DESIGN AND ANALYSIS OF HIGH-PRESSURE CASING OF A STEAM TURBINE

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 7, July 2017, pp , Article ID: IJMET_08_07_002 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed DESIGN AND ANALYSIS OF HIGH-PRESSURE CASING OF A STEAM TURBINE Chelamalasetti Pavan Satyanarayana and Dr YV Hanumantha Rao Department of Mechanical Engineering, KL University, Guntur, Andhra Pradesh, India ABSTRACT Steam Turbines are devices used to convert thermal energy of steam into mechanical energy and then into electrical energy by connecting rotor shaft to generator. Generally steam turbines are carried out in three different stages, each stage helping to extract little more energy from steam before its exhausted. All three stages HP, IP & LP turbines are mounted on same rotor axle and all turning the generator at the same time. This multi-stage approach, invented by Charles Parsons, means each stage is slowing or reducing the pressure of the steam by only a relatively small amount, which reduces the forces on the blades an important consideration for a machine that may have to run for years without stopping and greatly improves the turbine's overall power output. Most modern turbines, with steam pressures over 100 bar and ratings greater than 100 MW, have HP casings of double-shell design. This has been adopted because of the difficulty of designing a single casing to withstand the thermal and pressure stresses and yet is capable of flexible operation. The decrease in efficiency of steam turbine impact the efficiency and reliability of power stations. So, any improvement in the design of steam turbine reduces steam energy losses results in decreased cost and one which parameter the efficiency of steam turbine depends is casing design. The high pressure steam at 565 C and 156 bar pressure passes through the high pressure turbine. The exhaust steam from this section is returned to the boiler for reheating. At this stage the steam pressure reduces relatively small amount where as changes in thermal energy is large. The reheated steam enters into intermediate pressure turbine at 565 C and 40.2 bar pressure and extracts energy. From the intermediate pressure turbine, the steam continues its expansion in the third stage Low pressure turbine at 306 C and 6.32 bar. Form this we can observe that, to get more power output the steam exhaust pressure is kept very low and this achieved by casing design. The casing thus witnesses, energy of the steam turned into work in HP and IP stages. So, the design of the casing is a very important aspect. During the design of steam turbine casing we need to consider the important parameters like contact pressure, bolt pre-tensions and thickness of casing. Contact pressure analysis of turbine casing is very important in steam turbine which needs to be calculated for structural integrity. During operating condition steam turbine casings are subjected to very high pressure and temperature which results in stress and strain 11 editor@iaeme.com

2 Design and Analysis of High-Pressure Casing of A Steam Turbine distribution. If the contact pressure is not achieved as per the standards then it leads to leakage of steam which causes explosion of casing. These effects are difficult to validate experimentally, since the setup is very costly. In the present work, one such analysis is carried out blending the hand calculations and steady-state finite element analysis to evaluate the contact pressure in a high pressure steam turbine casing. The work involves design considerations, design checks, validation and sensitivity analysis to achieve the design criteria to fulfill the structural requirements for mechanical integrity. Our aim of this project is to estimate the contact pressure so that there should not be any leak. Pretension in bolts is considered to achieve a firm contact between the casings. The three dimensional model of steam turbine casing were created using Hyper mesh Software. The cad model created was meshed using ANSYS and results are obtained. Keywords: Cad Modeling, Steam Turbine Cite this Article: Chelamalasetti Pavan Satyanarayana and Dr YV Hanumantha Rao, Design and Analysis of High-Pressure Casing of A Steam Turbine. International Journal of Mechanical Engineering and Technology, 8(7), 2017, pp INTRODUCTION Generally turbine casings used are split horizontally and vertically. The casing houses the blades rotor, nozzles, and diaphragms. It also holds glands for steam sealing at each end for preventing leakage of steam from where the shaft passes through. Steam turbine generator designs for fast-start plants feature optimized casing designs to reduce the thermal stress during startup and rapid load changes. The use of higher grade material may be employed in the HP and IP casings and valves to reduce component thickness. In recent days, power plants feature includes a fully automated turbine startup and shutdown control system and integral rotor stress monitor. The rotor stress monitor is typically capable of limiting or reducing the steam turbine load or speed increase and is designed to trip the turbine when the calculated rotor stresses exceed allowable limits. Present steam turbine power plants are using warming blankets to reduce startup time and rotor stresses. Steam turbines have to be warmed slowly to avoid excessive differential expansion of the various components, rotor stress, and potentially reduced service life. For that reason, startup times must be extended under certain conditions to allow gradual heating of components, which can prevent plants from starting swiftly and seizing spikes in real-time market prices. For the most of LP turbine inner casing rings, uses cast carbon steel (ASTM A 356 Grade1for cast carbon steel). The reason is that cast carbon steel is weld repairable. Sometimes nodular cast iron as inner casing rings for old GE large LP steam turbines. However, weld repair from steam or water droplet erosion can be difficult on casing rings. For an IP outer shell you need a low alloy steel like 1Cr-1/2 Mo or 1.25Cr- 1/2 Mo (A 356 Grade 5 or 6) for elevated temperature strength and ductility to guard against thermal/mechanical fatigue cracking at shell transitions editor@iaeme.com

