Implementation of a Vibration monitoring system of a steam turbine for optimization of the maintenance

Transcription

1 Implementation of a Vibration monitoring system of a steam turbine for optimization of the maintenance Hassane Elmaati 1, Abdelkader Benbouaza 2, Bachir Elkihel 3, Fabienne Delaunois 4 1,2,3 Department Mathematic and Mechanic, National School of Applied Sciences, Oujda, Morocco 4 Department of Metallurgy, Mons University, Mons, Belgium Abstract: A steam turbine is the most strategic installation in an electricity production process. Its unforeseen unavailability will lead immediately to the loss of production and it can be the seat of incidents or severe accidents threatening the safe operation of the machine and personal security. The purpose of this article is to present the implementation of a vibration monitoring system of a steam turbine in the context to replace the routine maintenance of this machine by a conditional preventive maintenance to avoid serious human, material or economic losses, in case of incident or failure of this equipment. Keywords: Conditional Preventive Maintenance, Diagnosis, Optimization of the Maintenance, Vibration Monitoring. 1. INTRODUCTION A steam turbine is classified as vital machinery [1] whose failure will lead an immediate cessation of electricity production. Otherwise, any incident or accident at this installation threatens directly the safe operation of equipment and personal security. In other words, direct and indirect maintenance costs of this strategic machine are very important. In fact, its rehabilitation requires special spare parts and involves experts with extended shutdown. Thus, adopt adequate preventive maintenance at this turbine is indispensable in the objective to [2]: - Increase productivity, - Improve substantially decision support tools based on on the reliability, by providing capacity the optimization near-real time, - Ensure safe operation of the machine and personal security, - Reduce the risk of failure and increase the availability, - Plan maintenance, - Optimize maintenance resources, - Optimize the stock of spare parts. In this context, a vibration monitoring system has been implemented at the turbines of the power plant study site to replace comparative measurements of the vibration which the irregular use increases more and more the risk of unavailability or incident and they give no indication of defective components in case of any anomaly. This monitoring, which is done without disruption of electricity production [3], constitutes the pivotal role of a conditional predictive maintenance policy on this vital equipment. 2. PART OF THE STUDY The site of our study is a thermal power plant which includes three units. Each one is provided with a group comprising a turbine, a generator and an exciter. 2.1 Technical characteristics of the turbine The turbine is an external combustion engine which transforms the energy of the steam into mechanical energy in order to turn the alternator rotor. This transformation is occurred in two stages: - Transformation of the potential energy of the steam (pressure) into kinetic energy (speed), - Transformation of the kinetic energy into mechanical energy (rotation of the rotor) i.e. the transfer of kinetic energy of the tangent of impellers which entails a generator. The turbine covered in this study is a condensing steam turbine (see fig n 1&2). It is a single cylinder machine with a single shaft which includes a final control stage and 21 pressure stages. The first 19 rotor disks are forged integrally with the shaft and the last three are fitted on it. The rotors of the turbine and generator are joined by a half flexible coupling. The turbine is characterized by a: - Nominal Power of the turbine : 55 MW - Speed of rotor : 3000 rpm (a) View of side of the turbine Volume 2, Issue 6 November December 2013 Page 240

2 (b) View of side of the exciter Figure 1 Group turbine-generator-exciter Figure 2 Rotor of steam turbine 2.2 Technical characteristics of the generator The generator is a rotating machine comprises two cylindrical and coaxial plates, moving relative to each other (see fig n 3&4): - The fixed armature (stator) is constituted by a ring of magnetic sheet held by the frame. The stator carries a three-phase armature winding distributed on the inner periphery of the ring and housed in notches, - The moving armature (rotor) turns within the stator. The rotor carries a field winding traversed by a direct current. The generator used in thermal power plant has the following characteristics: - Nominal Power : P=55MW. - Stator voltage : V=10.5 KV. - Frequency : F=50 Hz. - Speed : N=3000 rpm. Figure 4 Rotor of generator 2.3 Technical characteristics of the exciter The technical characteristics of the exciter (see fig n 5) are as following: - Continuous Power : 470 KW - Short-term Power : 1380 KW - Long Term Voltage : 280 V - Short-term loads : 480 V - Current walk long : 1680 A - Current walk short : 2880 A - Speed of rotation : 3000rpm - Oil consumption for two bearings : 24 l/mn - Flow of cooling water : 15 m 3 /h - Weight of exciter with chassis : 6700 Kg - Weight of the exciter armature : 1460 Kg (a) View of side of the exciter Figure 3 Rotor of generator in the stator (b) View of rear of the exciter Figure 5 Exciter Volume 2, Issue 6 November December 2013 Page 241

