STRESS ANALISYS OF HIGH PRESSURE STEAMLINES IN THERMAL POWER PLANTS

Transcription

1 STRESS ANALISYS OF HIGH PRESSURE STEAMLINES IN THERMAL POWER PLANTS Aleksandar Jakovljevic, M.Sc.,Mech.Eng. Head of Study and Research Division Electric Power Industry of Serbia, Department for Strategy and Investments 412 Vojvode Stepe, Belgrade, Serbia ABSTRACT A reliable assessment of the condition and residual life of high pressure steam pipelines requires defining stress state, which will take into consideration not just the impact of internal pressure and temperature but the all applied loads. For that, usage of numerical methods for calculation and analisys of stress state is neccesary. This paper presents possibilities and disandvatages of finite elements methods in steam pipeline stress analysis, which is also shown in detail by an example of main steam pipeline of thermal unit TPP Nikola Tesla A1. Steam lines present statically undefined pipe structures allocating between connection points on the boiler and turbine; they consist of mutually welded elements connected to support structure by the support system. For this reason, it is necessary to bear in mind during the stress analyses of steam lines, an entire sequence of additional influences, which have considerable impact on the behaviour of steam lines, and are not directly related to the operating parameters of the fluid: pressure and temperature. Overstressing by permanent, thermal and dynamic loads is primarily caused by inadequate configuration of the steam line, irregular operation of support system and loads in transient states [1, 2]. High pressure steam pipelines are critical components of thermal power plants, which have significant influence on reliability and availability of plants as well as staff safety. Therefore, during operation, it is necessary to monitor their actual state and also to provide preconditions for assessing high pressure steam pipeline s behaviour during further exploitation in order to assess residual life in a proper way. In service conditions, high pressure steam pipelines are exposed to creep and low cyclic fatigue, which lead during the time to microstructural degradation as well as initiation and development of steel damage. Methods for quantitative estimation [3-7] of the remaining operation life are mainly based on the assumption that component is exposed only to the effect of internal pressure, which simplifies the problem. This is generally the reason why analyses are started with stress values, which do not represent stress to which the steamline component is exposed to during operation. For this reason, after performed analyses and consideration of observed limitations, modified method of determination of relevant stress was defined. For example, in Germany in FBRD recommendations [8] relevant stress consisted of stresses occurring under the effect of external load in addition to the effect of internal pressure: p d = u 4 tn p 0, 75 i M + + A 2 W 0, 75 i M + C 3 W σ (1) where: p fluid pressure in a pipeline; d i - internal diameter of a pipe; t n - nominal thickness of a pipe wall; i- stress intensification factor; M A - moment resultant to steady loads in a cross section; M c - internal moment resultant to thermal loads; W - resistance moment of the cross section.

2 Equation 1 represents a connection between structural calculations and calculation of strength and in this way it provides more realistic stress values in the element of the steam line system. This enables better consideration and estimation of the operation life [9, 10]. Steamline has to be cconsidered during the analysis as a complex structure, which is exposed, besides the action of internal pressure of the operation fluid, also to the action of additional external load. It greatly depends on configuration and support system of the steamline and it causes the appearance of stress state significantly different from the design ones. Their negative impact on the service life of the steamline and correct estimation of the remaining service life require the consideration and quantification not only the impact of internal pressure and temperature but also all affecting loads [1, 2]. For that, usage of numerical methods for calculation and analisys of stress state is neccesary. Most of numerical methods for the calculation of stress state are based on the application of finite elements method, which can be used in two ways, from the aspect of steamline analysis: for structural calculations of the complete structure, as well as for the calculation and analysis of individual components. The paper presents an example of the performed stress analysis of the main steamline at the unit A1 TPP Nikola Tesla in Obrenovac, which is used to demonstrate the importance of stress analyses and the possibilities and restrictions of numerical methods for the performance of such analyses (Fig. 1). Considered steamlines were selected for the analysis because they, exhibiting nearly operation hours (designed for hours), represent one of the oldest steamlines in Electric Power Industry of Serbia plants. Analysis were performed mainly on the basis of data obtained through investigations during the overhaul in Steam lines were replaced in 2003 after nearly operation hours. Basic data of the steam line are shown in Table 1. Table 1: Basic data of the main steamline (RA) Unit A1 TPP "Nikola Tesla" Material Dimensions Design Ddesign Operational Operational pressure temperature pressure temperature - mm bar o C bar o C (ČSN) Ø324x36; Ø219x25 141,

