vibration diagnosis approach for industrial gas turbine ... · compressor and turbine are due to...

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British Journal of Applied Science & Technology 14(2): 1-9, 2016, Article no.BJAST.23163

ISSN: 2231-0843, NLM ID: 101664541

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Vibration Diagnosis Approach for Industrial Gas Turbine and Failure Analysis

Sulaiman Khalifa Saif Al Adawi1* and G. R. Rameshkumar1

1Department of Mechanical and Industrial Engineering, Caledonian College of Engineering,

P.O.Box 2322, CPO 111, Muscat, Sultanate of Oman.

Authors’ contributions

This work was carried out in collaboration between both authors. Author SKSAA designed the study, performed the statistical analysis, wrote the protocol, carried out the practical experimentation in the field, data collection, wrote the first draft of the manuscript and managed literature searches. Author GRRK managed the analyses of the study and literature searches. Both authors read and approved

the final manuscript.

Article Information

DOI: 10.9734/BJAST/2016/23163 Editor(s):

(1) Elena Lanchares Sancho, Department of Mechanical Engineering, University of Zaragoza, Zaragoza, Spain.

Reviewers: (1) Chengwei Fei, Hong Kong Polytechnic University, Hong Kong, China.

(2) Bharat Raj Singh, Dr. APJ Abdul Kalam Technical University, Lucknow, India. Complete Peer review History: http://sciencedomain.org/review-history/12930

Received 17th

November 2015 Accepted 18

th December 2015

Published 9th January 2016

ABSTRACT

This study is carried out to analyze the problem of high vibration in gas turbine installed in power and desalination plant. This study is carried out in Al Ghubra Power and Desalination plant. Three case studies related to vibration problem were presented to diagnosis the cause of vibration and to find the root cause for failures that occur in the gas turbine. The vibration data were collected using accelerometer at three directions for the rotational speed of 3000 rev/min at different load conditions. The date was presented in the form of graphs and comparative study was made. It has been observed in all the three cases, the high amplitude of vibration. Data were analyzed for this high vibration and corrective action initiated to rectify the problem to reduce vibration level. In the first case study the high vibration in the generator rotor side and is due to unbalance condition of rotor. The vibration level is reduced after performing balancing of rotor. In the second case study it is to find the reason for high vibration in the gas turbine during the first run up at Full Speed with No Load. It is observed that the high vibration in the generator is due to wear out parts and damage of pinion in the gear box. It is suggested to

Original Research Article

Al Adawi and Rameshkumar; BJAST, 14(2): 1-9, 2016; Article no.BJAST.23163

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clean and replace the wear out part, and performed the same test again. It is interesting to note that the vibration level is minimal and it is within the acceptable level. In the third case, the flow back of gas to the machine casing resulting in accumulation of gas and when ignited caused the explosion in the exhaust side. Detailed analysis were made to analyze the problem and rectified. In all the three case studies the investigation has shown that the high amplitude of vibration is due to many reasons hence it is suggested to implement a proper way of regular maintenance work to allow gas turbine to run smoothly without any problem. Few recommendations are proposed for preventing similar failures in the future. It is also suggested to use online condition monitoring tool to assess the operation condition and to decide the time of maintenance work.

Keywords: Vibration; gear box; generator; industrial gas turbine.