3 Chelamalasetti Pavan Satyanarayana and Dr YV Hanumantha Rao Cut section showing casing and holes for mounting Bolts 2. ADVANCING STEAM TURBINE TECHNOLOGY Looking ahead, Voelker suggests further improvements in performance are anticipated: "What we are working on currently is, of course, some further improvements in efficiency, and strong interest and activity in the field of further development in the last stage blading area. That is one key component in the overall steam unit, to reach higher performance levels. "In mid-2015 Siemens also delivered a steam turbine that operates almost entirely without lubricants, with the bearing systems consisting of air-cooled, active electromagnetic bearings. The first 10 MW turbine equipped with magnetic bearings was installed at Vattenfall's lignite-fired Jänschwalde steam power plant in the German state of Brandenburg. Voelker concludes: "We believe that flexibility, going forward, is key in both dimensions: cost and efficiency. That needs to be supported as well by a flexible manufacturing system." 3. CAD MODELING It is very difficult to exactly model the Steam Turbine casing, in which there are still researches are going on to find out transient thermo mechanical behavior of casing during operating under higher temperature and pressure. There is always a need of some assumptions to model any complex geometry. These assumptions are made, keeping in mind the difficulties involved in the theoretical calculation and the importance of the parameters that are taken and those which are ignored. In modeling we always ignore the things that are of less importance and have little impact on the analysis. The assumptions are always made depending upon the details and accuracy required in modeling editor@iaeme.com

4 Design and Analysis of High-Pressure Casing of A Steam Turbine Cad model The assumptions made which are made while modeling the process are: 1. Casing material is considered as homogeneous and isotropic. 2. Inertia and body force effects are negligible during the analysis. 3. Structural analysis is carried out to find out the contact pressure 4. Thus stress level below yield stress is considered. 5. The analysis does not determine the life of the casing. 4. FINITE ELEMENT MODELLING AND ANALYSIS The finite element method (FEM) is the dominant discretization technique in structural mechanics. The basic concept in the physical interpretation of the FEM is the subdivision of the mathematical model into disjoint (non-overlapping) components of simple geometry called finite elements or elements for short. The response of each element is expressed in terms of a finite number of degrees of freedom characterized as the value of an unknown function, or functions, at a set of nodal points. The response of the mathematical model is then considered to be approximated by that of the discrete model obtained by connecting or assembling the collection of all elements. The disconnection-assembly concept occurs naturally when examining many artificial and natural systems. For example, it is easy to visualize an engine, bridge, building, airplane, or skeleton as fabricated from simpler components. Unlike finite difference models, finite elements do not overlap in space. The meshed model of steam turbine casing is imported to ANSYS and temperature distribution along the inner casing is performed at 200 C and observations are made for different conditions are shown below. Temperature distrubution along casing 14 editor@iaeme.com

5 Chelamalasetti Pavan Satyanarayana and Dr YV Hanumantha Rao Temperature Distribution in inner casing in unsteady (Transient) state condition after 2000s Temperature Distribution in inner casing in unsteady (Transient) state condition after 6000s Temperature Distribution in inner casing in unsteady (Transient) state condition after 12000s 5. CONCLUSION To maintain a high level of availability and reliability in a fossil power plant, substantial consideration of failure by repeated thermal loading should be carried out. In this study, the transient temperatures and stresses distributions within a turbine inner casing were achieved from actual operation data during cold start-up. Casing heat flux and distribution of total temperature is observed. The maximum deformations are calculated in transient state condition within inner casing editor@iaeme.com

6 Design and Analysis of High-Pressure Casing of A Steam Turbine Equivalent (von-misses) Stress distribution in Transient condition. Total deformation and stress values are compared with analytical results calculated for 2D geometry. If the thermal gradient is great enough, the stress at the bottom of the threads may be high enough to cause the carking. The result shows the casing develops higher stress levels in startup condition. REFERENCES Table shows thermal stresses values obtained at different conditions [1] W. S. Choi, E. Fleury, G. W. Song and J.-S. Hyun, A life assessment for steam turbine rotor subjected to thermo mechanical loading using inelastic analysis, Key Eng. Mat , (2006). [2] Lucjan Witek, Daniel Musili Ngii, thermal fatigue problems of turbine casing Vol. 1 (2009) [3] Maneesh Batrani, BHEL Haridwar, Hypermesh an effective 3-D CAE Tool in Designing of complex steam turbine low pressure casing in [4] T.Stubbs, the role of NDE in the life management of steam turbine rotors, Swindon, England [5] K. Fujiyama, Development of risk based maintenance planning program for Power Plant steam turbine, Final report on the Joint Project, pp (2007). [6] Kiyoshi SAITO, Akira SAKUMA and Masataka FUKUDA, Recent Life Assessment Technology for Existing Steam Turbines, JSME International Journal Series B, Vol. 49, No. 2 (2006), pp [7] Development of Life Prediction System for Thermal Power Plant Based on Viscoplastic Analysis, Final report, KERPI (2007) editor@iaeme.com