3 3. POSITION OF PROBLEM The technique of preventive maintenance initially used to monitor in operation the health of the steam turbine purpose of this study consists of collecting irregularly vibrations at different bearings of this equipment. Any evolution by comparing these vibration readings in relation with reference values which are the danger and alert thresholds allowed alerting a degradation operation without designate the defective component at the bearings of turbine. The reference values constituting the signatures alert and danger thresholds at this steam turbine are taken from the scale adopted in thermal power plant site of our study. These thresholds assessment of vibration are determined according to speed of rotation [4]. Thus, this technique practiced at this strategic installation which is based on the method of vibration analysis method called "global measure" let incur a very high risk of shutdown of electricity production and mean a daily shortfall of about 660 KDH (KDH = 10 3 Moroccan Dirham). Indeed, this method of comparative measurements of vibration with an irregular manner is not adequate to detect at the appropriate time an abnormal functioning of the turbine and limit any investigation as it absolutely impossible to identify the possible causes of excessive vibration. So, the information collected by this monitoring is incomplete to plan and to optimize maintenance at this strategic installation. 4. MAIN POSSIBLE DEFECTS IN A STEAM TURBINE There is no predictive maintenance without a minimum diagnosis of the defects and their severity. For this reason, the first step of a monitoring action is to ask what the potential defects of the monitored machine are. The second concern the manifestation of these defects. What information, what descriptor parameters of the defect must be elaborated up and be measured to require solid information which will allow us to tell if the situation is normal or not (detecting abnormality), but also those that will allow us to found subsequently their origin (diagnosis of the origin and the severity of abnormalities) [2]. The main vibration manifestations of malfunctions of steam turbine are as follows: 4.1 Massic unbalance The mass unbalance is eccentricity or excess of material (bad machining shaft, etc...), Thus creating a mass imbalance in the rotor. [5] This imbalance creates a centrifugal force thus increasing the amplitudes of vibrations at a frequency equal to the frequency of rotation (1xRPM) in the spectral domain. The unbalance is distinguished from other defects by a phase shift of 90 between two radial sensors orthogonal because it is synchronized with the rotation of the mobile. 4.2 Magnetic unbalance The magnetic imbalance is the result of a defect in the in the electrical circuit affecting the magnetic field that keeps the rotor centered in relation to the stator. This is characterized by the failure of a rotor pole (or electromagnet) pole that disrupts the electromagnetic force field. This disturbance is synchronized with the rotation speed of the mobile. A magnetic unbalance will be prominent on the radial accelerometers near the generator and appear at the frequency of rotation at the frequency spectra [5]. 4.3 Misalignment The misalignment (or alignment problem) occurs when the rotation axes of the two coupling components form an angle and / or shift of one in relation to the other. The consequence of the misalignment is the appearance of severe vibrations in the axial direction and / or radial. The angular misalignment produces mainly axial amplitudes with 180 phase shift between the two sides of the coupling. The parallel trimming produces radial amplitudes with a phase shift of 180 between the two sides of the coupling. An alignment problem manifests in the spectral domain at 1xRPM and / or 2xRPM, depending on the type of coupling used [4]. 4.4 Tightening Mechanical loosening occurs when rotating components are not attached correctly. Structural loosening can be caused by loosening or deterioration of machine feet (antivibration mounts), unsuitable foundations, bolts or nuts loosened. These conditions occur at 1xRPM and / or 2xRPM in the radial direction in the spectral signal (vertical amplitudes may be more important), and depending on the type of structural problem [4]. 4.5 Oil whirl This phenomenon appears on the weakly loaded smooth bearings slightly polluted lubricated in the hydrodynamic regime. It occurs slightly below half the frequency of rotation of the shaft (from 0.42 to 0.48 times the frequency rotation according to the bearing characteristics). This along imbalance of the rotor and a resonance frequency nearly the rotation speed, led to the whipping phenomena of rotor [4]. 4.6 Couplings In our case, the coupling is of plate type, the coupling a plateau type which should allow the axial expansion of the turbine, or radial movements. The defects of this type of coupling are mainly: - Bad centering plates, - A parallel defects (a non-perpendicularity of one of the plates relative to the axis of rotation). These two defects create an unbalance and Volume 2, Issue 6 November December 2013 Page 242

