on an assumption that the logged-in plant thermal-hydraulic parameters are representative of past plant history. Thus the total usage factor data due to creep and fatigue are subsequently extrapolated for the service life of the components by the monitoring system using on-line acquisition of process data. The contours of the stress intensity and usage factor due to creep and fatigue
424 Rajesh Daga and Mahendra Kumar Samal / Procedia Engineering 55 ( 2013 ) 421 427 obtained from finite element analysis of superheater outlet header, reheater outlet and inlet headers, and hot reheat pipe bend (left) before control valve under monitoring are depicted in Figures 3-6. Fig. 2. Various Stages of on-line creep-fatigue monitoring system BOSSES. Stress intensity Fig. 3. Super-heater outlet header contours after 23847 hours of monitoring. Stress intensity Fig. 4. Re-heater outlet header contours after 23840 hours of monitoring.
Rajesh Daga and Mahendra Kumar Samal / Procedia Engineering 55 ( 2013 ) 421 427 425 Stress intensity Fig. 5. Re-heater inlet header contours after 23656 hours of monitoring. Stress intensity (MPa) Fig. 6. Hot reheat pipe bend contours before control valve (left) after 23838 hours of monitoring. The computed values of material temperature, maximum stress intensity and usage factor history at a shellnozzle junction of a superheater outlet header of a unit after 23847 hours of monitoring are shown in Figures 7 and 8. The remaining life assessment of the components is subsequently performed on the cumulated usage factor and is shown for one of the units in Figure 9. Fig. 7. Recorded main steam temperature history over time.
426 Rajesh Daga and Mahendra Kumar Samal / Procedia Engineering 55 ( 2013 ) 421 427 Fig. 8. Computed material stress intensity, temperature, usage factor history at a shell-nozzle junction of superheater outlet header. Fig. 9. Remaining life assessment of the components under monitoring. 6. Consumed creep life fraction assessment of super-heater outlet header Super-heater header, one of the critical components operating round-the-clock in the creep regime in a coalfired utility, is a thick cylinder with outer diameter of 406.4 mm and 75 mm thickness. It was found that one set of stub-header weld joints of the in-service header in higher heat flux zones of the boiler had indicated possibility of onset of crack as per ASME design code calculations on extrapolation of the usage factor after 1,12,447 hours of operation. The header was taken up for surveillance programme as per the statutory requirement of IBR Act 391A of 1998 for its design life exhaustion. The cryogenic in-situ metallography technique [5] was adopted for assessment of accessible locations closest to the identified weld joints of the header [6-9]. The qualitative assessment using high resolution microscopy indicated that the creep cavities correspond to stage II of Neubauer s classification diagram. To evaluate the extent of life exhaustion, the insitu replicated micrographs were quantified for accrued creep damages. Such re-inspection of the identified location would be conducted in the next four-five years.
Rajesh Daga and Mahendra Kumar Samal / Procedia Engineering 55 ( 2013 ) 421 427 427 7. Conclusions The implementation of a real-time monitoring system for estimating the creep-fatigue usage factors and assessing the integrity of the structure of critical components of thermal power plants will aid in minimising frequent off-line inspections and reducing operation and maintenance expenses. This will substitute the approach of the utilities performing inspection on the basis of past operation and maintenance experience which often cause unexpected failures. The present approach of monitoring critical components would be applied along with regular operation and maintenance planning and scheduling activities to provide a cost effective plant management and thereby reducing uneconomical decisions of arbitrary inspections. The evaluation procedure for creep and fatigue usage factor is according to relevant API and ASME codes. The predicted usage factors may be improved by the use of specific material data (i.e., mechanical, creep and fatigue properties) for the component under consideration along with the provision to account for the servicerelated degradations. This has scope for future research. It is also being planned to extend the present damage monitoring system to all ageing critical components with updated material and component specific database. Acknowledgement The authors are indebted to Mr. K. K. Vaze, Director, Reactor Design and Development Group, BARC and Mr. A. K. Jha, Director-Technical, NTPC Ltd. for supporting real-time monitoring activity in the respective organizations. The authors also gratefully acknowledge the encouragement given by Mr. D. K. Agrawal, Executive Director, NETRA, and Mr. A.K. Mohindru, General Manager, NETRA. The authors acknowledge the support and necessary guidance extended for the activity by Dr. Pradeep Jain, Additional General Manager, NETRA. The cooperation extended by Site Management and Engineers of the power utility is highly appreciated. References [1] N.K.Mukhopadhyay, B.K.Dutta, H.S.Kushwaha, S.C.Mahajan, A.Kakodkar, On line fatigue life monitoring methodology for power plant components. Int. J. Pres. Ves. Piping 60(1994)135-43. [2] N.K.Mukhopadhyay, B.K.Dutta, H.S.Kushwaha, On line creep-fatigue life monitoring system for components at elevated temperature. 3 rd Workshop on Creep, Fatigue and Creep-Fatigue Interaction, IGCAR, Kalpakkam, 1999, C349-358. [3] Assessment of components operating in the creep regime, Section 10 of API-579 document, June 2001. [4] R.Daga, G.Bandyopadhyay, M.K.Samal, B.K.Dutta, J. of Power Plant: Operation, Maintenance and Materials Issues 2008; 5(1): 1-19. [5] R.Viswanathan, Damage Mechanisms and Creep Life Assessment of High Temperature Components, ASM International, Ohio, 1989, 182. [6] G.Venkataraman, In-situ Metallography and Replication of Microstructures for CA and RLA of Boilers, Proceedings of Thermal Babcock and Wilcox workshop on Condition Assessment and RLA of Boilers, Pune, India, 24-25 January, 2001. [7] M.K.Samal, B.K.Dutta, S.Guin, H.S.Kushwaha, A finite element program for online life assessment of critical plant components. Engineering Failure Analysis 16(2009)85-111. [8] M.K.Samal, B.K.Dutta, H.S.Kushwaha, R.Daga, G.Bandyopadhyay, Creep damage evaluation of a power plant header using combined FEM analysis and quantitative metallography. Transactions of the Indian Institute of Metals 2010; 63(2-3): 411-6. [9] R.Daga, G.Bandyopadhyay, M.K.Samal, B.K.Dutta, A.K.Mohindru, Consumed creep life fraction assessment of critical locations of an in-service superheater outlet header under surveillance programme. Transactions of the Indian Institute of Metals 2010; 63(2-3): 423-9. [10] B.K.Dutta, M.K.Samal, S.Guin, On-line remaining life assessment of piping components in power plants. International conference and exhibition on pressure vessels and piping (OPE 2006), 7-9 February 2006, Chennai, India. [11] S.Guin, M.K.Samal, B.K.Dutta, H.S.Kushwaha, Stress intensity factors for shell-nozzle junctions with part-throughwall axisymmetric cracks under internal pressure. International conference and exhibition on pressure vessels and piping (OPE 2006), 7-9 February 2006, Chennai, India. [12] M.K.Samal, S.Guin, B.K.Dutta, H.S.Kushwaha, Development of 3-D online damage monitoring system for heavy water plant Tuticorin, International conference and exhibition on pressure vessels and piping (OPE 2006), 7-9 February 2006, Chennai, India