VIBRATION AS A DIAGNOSTIC TOOL FOR HEALTH MONITORING OF MACHINES, STRUCTURES AND EQUIPMENTS

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1 VIBRATION AS A DIAGNOSTIC TOOL FOR HEALTH MONITORING OF MACHINES, STRUCTURES AND EQUIPMENTS A. Rama Rao Reactor Engineering Division Mr.. A.. Rama Rao is the recipient of the Homi Bhabha Science & Technology Award for the year 2008 Abstract The need for vibration diagnosis for machinery, structures and equipments is a growing requirement in all types of industries including in nuclear power plants. This is largely because of the inherent benefit of being able to promote predictive steps well before the distress can manifest into failure. Response of every machinery, structure, equipment, component or system as a whole under dynamic loading is uniquely its own. Detection of such signals and monitoring the changing conditions is a technique involving the knowledge of engineering art combined with the mathematical background. This blend of sound engineering judgment and the skill of vibration data interpretation is in fact the basis of vibration diagnostic techniques. Key ey words: vibration, machinery, diagnostics, signals, turbine blades Introduction The need for vibration diagnosis for machinery and structures is a growing requirement in all types of industries including in nuclear power plants. This is largely because of the inherent benefit of being able to promote predictive steps well before the vibration related distress can manifest into failure. It is known that every moving element in a rotating machine generates vibration signal that is uniquely its own. Likewise response of every structure, equipment, component or system as a whole under dynamic is uniquely its own. Detection of such signals and monitoring the changing conditions in a machine through vibration analysis is a technique involving the knowledge of engineering art combined with the mathematical background. Vibration technique offers cost effective and reliable method of condition evaluation. It helps in taking corrective measures much before damage/breakdown can occur. This blend of sound engineering judgment and vibration data interpretation skill is in fact the basis of vibration diagnostic techniques. Some of the important vibration diagnoses are explained below with illustration. Detection and prevention of water hammer in moderator system of MAPS [1] After the damage to the inlet manifold, the plant was operating at lower power. With an aim to restore the generation capacity, sparger tubes were introduced in the calandria in moderator system. Since for the first time perforated tubes were being used in the core for moderator flow distribution, extensive pre-commissioning tests/trials on full scale sparger were carried out in RED. As there were number of issues on fluid induced forces in the sparger, many vibration qualification tests were carried out. After the spargers were introduced in MAPS core, before the final acceptance, in-situ dynamic condition was monitored. During the commissioning with moderator flow and dumping trials, a loud sound was heard propagating in the moderator piping. Fig. 1 shows the 12

2 Fig. 1: Flow sheet of Moderator system in MAPS and Shock due to water hammer flow sheet of moderator system. It was found out that, just before dumping the moderator from the core, two pumps are tripped and the control valves are closed simultaneously. From the data analysis, it was found that the valve was being closed against high flow in the line causing water hammer. To avoid high shock loading to the piping, new sequence of dumping was suggested. After implementation, the solution was verified and found effective. This diagnosis had high impact in smooth operation of moderator dumping which is done on every shut down. Girder Modification in Critical facility for Seismic Qualification [2] It was required to qualify the design of shut off rod guide tube, girders and drive mechanism for seismic loads. Set 13

3 of nine girders on top of the reactor vessel supports the entire in-core components. From analytical results it was found that with the mass loading, the girders were not qualifying. In situ tests were carried out to verify the dynamics of the girder. To meet the safety requirement, it was required to modify the girder. (Fig. 2). Suggestion was given based on modal modification analysis to add additional bridge plates. Bridge plates were fabricated, installed and verified by test for the adequacy of seismic loads. vibration required for working of sensitive NMR. Realizing the design deficiency, the supplier replaced the isolators with required size. With new isolators, NMR was declared as commissioned. (Fig. 3). Diagnosis of Isolators for 800 MHz Nuclear Magnetic Resonance (NMR) at TIFR TIFR has established a national NMR facility in their campus for research and industrial application. The 800 MHz NMR was purchased from Switzerland after inspection and testing. After commissioning in the campus, the results showed extraneous disturbance not understood by the supplier and the user. The civil structure and the NMR support structure were analyzed to identify the source. Isolators supplied with the NMR were found to be inadequate for high level of attenuation of building Fig. 3: Showing NMR with isolators Fig. 2: Cross Section View of Critical Facility and the Modified Girder (green bridge plates) 14

