Title of Invention

"A METHOD AND SYSTEM FOR MONITORING VIBRATIONS OF ROTATING BLADES OF TURBINES."

Abstract

This invention relates to a system for monitoring vibrations of rotating blades of turbines comprising: a reference sensor probe mounted on the shaft of the turbine for providing a reference signal at the start of each revolution of the turbine, three sensor probes inside the casing of said turbine for determining the arrival time of each blade of said turbine, each said sensor is connected to a signal conditioner to condition the received signal for conversion to a digital value, the said signal conditioners are connected to at least one analogue to digital converter that provides a digital value of the received signal, the said analogue to digital converter is connected to a processor means for computing the maximum amplitude by determining the value of vibration frequency of the blade ω from equation B and then applying the value of the vibration frequency of blade (ω) in equation A wherein t1, t2, t3 is the arrival time of blades at the sensors,ωΔt1, Δt2, Δt3 is undeflected arrival time for each blade, θA = blade vibration amplitude, Ω2=speed of rotator in radians, ω = frequency of blade vibration. The invention also provides a method for monitoring vibrations of rotating blades of turbine

Full Text

Field of the invention This invention relates to a method and system for monitoring vibration of rotating blades of turbines Background of the invention Blade failure in steam and combustion turbines has been the leading cause of unplanned outage at power utilities. For the safe operation of thermal turbo machines it is essential not to subject the blades to any dangerous vibrations. The problem of blade failure due to excess vibration is particularly acute in the last row of turbine blades. These final stage blades, which are thin and flexible due to their blade geometry, are subjected to high mechanical stress imposed by centrifugal forces. Therefore, vibration monitoring of rotating blades of steam or other turbines is considered important for determining the vibration levels of the blades for indicating the blade condition. Such monitoring makes it possible to predict an impending failure of any of the blades due to excess vibration saving a lot of restoration effort and money. Historically, the vibrational modes of steam turbine blades have been measured by placing strain gauges on the rotating blades and telemetering the information to a stationary receiver. This method is not very desirable because the gauges have short lives. It is also necessary to mount a strain gauge on each blade that is to be monitored. US Pat. No. 4,593,566 discloses a capacitive type of sensor. As each blade tip passes the sensor, there is a change in the capacitive coupling between the sensor and the blade. This results in small capacitive currents, as each blade passes the sensor. These signals are passed through two threshold detectors. The first, a variable positive level comparator, gates a second close-to-zero level comparator. This allows precise timing through the zero-crossing when the signal-to-noise ratio is high but inhibits the zero crossing comparator during periods when a blade tip is not adjacent to a sensor and the signal-to-noise ratio is zero. Variable capacitance sensors, although inherently fast and rugged, do not give usable results in, two-phase flow. Changes in capacitance due to flow induced changes in dielectric properties can cause spurious results. Optical sensors have been used for blade vibration monitor applications. However, optical sensors are not suitable for long term installation in an operating steam turbine environment because of poor performance in steam containing droplets, refraction effects of surface water, erosion, and contamination of the optical surfaces by oxides and other particles. US Patent No. 4,573,358 relates to a turbine blade vibration detection apparatus. In this patent a plurality of sensors are disposed equally about a blade row. As a blade passes the sensor, a gated pulse is generated. The rotational motion of the turbine generates a pulse train at each sensor that is frequency modulated by the blade vibration. A FM demodulator is used to recover the vibration information. It may thus be seen that in this method a large number of sensors are required to characterize the vibration accurately. US Patent 4,518,917 relates to plurality of proximity sensors for monitoring turbine. As each blade deflects the gap between the blade tip and the casing varies causing a change in the signal level at the proximity sensor. The change in the value of sensor signal is correlated to the blade vibration for estimation of vibration. US Patent 4,922,757 