4 therefore vibration at the frequency of rotation fr [2]. 5. DEVELOPMENT OF A VIBRATION MONITORING SYSTEM AT THE TURBINE Vibration analysis at the steam turbine will no longer stop at the observation of the phenomenon, it will also indicate the type of deterioration and which organ is concerned. Thus, to have a complete and accurate view of the vibrations at different bearings of the turbine, a vibration monitoring system has been established to monitor vibration evolution and make a diagnosis when monitoring detects an anomaly or a dangerous evolution of the vibration signal [1]. To establish this system, supply, installation and implementation of on-line vibration monitoring of the group (turbine, generator and exciter) were performed with a quantitative of supplies as follows: 12 accelerometers: 2 accelerometers per bearing mounted on two perpendicular axes. The accelerometer provides a signal at 100mV / g be transmitted over a long distance without attenuation. Its frequency range is high at 2 to Hz (+/- 3dB). The sensor is supplied by a constant current at 4mA, Shielded cables, The junction boxes, 6 proximity sensors for contact-free measurement of relative displacement of moving parts on machines. They are particularly well adapted to measure relative vibrations and axial position of the shafts of steam turbines. The characteristics of these sensors are as follows: o Contact-free measurement, o Principle of the eddy current, o Measuring range: 2mm, 4mm or 12mm, o Sensitivity: 1.33 m V / µm, 4m V / µm or 8m V / µm, o Frequency Response: DC to 20 khz (-3dB). These proximity sensors have been implemented as follows: A proximity sensor with integral cable mounted at the end of radial shaft and measures the variation in radial position due to its eccentricity, o Two proximity sensors for monitoring of speed of rotation and measures phase reference, o A proximity sensor with integral cable mounted parallel to the axis of rotation for the monitoring of the differential dilatation, o Two proximity sensors mounted parallel to the axis of rotation in opposition to 180 relative to the other for monitoring the axial displacement. 1 Electronic monitoring system composed of 2 racks with data loggers memo graph (see fig n 6&7): o 1 rack for vibration monitoring, o 1 rack for monitoring position and speed of rotation. (a) Electronic monitoring system for unit n 1 (b) Electronic monitoring system for unit n 2 (c) View of rear of the Electronic monitoring system Figure 6 Electronic monitoring system (a) Registration of absolute vibration Volume 2, Issue 6 November December 2013 Page 243

5 (b) Registration of Differential Dilatation, Axial displacement, Eccentricity and Speed of rotation Figure 7 Data loggers of measured values The figures (n 8, n 9 and n 10) below show the location of measurement points and the type of measurement for monitoring the turbo generator. - VA6 to VA1: measuring absolute vibration by accelerometers, radial vibrations in two perpendicular axes X and Y, o Bearings of steam turbine ( Hz frequency, full-scale 0-100μm peak), o Generator Bearings ( Hz frequency, fullscale 0-100μm peak), o Exciter bearings ( Hz frequency, full-scale 0-100μm peak). It is also expected to be able to measure axial vibration on exciter. - DD: measuring Differential Dilatation of the shaft by proximity sensor: o The turbine shaft (full-scale + /-6mm). - DA: measuring Axial displacement (wear thrust) by two proximity sensors: o Thrust bearing (full scale + /-2mm). - EX: measuring Eccentricity by a proximity sensor: o End of the turbine shaft (full scale 500 μm peak to peak). - VP1 and VP2: measuring Speed of rotation and reference phase by proximity sensors (full scale 4000 rpm). (b) Accelerometers vibration sensors implemented on the bearing n 2 of the turbine Figure 8 Accelerometers vibration sensors implemented on the bearings of the turbine (a) Accelerometers vibration sensors implemented on the bearing n 1 of the generator (b) Accelerometers vibration sensors implemented on the bearing n 2 of the generator Figure 9 Accelerometers vibration sensors implemented on the bearings of the generator (a) Accelerometers vibration sensors implemented on the bearing n 1 of the turbine (a) Accelerometers vibration sensors implemented on the bearing n 1 of the exciter Volume 2, Issue 6 November December 2013 Page 244