3 Figure 1. Stress state of the main steam line in hot state Structural analysis of stress state of these steamlines was performed according to ASME/ANSI B31.1 standard [11] using data obtained through recording of displacements and geometry of the steam line in the hot and cold state. Analysis was performed for the case of: permanent load from pipe weight, insulation, internal pressure and other constant loads; thermal load; and simultaneous activity of permanent and thermal load. Then, two pipe elbows of the same size were chosen from steamline. Selection is made so that one of them belongs to the group of pipe elbows with the highest load (K31) and the other to the group with the lowest load (K11) on the steamline and that there is sufficient data from previously performed investigations. Figure 1 represents stress state of the main steam line in the hot state, through the presented relation of the calculated and allowable stress for the given loading case, with separated pipe elbows to be analyzed later. Significant difference of stress level can be seen along the steam line. Detailed analysis of the stress state of chosen elbows by the finite elements method was performed using linearelastic analysis, for three different load cases. Mathematical models of their geometry were formed based on size of pipe elbows measured during investigations performed in 2000 (Table 2) by defining of a unique cross-section with: minimal values of wall thickness (from the five measures) for each zone; mean values of diametars of pipe elbows; symmetrical models of pipe elbows (the same wall thickness for both neutral zones) Table 2: Dimensions used for mathematical modeling of pipe elbows Diameter (mm) Wall thickness (mm) Pipe elbow Compressed d 1 d 2 Extended zone Neutral zone 1 zone Neutral zone 2 RA-K11 327,3 322,7 33,0 35,0 38,6 35,0 RA-K31 323,4 319,7 31,6 34,0 35,5 34,0 Structure of pipe elbows of the main steam line was discretized to 5760 elements through the application of finite elements method. Detailed analysis of the stress state through the finite elements method was performed by means of linear-elastic analysis, with utilization of PAK S software for stress state calculation itself, and FEMAP during the phase of pre- and post- processing, for three different load cases: action of the internal pressure only; cold state (action of forces and moments obtained as the result of structural analysis); hot state (action of forces and moments obtained as the result of structural analysis). Used loads (forces and moments) were obtained from the structural analysis of the steam line as a whole in cases of cold (permanent loads and overpressure) and warm (permanent and thermal loads) state of the steam line, in accordance with ASME/ANSI B31.1 standard. Obtained results from the stress analysis show the following: Complex distribution of stress is obtained during the activity of internal pressure. In this case maximum value of the effective and hoop stresses occurs in the thin surface zone from the inner side of the pipe elbow, around 15 from the neutral axis towards the extended zone, for both pipe elbows. Values of effective and hoop stresses are practically the same: difference amounts to ~4% (effective stress is higher). Values of longitudinal and shear stress are considerably lower than effective and hoop stresses. It can be seen that the nature of distribution is the same for both pipe elbows. From the analysis of the stress state under the load that corresponds to the cold state, it can be concluded that stress state is almost identical as in the case of only internal pressure. However, during the analysis of load active in the hot state, a phenomenon of significantly changed distribution of stress state is noticed. Difference in stress distribution can also be seen during the comparison of pipe elbows. In this way, in the case of K31 pipe elbow (which belongs to the group of the highest loaded in the main steam line) there is a high increase of maximum values (compared to the effect of internal pressure only) of effective stress (for 39% from MPa to MPa), hoop

4 stress (for 20% from 101 MPa to MPa), and especially for longitudinal stress (for over 550% or 6.5 times from 9.2 MPa to 60.5 MPa). Maximum values of hoop stresses on the external surface of the pipe elbow occur in the neutral zone of the pipe elbow, while in the case of effective stress, maximum values occur both on the external and internal surface in the pressed zone of the pipe elbow! For the elbow K11 there is no important change of stress state in hot state compared with the case of internal pressure effect only (increase is on the level of 5 %), with the same character of stress distribution in the hot state as during the action of internal pressure only or in the cold state. internal pressure hot state "D " hoop stress UPOREDNI NAPON UPOREDNI NAPON longitudinal stress AKSIJALNI NAPON "D " AKSIJALNI NAPON Figure 2: Stress analysis results for pipe elbow K31 hoop and longitudinal stress Considerable increase of stress in the hot state, in the case of K31 pipe elbow could be explained by the increase of forces and moments affecting it, and primarily by extremely high increase of Mx moment, caused by the existence of fixed point at m elevation, which does not permit rotation around x axis, Table 3. Table 3: Forces and moments which have an impact on the pipe elbow RA-K31 RA-K31 Cold state Hot state Cross-section A Cross-section D Cross-section A Cross-section D F x (N) F y (N) F z (N) M x (Nm) M y (Nm) M z (Nm) Maximall stresses are only active in thin surface layers, which comprise of around 20 to 40, so that average stresses in the cross-section are significantly lower than maximum values. Presence of local stresses of this magnitude (over 140 MPa) which considerably exceed material characteristics anticipated in the standard (R m/100000/540 C = 100 MPa; R m/200000/540 C = 76 MPa), after the operation of more than hours, indicates that applied linear-elastic analysis does not fully describes the stress state in analyzed components. Stress and displacement redistribution process during relaxation caused by creep and the reduction of load in conditions of increased temperature, cannot be included in the linear-elastic analysis. For more precise analysis, it is therefore necessary to use elasto-plastic or plastic method instead linear-elastic method, which takes into consideration the mentioned processes. However, this kind of analysis requires additional, more precise, input