relates to an apparatus for precise detection of blade passing timing in a steam turbine. This patent also includes a method of finding the time difference between the expected arrival time and the actual arrival time by using a specialized electronic circuit only. The limitation in this invention is that it merely detects the blade passing time and does not determine the vibration amplitude The Object and Summary of the Invention The object of this invention is to overcome the above mentioned drawbacks and determine the amplitude and frequency of the asynchronous blade vibration using proximity sensors for preventing blade failure. To achieve this object the invention describes a system for monitoring vibrations of rotating blades of turbines comprising: a reference sensor probe mounted on the shaft of the turbine for providing a reference signal at the start of each revolution of the turbine, three sensor probes inside the casing of said turbine for determining the arrival time of each blade of said turbine, each said sensor is connected to a signal conditioner to condition the received signal for conversion to a digital value, the said signal conditioners are connected to at least one analogue to digital converter that provides a digital value of the received signal, the said analogue to digital converter is connected to a processor means for computing the maximum amplitude by determining the value of vibration frequency of the blade ω from equation B and then applying the value of the vibration frequency of blade (ω) in equation A (Equation Removed) wherein t1, t2, t3 is the arrival time of blades at the sensors, Δt1, Δt2, Δt3, is undeflected arrival time for each blade, ΘA = blade vibration amplitude, Ω2=speed of rotator in radians, ω = frequency of blade vibration. The reference sensor probe is a Hall Effect sensor. The said three sensor probes inside the casing of the said turbine are eddy current probes. The said reference sensor probe and three sensor probes are proximity sensor probes. The said signal conditioner comprising a proximity sensor biasing means for said sensor probes and a filtering means for eliminating the DC component of the signal from the sensor. The said processor means is a computer. The said sensors mounted in the casing are not at equi-distant from each other to avoid inaccuracy in computation. The said turbine is a steam turbine. The present invention further provides a method for monitoring the vibrations of rotating blades of turbines comprising the steps of; generating a reference signal at the beginning of the revolution of the turbine, sensing the arrival of each blade of the turbine at predefined points in the casing that are not equi-distant from each other, conditioning the sensed signal for conversion to a digital value, converting the said signal to a digital value to compute the maximum amplitude by determining the value of vibration frequency of the blade ω from equation (B) and then applying the value of the vibration frequency of the blade ωin equation (A). (Equation Removed) wherein t1, t2, t3 is the arrival time of blades at the sensors, Δt1, Δt2, Δt3,, is undeflected arrival time for each blade, θA = blade vibration amplitude, Ω2=speed of rotator in radians, ω = frequency of blade vibration. Brief description of the drawings The invention will now be described with reference to the accompanying drawings. Figure 1 shows the system for monitoring vibrations of the rotating blades of turbines, according to this invention. Figure 2 shows the flow diagram of method for monitoring vibrations of the rotating blades of the turbine, according to this invention Figures 3, 4 and 5 show the proximeter, its connections with RC filter / high pass filter. Figure 6 shows the data acquired at the rate of 100,000 samples/sec. in the data shown. Figure 7 shows the data acquired at the rate of 300,000 samples/sec. in the data shown. Detailed description of the invention Referring to the figure 1, item (BD) shows the blade disc attached to a shaft (S) of the turbine. Three sensor probes (1, 2, 3) are provided inside the casing of the turbine (not shown) at unequal distances from each other. The reference sensor probe (4) is located in proximity to shaft (S). The reference signal sensor probe (4) is a Hall effect sensor. The reference signal is used as the time base from which all the measurements are made. Each of said sensor probes 1, 2, 3 are connected to signal conditioners (C1, C2, C3) consisting of a proximeter (proximity sensor biasing means) (5, 6, 7) and filtering means (8, 9, 10) for eliminating the DC component of the signal from said sensor probes (1, 2, 3). The filtering means (8, 9, 10) are high pass filters. The output signals from the