1. INTRODUCTION The gas turbine is the most common unit in production of power and the most important units in the industry to produce energy. The gas turbine engines are more critical to the operation of power plants, aircraft and heavy vehicles. The gas turbine consists of three main parts, a compressor, a combustion chamber and the turbine in addition to the generator, which is associated with it through the gearbox. Many industrial gas turbines have a direct coupled, integral power turbine that extracts all the energy needed to both drive the compressor section of the turbine and transmit torque to the driven equipment through an output shaft. The failure of gas turbine causes the effect on generation of power. There are many reasons for failure of components of gas turbine. Vibration Analysis is a very powerful tool which allows early detection of faults in most types of rotating equipments such as motors, compressor, gear boxes and turbine etc. If any rotating equipment operating at normal condition, the level of vibration amplitude will be within the acceptable level as specified in standard vibration level for various rotating equipments. If the machine is running at very high speed and if there is any fault in the machine causes high vibration level, if not rectified the problem grow further and finally equipment fails. General defects that cause high vibration in rotating equipments are unbalance of rotating parts, misalignment of couplings and bearings, bent shafts, mechanical looseness, rubbing, resonance etc. The most common problems which lead to high vibrations are rotor unbalance and shaft misalignment. The symptoms observed in vibration spectrum as high amplitude of vibration at 1X (one time rotation speed) in radial direction for unbalance fault whereas for misalignment fault high amplitude of vibration at 2X (two time rotation speed) in axial direction. There are earlier studies discussing the most important reasons for failure and analysis of the

major causes [1]. And as well as the ways that has been followed to ensure the continuity of the work of the turbine. These studies are concentrated most of the issues due to vibration that affects more components of the turbine [2]. Consequently, the focus of the research will more towards the condition monitoring methods and the vibration diagnostic approach to monitor Industrial gas turbine and to analyze root cause for failure [3].

There are many earlier studies that carried out in diagnosing the causes of failure of components in the gas turbine. Carazas and de Souza [4] presented a method for assessing the reliability and availability of gas turbines installed in power plants which uses the application of the concepts of reliability centered maintenance policies to improve the maintenance of a complex system that aims to reduce accidents in the unexpected failure of critical components. Guo [5] conducted extensive experiments and used the FEM model to study the vibrations as well as identifying ways which are most likely to occur. Their results showed that fluctuations in torque and pressure fluctuations are most likely to excite resonance blade shaft coupled modes. Non-intrusive measurement of the case within the gas turbine blade is of major interest in all areas of use. Forbes and Randall [6] suggested that the measurement of vibration casing, due to the aerodynamic, structural interaction within the turbine, can provide a way to monitor the status of the blade and the modal parameter estimation. Ogbonnaya, et al. [7] studied the properties of compressor associated with turbine. Surge and stall are two major types of instability that often occur in the compressor system that influence on the gas turbines. This instability often leads to excessive vibration due to the pressure again to impose order by this phenomenon. In this work, it was observed fouling, as the main cause of instability of compressor. They also provided steps to analyze how it can control this


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phenomenon with the continuous examination of the vibration amplitude using a computational approach has led to the implementation of this work. Clifton, et al. [8] present a novel method for the identification of abnormal episodes in gas-turbine vibration data, in which they showed how a model of normal engine behavior is constructed using signatures of normal engine vibration response, the extreme value theory (EVT), a branch of statistics is used to determine the expected value of extreme values drawn from a distribution, and these can be used to set novelty thresholds in the model, which, if exceeded, indicate an abnormal episode and application to large data sets of modern gas-turbine flight data, which shows successful novelty detection results with low false-positive alarm rates. Ogbonnaya, et al. [9] discussed the case studies of gas turbine engines and they use step-by-step analysis of the plant parameters to develop a computer program to determine the condition of the components of the plant. Choi, Y. and Lee, K. [10] studied a gas turbine and investigated the broken surface of the blade and stress analysis of the blade and they observed that the maximum stress occurred due to the pressure profiles developed during operation. In this work the vibration monitoring method is adopted to analyze the operational condition of gas turbine and to find out the root cause for failure of gas turbine components. This work is presented as three case studies to understand the operational condition of gas turbine and to analyze the causes of gas turbine components failure. 2. METHODOLOGY The gas turbine works on the principles of change of kinetic energy of the gas to a special kind of kinetic energy which is the rotational energy and which is in turn used to rotate the machine blades. Turbine provides the mechanical energy to other components through the rotation of the rotor shaft. Turbine provides energy for different machines, including generators. In fact, it produces most of the electricity used in lighting homes and supplies to operate industries. The Gas Turbine consist mainly three parts air compressor, the combustion chamber and Turbine. Gas turbines operate at a higher temperature of the steam turbine. And increase the effectiveness of the turbine higher temperature operation; the temperature of operation of most of the gas turbine is an 875°C or more. The turbine is often