5 data related to material characteristics and operating conditions, that most frequently cannot be provided (this is proven in the case of TPP Nikola Tesla A1 steam line). Additional analysis was also performed for the case when the pipe elbow is only affected by bending moments in pipe elbow plane and out of the pipe elbow plane, with the aim of pointing out the complexity of stress state during the activity of basic loads and for control of the applied model. If compare theoretical stress distribution [13] with distribution obtained by the finite elements method it could be seen that character of stress distribution is the same. Different values are consequence of different geometries and geometrical proportions (thikness, diameters...). Theoretical stress distribution Stress distribution obtained by the finite element method hoop stress (MPa) angle (degree) longitudinal str. (MPa) angle (degree) Figure 3: Comparison of theoretical stress distribution and stress distribution obtained by finite elements method on the perimeter of pipe elbow Conclusions Steam line has to be considered during the analysis as a complex structure, which is exposed, besides the effect of operational fluid internal pressure, also to the action of additional external load. It greatly depends on configuration and support system of the steam line and it causes the stress state that significantly differs from design ones. Having in mind, their negative impact on the service life of the steam line, correct assessment of the remaining service life requires their consideration and quantification. Most of numeric methods for the calculation of stress state are based on the application of finite elements method, which can be used in two ways, from the aspect of steam line analysis: for structural calculations of the entire structure, as well as for the calculation and analysis of individual elements. Application of finite elements method, although with some simplifications, enables the obtaining of a clearer picture and better consideration of the behavior of analyzed steam line elements. Performed stress analyses enable the identification of critical points with increased stress in the steam line and its components, which especially have to be taken into consideration during the monitoring of steam line condition. Impact of external load on the appearance of considerably different stress states in certain components with nominally the same geometry, operating on the same pressure and temperature, as well as the possibility of appearance of the most unfavourable stress state outside the anticipated zones was clearly demonstrated.

6 References 1. Jakovljević. A., Material Damage and Stress Condition Influence to Residual Life of High Pressure Steam Pipelines, Master thesis, Faculty of Mechanical Engeenering, Beograd, Jürgenson H., Elastizität und Festigkeit im Rohrleitungsbau, Springer Verlag, Berlin, Cane B.J., Townsend R.D., Prediction of Remaining Life in Low-Alloy Steels, Proc. Seminar on Flow and Fracture on Elevated Temperatures, ed. R.Raju, Philadelphia, USA, pp , TRD 508, Zusätliche Prüfungen an Bauteilen berechnet mit zeitabhängigen Festigkeitskennwerte, TRD, ASME Boiler and Pressure Vessel Code, Section III, Division 1, Case N-47-28, Class 1 Components in Elevated Temperature Service, The American Society of Mechanical Engineers, New York, Standard JUS M.E2.213, Boiler instalations: Additional testing: parts calculated based on characteristics of time dependent strenght, Federal Standardization Institute, Belgrade, Šijački-Žeravčić V. et al., Limitations in application of Larson-Miller parameter for determining of remaining working life for components exposed to longterm high temperature creep, Zbor. ENYU 99, Zlatibor, 1999, page FDBR Richtlinie: Berechnung von Kraftwerksrohrleitungen, FDBR, Bühl G., Kaum M., Reiners U., Weber J., Rohrleitungsüberwachung mit dem Mannesmann Lifetime Monitoring System (MLM), 3R International, H.9 34, pp , Becker U., Schepers H., Zustands- und Restlebensdaueruntersuchung der Kesselanlagen und HD Rohrleitungsysteme im Kraftwerk Kosovo A, Blöcke 1 bis 4, VGB Kraftwerkstechnik, H.4 71, pp , ANSI/ASME B31.1, New York, Elaborat No 155/2000, Examinations of unit 1 TPP Nikola Tesla A, Technical centar of Termoelektro, Obrenovac, Oude-Hengel H.H., Rohrleitungen in Kraftwerken, Verlag TÜV Rheinland GmbH, Köln, 1978