4 Turbine Blade Diagnostic system for Power Plant [3] Heavy duty rotary machines are prone to vibration related problems at almost every stage of it useful life. The teething problems appear during commissioning and then wear and tear due to high duty cycle and subsequent ageing process demand constant monitoring for continued operation. Machines such as turbo generators are most critical equipment in the entire power plant. The turbine blades work under hostile working condition. Among hundreds of blades of different size and geometry rotating in the turbine, even if one blade fails the damage and the consequence is very large. In spite of thousands of turbines operating in the world, there are still many unknown forces and synergies in the machine that can manifest into wide scale damage. Blades in a turbine vibrate due to many reasons. While all care is taken to design the blades to endure all the known forces of excitation, still loads resulting from blade vibration remain elusive. Though new blades promises fail free life, the improvements are still far away from the expectation of shop floor. For a power plant, the most important diagnostic is to monitor the health of turbine blades. With innovative technology, low pressure turbine in TAPS-3, and gas turbine in one of the NTPC plant has been equipped with on line monitoring system.(fig. 4). The system identifies blade signals from the turbine casing vibration and correlates it with the process to establish the cause and effect relationship. Such a system is unique and first in the country. Vibration Technology Transfer[4] Vibration based technologies have proved to be very effective in industries in dealing with complex shop floor problems. Vibration meters, shock pulse meters, kurtosis meters, acoustic meters, sound level meters and many others are built on vibration technologies. A strong Fig. 4: Turbine Blades and Shaft Health Monitoring System in TAPS and NTPC Gas Plant 15

5 Section of RED. The sustained encouragement given by Dr R.K.Sinha Director, BARC and Director, RD&DG and Mr. D. Saha Associate Director, RD&DG has played an important role in breaking new grounds in developing diagnostic techniques. References 1. Sparger Vibration Measurement at MAPS-2. RED/VLS/ 4009/2003. Fig. 5: Hardware under testing for checking the quality of brazed joint below the surface correlation between the mechanism and the resulting vibration is all that is required to build such technology. Two technologies were developed, implemented and inducted for testing the hardware developed for important projects. Both are unique in characteristics and have nothing common with machinery diagnostics. One was for testing the quality of brazed joints (Fig. 5) that are not visible on the surface of the hardware and the other was to check good bonding between two dissimilar materials. Both the technologies were based on rigorous vibration analysis of high frequency signals captured during testing. The repeatability and ease of operation was highlight of the technologies. 2. Experimental Modal Analysis and Finite Element Analysis of Lattice Girder and Guide Tube of Shut off rod of AHWR Critical Facility for Seismic Qualification. RED/VLS/03-08/ EPRI proceeding: 3rd Incipient failure detection conference August Technical Guidance for setting up test facility at Godrej for carrying out Acoustic Topography test on stabilising wings and panels and for testing quality of Bokmy coating on F3 section. MoU signed between BARC and Godrej May Conclusion Machinery diagnosis in totality is a fine blend of art and science of vibration analysis. The cost of machinery outage, together with stringent safety considerations makes it essential that good operating condition of all the machinery, vital to the plant operation, is maintained. The reason that more and more industries are investing in vibration monitoring program is a proof that the investment is translating into increased production and profit. However, there is a strong need to spread the skill of machinery diagnosis to the entire enthusiastic and initiated scientific community. Acknowledgement Author would like to acknowledge the support and assistants provided by colleagues in Vibration Laboratory 16