connected o the horizontal axis of the air compressor on the other hand, with the mechanical load to be recycled (e.g., a generator) and through the gearbox which is used to reduce speed because the speed of rotation of the turbine is very high compared to generator. In this work three case studies have been studies at Al Ghubra Power and Desalination Company SOAC which is established in 1975 with the aim of producing electricity and water desalination. Al Ghubra power station faces problem of high vibration of gas turbine and also in other associated equipment’s. The most important common problems in gas turbines are compressor and turbine vibration, generator rotor vibration, gear box vibration. High vibration in compressor and turbine are due to lack of air circulation inside the combustion chamber leads to problem in Inlet Guide Vane (IGV) which is the distribution of the air in equal quantity to all stages of the compressor, causing the back pressure or surge to the compressor or turbine. This causes damage to turbine blades such as twisting blades or broken blades. Most of the time, the problem of vibrations occurs in the generator's rotor due to imbalance, shaft crack, bent shaft, looseness, misalignment and other malfunctions. These factors lead to increase the vibration in the gas turbine; this is because of wrong installation of the generator's rotor. Also, there is a problem such as cracks in the bearing where they are two bearings in the generator one called drive bearing coupling with the gearbox and thrust bearing and other one is non-driving bearing. The gearbox is the important parts in the gas turbine, where it reduces the speed of the turbine, which is estimated at 5000 rpm to run in parallel with the speed of the generator, which is running at 3000 rpm. The gearbox causes high vibration due to misalignment of gears and connecting shafts. Vibration diagnosis and failure analysis is now more effective than ever, in order to ensure reliable operation of rotary machines. The operating conditions of Turbines, pumps, generators, compressors, engines, gearboxes, etc. should be monitor to ensure safety and continued operation with out any failure. The vibration monitoring is one of the best methods used to analyse the condition of rotating machinery. In this study vibration monitoring is used to analyse the critical components of gas turbine. The real time vibration data is acquired from the gas turbine in Al Ghubra power and desalination plant. This study is conducted in


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cooperation with engineers at the desalination and power plant to identify the root cause of vibration and subsequent failure of the gas turbine components.

The Gas turbine (GT5) with 17.5 MW generating capacity has been selected for this study. It was commissioned in 1979 and the unit is of two stage turbine with three main radiator fans and one minor radiator fan. The turbine speed is 5120 rpm and reduced to 3000 rpm as the generator is 3000 rpm a reduction gear (VF 63).

3. RESULTS AND DISCUSSION

The detailed procedure and analyses of all three cases presented and discussed in this section.

3.1 Case Study 1

This study is carried out to analyze the reason for high vibration in the generator side during the first run up at Full Speed with No Load (FSNL). The vibration pick up instrument with accelerometers are used to measure the vibration. The accelerometers were mounted at four locations of the gas turbine assembly and these locations are tabulated in Table 1 and corresponding vibration readings required for investigation were collected at these locations.

The amplitude of vibration recorded at rotor rotational speed of 3000 rpm is shown in Fig. 1(a). It is observed that high vibration at front and rear bearing locations in horizontal direction. To identify the cause for this high vibration, the unit put in crank mode to check if any damage in the compressor, generator or in the turbine. Initially it is noticed that the damage in the coil, even after cleaning and replacing the coil it is observed the high vibration as shown in Fig. 1(b).

Table 1. Location of Accelerometer

Location of Accelerometer

1 Front Bearing – Vertical

3 Rear Bearing - Vertical

2 Front Bearing - Horizontal

4 Rear Bearing - Horizontal

Further analysis shows that this is due to unbalance in the rotor of the generator. This is due to accumulation of dust and additional weights added to balance the rotor year after year. As a corrective measure, it was decided to remove weights added at exciter (rear bearing) of 4 weights at 348° and 3 weights at 0° of weighing 25 gram each as shown in Fig. 2(a). The vibration reading were recorded and represented in Fig. 2(b).