reference signal sensor probe and the three signal conditioners (C1, C2, C3) are passed to analogue-to-digital converter (11) for computing all the data in the PC (12) using MATLAB programme means. As each blade of the turbine passes the sensor probes (1,2, 3), a sharp peak is obtained. There is a cluster of three peaks for each blade. Each sensor in the assembly produces a peak, as the blade passes it. The cluster is repeated for all the blades. The reference signal probe (4) and the three sensor probes (1 to 3) are proximity sensor probes. The peaks, though they appear as sharp lines on the drawings, are actually rectangular bands as shown in figure 6 & 7. The data has been acquired at the rate of 100,000 sample/sees and 300,000 samples/sees. The method provided by the present invention is based on differential time measurement, wherein the time difference between a blade pass and a reference signal is measured and the difference is compared with the time difference between the reference signal and an un-deflected blade. The comparison yields a measure of the instantaneous blade deflection. A minimum of three probes is required for measurements to compute the phase, amplitude and frequency of the vibrations. The steps of the method for monitoring the vibrations of rotating blades of turbine are explained with the help of flow diagram shown in figure 2. The steps of the method are: a. When the rotor moves, once every revolution, the reference sensor probe (4) gives a rising edge reference signal. The rising edge is detected and a timestamp is generated in software. This initiates the time count for every cycle. The time interval between successive timestamps is also used to calculate the speed of rotation of the shaft. b. When a blade passes a sensor probe 1, a rising edge is generated in the sensor output. The time delay of this signal from the rising edge of the reference sensor is stored as ti. c. The time delay with which an undeflected blade passes that particular sensor probe is known as a product of standard time and is the number of blades and the reciprocal of the speed of the shaft. d. The time difference between the standard time and t1 is Δt1. e. Repeat steps b to d for sensor probes 2 and 3 for one blade to get At2, t2, At?, t3. f. From the values of Δt1, t1, Δt2, t2, Δt3, t3, use the equations A and B to calculate the vibration frequency and amplitude for one blade. g. Repeat steps b to f for every blade. h. Repeat steps a to g at the rate at which the blade vibration amplitude and frequency are desired to be monitored. Vibrations have been measured using non-contact type Bently Nevada's 3300 series 8-mm proximity transducer system, which consist of proximity probe, extension cable, proximeter and monitor. A 24 V.D.C supply is required as per the manufacturer's specifications. The transducer system is generally used for measuring radial and axial vibrations of rotating shafts but has been used in the present invention for indicating positions of blades. The recommended blade-casing gap is 50 mils (1.27 mm). The system scale factor is 200 mV/mil and frequency response is satisfactory till 6.5 K. Hz. The frequency response of the sensors has to be higher than 3.6 KHz. The probe signals and the input power are transmitted between the proximeter and a standard monitor through a 3-conductor shielded signal wire. The proximeter sensor can be up to 305 meters from the monitoring apparatus without degradation of performance. Figure 3 gives details of the connections of each proximeter (5, 6, 7). A High Pass Filter is required to filter or eliminate the DC component coming from the output of each proximeter. The connections to the high pass filters are shown in Figure 4 while the internal circuit is shown in Figure 5. Reference signals are generated using Panasonic's DN6851 type Hall effect sensor as shown in Figure 1. This sensor is designed particularly for operating at a low supply voltage in an alternating magnetic field. The operating supply voltage range Vcc = 3.6 to 16V, while operating in an alternating magnetic field. An A/D (analogue-to-digital) card interfacing board (11) is interfaced directly to the setup with the help of a MATLAB program means in PC (12). An A/D card that meets the specification required in the present invention's setup is AD link Technology Inc's NuDAQ PCI-9118 DG/HG/HR type 330 KHz data acquisition card or analog to digital card (A/D card). The maximum voltage range is 5V. This card is connected to the setup through an interface board. The sensor probes (1, 2, 3) are located at three places as shown in Figure 1. The probes are connected to the proximeter through extension cables and from the proximeter to the A/D card (11). The gap between