6 vibration monitoring system wireless to reduce more and more conditional predictive maintenance cost. (b) Accelerometers vibration sensors implemented on the bearing n 2 of the exciter Figure 10 Accelerometers vibration sensors implemented on the bearings of the exciter The Table below shows the alert and danger thresholds of different parameters monitored (absolute vibration, differential dilatation, axial displacement, eccentricity and speed). Table 1: Alert and Danger thresholds This current method of vibration monitoring at the group of turbine-generator-exciter constitutes a detecting tool and diagnostic of malfunctions and contributes to reduce overall costs for an optimal maintenance of this installation [2]. Indeed, it ensures: - Visualize in real-time the state of the group, - Monitor and manage alarm levels, - Increase the availability and productivity, - Establish a precise diagnosis by identifying the defective component, - Reduce the risk of failure and incident, - Plan maintenance interventions and limit the level of preventive. 6. CONCLUSION AND PROSPECTS To establish a continuous vibration monitoring at the elements of the turbine-generator-exciter group rather than just comparative measurements of vibration levels, leaving to greater important risk of failure and incident, it possible to plan and to optimize maintenance of this group [6]. Otherwise, many prospects are envisaged after the completion of this system. Thus, it is possible to define and establish an adequate equipment to make the current References [1] D. AUGEIX, " Analyse vibratoire des machines tournantes" Techniques de l Ingénieur, traité Génie mécanique BM [2] J. MOREL, "Vibration monitoring and predictive maintenance", Technique de l ingénieur, traité measures et contrôle, R [3] H.P. RAMELLA, "Maintenance des turbines à vapeur ", Technique de l ingénieur, traité Génie mécanique, BM [4] H.ELMAATI, A.BENBOUAZA, B.ELKIHEL, F.DELAUNOIS, "Development of a Vibration Monitoring System for Optimization of the Electrical Energy Production ", International Journal of Emerging Technology and Advanced Engineering, Volume 3, Issue 6, June [5] E. BENEVISE, "Etude de la corrélation entre les mesures vibratoires et l état réel des groupes de production hydro-électriques", INSA Strasbourg, Septembre [6] A. Benbouaza, B. Elkihel, and F. Delaunois, "Analysis and diagnosis of the different defects of asynchronous machines by vibration analysis ", International Journal on Computer Science and Engineering vol. 5, issue 4, April AUTHORS Hassane ELMAATI received his engineer degree in Materials engineering in 1997 at the National School of Mineral Industry (ENIM), Rabat, Morocco. Since 1998 until today, He s an engineer of maintenance in a thermal power plant in Morocco. Head of technical division. He is presently pursuing his PhD program in industrial engineering Department, National School of Applied Sciences University Mohammed I, Oujda, Morocco, on the theme Optimization of the production by an intelligent maintenance. Abdelkader Benbouaza Research engineer in industrial engineering on the theme maintenance of the complex installations industrial by vibration analysis. Bachir El Kihel Professor of Higher Education Grade: C, head of department math and mechanical. PhD status in Structural Design. PhD University of Compiegne engineering school France in Vibration and control non destructive. Fabienne DELAUNOIS Associate Professor- Head of Service Metallurgy University of Mons Belgium. Volume 2, Issue 6 November December 2013 Page 245