Fig. 1. a) During run up b) Initial after replace of coil at 3000 rev/min

Fig. 2. a) Removal of weights at generator side b) vibration reading after removal of weights at rotational speed of 3000 rev/min


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However it is observed only small reduction in vibration level, hence it is suggested to do balancing of rotor. To achieve proper balance, further five weights with a total of 125 grams were added at angle 330° on the generator rotor as shown in Fig. 3(a). It is observed that the vibration level is reduced to acceptable level as shown in Fig. 3(b).

After balancing the vibration reading were collected with different operating conditions. The results of tests performed before and after the balancing of the generator rotor in the gas turbine to see the effectiveness and success of the process of balancing. To make sure the last balancing step was successful, we do the other test by the run-up gas turbine and put at load 10 MW and take a reading of vibration and the result as show in Figs. 4(a) and (b). If we compare the results of vibration between the after balancing on FSNL and after base load 10 MW we can see how the result observed is very close to the results obtained which is shown in Fig. 3(b). This means that the balancing process is completed perfectly and the gas turbine now operating in good condition. Also can be seen from these graphs, the difference in vibration amplitude

before and after the balancing position in the FSNL and this will improve the performance of the turbine.

3.2 Case Study 2 This study is carried out to analyze the reason for high vibration in the gas turbine during the first run up at Full Speed with No Load (FSNL). Seven locations were identified to acquire vibration readings using accelerometer on the gas turbine and gearbox as shown in Fig. 5. The accelerometers were mounted at these seven locations of the gas turbine assembly and these locations are tabulated in Table 2. Vibration readings were acquired in all three directions for three different load conditions such as Full Speed with No Load (FSNL), at 10 MW, at 15MW and at 20 MW load to investigate the problem. The amplitude of vibration recorded at rotor rotational speed of 3000 rpm and these are shown in Fig. 6. It is noted that the high vibration amplitude at reduction low speed compound thrust journal bearing (in gearbox) which is coupled to generator rotor in all the three directions. As the load increases the amplitude of vibration is also proportionally increased.

Fig. 3. a) Addition of weights at generator side b) vibration reading after added weights at rotational speed of 3000 rev/min

Fig. 4. After balancing a) For Full Speed with No Load (FSNL) b) With 10 MW load


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Fig. 5. Locations of accelerometers in the Gas turbine coupled to gearbox for FSNL, 10 MW and 20 MW at rotational speed of 3000 rev/min

During inspection of the pinion, gear seals, thrust bearings and journal bearing, it is observed that a broken in the low speed gear (pinion) as in Fig. 7(a) and fragments of them gone outside the casing. The hub shift linked with thrust bearing, journal bearing and gear had also worn out as observed is shown in Figs. 7 (b) and (c).

Table 2. Location of Accelerometer

Location of Accelerometer

1 Compressor bearing

2 Reduction high speed journal bearing [Rear]

3 Reduction high speed thrust bearing [Rear]

4 Reduction gear

5 Reduction low speed journal compound thrust journal

6 Reduction low speed journal bearing [Rear] or Generator bearing [Front]

7 Generator bearing [Rear] The Fig. 8 shows the interconnection of parts together gas turbine. After taking corrective action and change of damaged parts, the test is carried out with the same conditions and the

required vibration readings were acquired and shown in Fig. 9. The vibration level now reduced to acceptable level that indicated the good condition of gear box.