the target and the probe should be below 2mm. When the vibrating blade comes in front of the probes, the sensor probe gives a signal. Thus, for every rotation of each blade, four signals are generated - three from the sensor sensors giving the blade position and the fourth from the Hall sensor on the shaft as a reference signal. The signals are transferred to the A/D card and then to a computing device (PC, 12) for measuring the time differential between the sensing at the sensor probes and the reference signal. If the four blades in the setup are not vibrating then the time interval between two peaks is same and is equal to the time interval required by the blade for travelling a distance of one-fourth revolution. If the blades are vibrating, as is the case with the blades of the steam turbine, then the time interval between two consecutive peaks varies. The reference signals from the Hall Effect sensor as shown in figure 1 are used to recognize the peaks of the corresponding blade. The samples captured at different time intervals are stored in a file. Then, programmer means written using MATLAB is used to calculate the time intervals between the peaks. The MATLAB programme means is used to calculate the timer intervals between the peaks. The LATLAB programme means fine the positions of various blades as a fraction of one complete revolution for the reference case, i.e. when the blades' are not vibrating. The peak data for the case when the blades are vibrating is compared with the peak data. We claim: 1. A system for monitoring vibrations of Rotating Blades of Turbines comprising: a reference sensor probe mounted on the shaft of the turbine for providing a reference signal at the start of each revolution of the turbine, three sensor probe inside the casing of said turbine for determining the arrival time of each blade of said turbine, each said sensor is connected to a signal conditioner to condition the received signal for conversion to a digital value, the said signal conditioners are connected to at least one analogue to digital converter that provides a digital value of the received signal, the said analogue to digital converter is connected to a processor means for computing the maximum amplitude by determining the value of vibration frequency of the blade TS from equation B and then applying the value of the vibration frequency of blade (ω) in equation A. (Equation Removed) wherein t1, t2, t3 is the arrival time of blades at the sensors, Δt1, Δt2, ωt3, is undeflected arrival time for each blade, ΘA = blade vibration amplitude, Ω2=speed of rotator in radians, = frequency of blade vibration. 2. The system for monitoring vibration of rotating blades of turbines as claimed in claim 1 wherein such reference sensor is a Hall Effect sensor. 3. The system as claimed in claim 1 wherein said sensors inside the casing of the said turbine are eddy current probes. 4. The system as claimed in claim 1 wherein said signal conditioner comprising a biasing means for said sensors and a filtering means for eliminating a DC component of the signal from the sensor. 5. The system as claimed in claim 1 wherein said sensors mounted in the casing are not at equi-distant from each other to avoid inaccuracy in computation. 6. The system as claimed in claim 1 wherein the said turbine is a steam turbine. 7. A method for monitoring the vibrations of rotating blades of turbines, using the system as claimed in preceding claims, comprising the steps of: generating the reference signal at the beginning of the revolution of the turbine, sensing the arrival of each blade of the turbine at predefined points in the casing that are not equi-distant from each other, conditioning the sensed signal for conversion to a digital value, converting the said signal to a digital value to compute the maximum amplitude by determining the value of vibration frequency of the blade and then applying the value of the vibration frequency of the blade as herein described. 8. A system for monitoring vibrations of rotating blades of turbines substantially as herein described with reference to the accompanying drawings. 9. A method for monitoring vibrations of rotating blades of turbines substantially as herein described with reference to the accompanying drawings.

Documents:

1178-del-2002-abstract.pdf

1178-del-2002-claims.pdf

1178-del-2002-complete specification (granted).pdf

1178-del-2002-correspondence-others.pdf

1178-del-2002-correspondence-po.pdf

1178-del-2002-description (complete).pdf

1178-del-2002-drawings.pdf

1178-del-2002-form-1.pdf

1178-del-2002-form-19.pdf

1178-del-2002-form-2.pdf

1178-del-2002-form-24.pdf

1178-del-2002-form-3.pdf

1178-del-2002-form-4.pdf

1178-del-2002-gpa.pdf

1178-del-2002-petition-138.pdf