3.3 Case Study 3 This study is carried out to analyze the reason for Gas turbine tripping during start-up due to servo valve problem that was created a very loud sound in the technical building and observed heavy dust came-out from the chimney. Immediate action initiated to put the unit into crank mode to check if any damage in compressor or in the gas turbine. The investigation was started with a thorough visual inspection of the turbine and the blades surfaces; the observation showed that a serious damage in the diffuser as represented in the Fig. 10. However there is no clear evidence that is was caused due to this incident and it is not unusual to find this kind of damages during planned annual maintenance. A little shift of silencer was observed and also a buckling and a minor crack were observed on the exhaust stack wall outer surface as shown in Fig. 11. No evidence of a possible oil leak was found in the exhaust stack

Fig. 6. Amplitudes of vibration in mm/s at rotational speed of 3000 rev/min for 10 MW, 15 MW and 20MW load on Gas Turbine


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or anywhere in or around the unit. All compartments damper was found closed. The firefighting system was checked and found that Co₂ cylinders were not released indicating that the dampers were closed due to the high vibration during the incident. The turbine was

inspected by bore scoping (1st and 2nd stages) and no damage was found. Combustion system was inspected and four liners was removed and inspected. The liners showed a clear evidence of overheating on the surface as shown in Fig. 12, but no damage on was found.

Fig. 7. a) Broken pinion b) Thrust bearing wear c) Hub shift wear

Fig. 8. Amplitudes of vibration in mm/s at rotational speed of 3000 rev/min for 10 MW load on Gas Turbine

Fig. 9. Amplitudes of vibration in mm/s at 3000 rev/min for 10 MW load on Gas Turbine after corrective action

Fig. 10. Piece of damaged diffuser, and little shift of silencer


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Fig. 11. Bucket in good condition, crack in exhaust wall and bore scoping on the turbine

Fig. 12. Overheating in combustion liners and IGV in good condition

Considering the above observations, it was decided to check if any gas leakage, it was observed that the amount of gas leakage was too much during the start up. During startup, the air purge line NRV did not close fully as it is supposed to, which allowed gas to flow back to the machine casing, resulting in accumulation of gas and when ignited caused the explosion in the exhaust side. Hence, the following steps were carried out in order to check SRV and GCV:

1. The gas line was charged (unit in shutdown condition) to check if SRV and GCV are passing gas, and no gas leak detected.

2. The unit was put in crank mode and also no gas leak was found.

3. A temporary blind was fixed on the air purge line between purge valve and NRV with a pressure gauge fixed to detect any gas leak.

Dual fuel combustion turbines have a gas side air surge system to prevent fuel oil from dripping into the gas side of the nozzle during oil operation. When operating on gas, the purge system is closed by the NRV in the line running from the machine casing to the gas nozzles. The pressure gauge reads 6 bars during the test indicating that NRV is passing. To rectify this one permanent 4 mm blind was fixed with air purge line between purge valve and NRV and Fuel oil system was isolated temporary.

4. CONCLUSION

The gas turbine is one of the most important units in desalination plants for power generation. Maintenance of gas turbine is most important to run the plant without any failure. The presented three case studies provide insight to analysis and diagnose the most vibration problems. The first case study provides the analysis of unbalance problem in the rotor which is common in most of rotating machinery. Initially the high level of vibration amplitude was observed after initiating corrective action the vibration level reduced to acceptable level. The second case study presents vibration due to gear box problem and failure analysis is carried out to short out the problem by adopting proper maintenance and replacing damaged parts this gave good result in reduced level of vibration amplitude. The third case study provides the detailed investigation into the situation where the gas flow back to the machine casing resulting in accumulation of gas and when ignited caused the explosion in the exhaust side. The problem was analyzed and corrective action suggested the overcome this problem. As per the analysis carried out, it has been suggested to use regular maintenance work particularly online condition monitoring method to know the running condition of Gas Turbine in advance to plan maintenance activities to avoid sudden failure of Gas turbine.


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ACKNOWLEDGEMENTS Authors are grateful to Chief Executive Director, Al Ghubra Power and Desalination Company for giving permission and support to carry out this project work. COMPETING INTERESTS Authors have declared that no competing interests exist.

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_________________________________________________________________________________ © 2016 Al Adawi and Rameshkumar; This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Peer-review history: The peer review history for this paper can be accessed here:

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vibration diagnosis approach for industrial gas turbine ... · compressor and turbine are due to...

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