Patent Application
20210140337
This disclosure relates generally to diagnostic testing and monitoring of rotor blades. In particular, a method of monitoring rotor blades using a predetermined pattern on the blade tip in order to detect blade deformation and axial shift of the rotor blades is presented.
The compressor and turbine section of a gas turbine is composed of rings with blades that rotate around a joint shaft (rotor). The blades in the compressor reduce the volume of the working gas, thus increasing its pressure and temperature. After the compressor, the combustor section further increases the gas temperature by burning fossil fuels, for example. Thereafter, the blades in the turbine section extract energy from the working gas by translating its expansion to a rotational force on the shaft.
The combination of gas pressure, centrifugal force, pressure fluctuations or resonances, and temperature create significant stress on the blades that may result in blade deformations and breaking. Breaking of a blade may have a catastrophic effect on the gas turbine as the material that breaks off from the blade travels through the turbine causing damage leaving the turbine inoperable. Thus, it is imperative to monitor the stress on the blades during operation so that any deficiencies found may be fixed before catastrophic damage occurs.
Briefly described, aspects of the present disclosure relates to a method and system for monitoring a rotor blade.
A method of monitoring a rotor blade is provided. The method includes disposing a probe including an optical sensor within a mounting hole in a turbine casing of a turbine engine. A laser beam is them emitted by a light source radially inward from the probe position onto a rotor blade tip of the rotor blade. The rotor blade is positioned such that it periodically passes the laser beam. The rotor blade tip includes a predetermined pattern. The reflected light images from the rotor blade tip are received by the optical sensor. From the reflected light images, a blade profile is constructed. Based on this constructed blade profile from the reflected light images off the predetermined pattern, a position of the rotor blade is determined.
A rotor blade monitoring system is also provided. The system includes a rotating rotor blade having a rotor blade tip including a predetermined pattern, a light source that emits a laser beam radially inward onto the rotating rotor blade tip, a probe including an optical sensor disposed within a mounting hole of a turbine casing of a turbine engine, and a processor coupled to the optical sensor for constructing a blade profile from the reflected light images off the predetermined pattern. The optical sensor is configured to receive the reflected light images. From the constructed blade profile, the position of the rotor blade is determined.
To facilitate an understanding of embodiments, principles, and features of the present disclosure, they are explained hereinafter with reference to implementation in illustrative embodiments. Embodiments of the present disclosure, however, are not limited to use in the described systems or methods.
The components and materials described hereinafter as making up the various embodiments are intended to be illustrative and not restrictive. Many suitable components and materials that would perform the same or a similar function as the materials described herein are intended to be embraced within the scope of embodiments of the present disclosure.
Commonly, the stress on the blades is monitored by an optical tip timing system that measures the arrival times of each blade using a laser. A challenge with this approach is that the profile of the blade is very different depending on the position where the laser hits the blade. This position cannot be well controlled due to the thermal expansion of the shaft.
Similarly, vibrations of the blades are monitored using blade tip timing systems. The arrival times of the blades are measured through different physical effects including, for example, changes in capacitive field or optical reflections of a laser beam. The sequence of arrival times may be evaluated for each blade to resolve vibrations and frequencies of vibration. An advantage of using the capacitive method is the low cost of each sensor while disadvantages include sensitivity to the blade tip distance and the presence of noise in the signal, such that the signal is not smooth, resulting in a high uncertainty on the tip timing measurement.
There are multiple optical approaches that address various challenges. They generally function by emitting a laser beam through the casing of the turbine onto the blade ring. When a blade passes by the laser beam, the reflected light is measured and analyzed. An advantage of this approach is a sharp rise when a blade flank passes in front of the laser which translates to highly accurate tip timing measurements. However, this approach is sensitive to axial shift and blade bending as the blade profile varies for different axial positions and angles. Axial shift, or axial movement of the blade relative to the rotation axis of the blade, may occur during operation of the turbine engine. Axial movements of the blades during operation may occur as the rotor spins up or down, as the turbine warms up and cools down, as the load on the turbine changes, and due to torsional movements of the blades. Blade bending occurs when the blade deforms somewhat, twists, tilts or moves in a direction towards the casing.
In order to compensate for the uncertainty in the axial shift, multiple rows of blades may be monitored and a common shift in arrival time may be extracted and related to the rotor movement. In general, it is very difficult to accommodate for blade bending and determine the actual position of the measurement spot of the blade profile. Several laser beams may be used, for example, four for each blade ring. The tip arrival information from the different laser measurements may be used to reconstruct the blade profile. The reconstructed blade profile can then be used to compensate for the axial shift. These approaches, however, are significantly more expensive as they requires multiple sensor installations and high frequency data acquisition channels per blade ring and are also computationally challenging. There remains a significant uncertainty given the many possible bending modes of the blades as well as different blade distances to the casing.
Consequently, a method of monitoring rotating blades having better measurement accuracy of the blade deformation and axial shift using an optical tip timing system with an optical sensor is desired.
The inventors propose extending the capability of an optical tip timing system with at least one sensor per row of blades to better measure blade bending, blade vibrations, and axial shift. Currently, tip timing systems only measure the simple shape of the blade profile with uncertainty on the exact location where the laser crosses over the profile. This location is dependent on different bending modes of the blade and axial shift resulting from the thermal expansion of the turbine shaft. Due to the simple shape profile of the blade, it is not possible to extract all blade bending modes and axial shift from a single path laser measurement.
A rotating blade monitoring system 20 is also shown in
In an embodiment, the probe 22 includes an optical sensor 25 which may produce a signal in response to a passing rotor blade 14. The optical sensor 25 may include a fiber optic portion that detects blade passing events during blade vibration monitoring. In this embodiment, a light source 54 emits a laser beam through the turbine casing 36. The fiber optic portion may include an illumination conduit having a transmission end for projecting the light source onto the rotor blade tip and a receptor end for receiving the reflected light images from the rotor blade tip.
As is further illustrated in
In accordance with an embodiment, a rotor blade monitoring system 20 includes a rotor blade 14 having a rotor blade tip including a predetermined pattern. A top view of a rotor blade tip 100 is illustrated in
The predetermined pattern 120 may be created on the rotor blade 14 by laser cutting small structures into the rotor blade tip 100. In another embodiment, the predetermined pattern 120 may comprise reflective paint applied onto the surface of the rotor blade tip 100. In a further embodiment, materials with different reflection coefficients may be inlaid onto the surface of the rotor blade tip 100. In a further embodiment, the pattern may be created by additive manufacturing. Additionally, the predetermined pattern 120 may be created by a combination of the presented embodiments. As the rotor blade tip 100 is exposed to extremely high temperatures, the predetermined pattern 120 should be robust enough to withstand these extreme temperatures.
An effective pattern may include a simple pattern that accurately differentiates different angles and translations of the laser path over the pattern 120. A simple design may be advantageous as a more complex pattern with fine structures could be harder to reliably read or be more easily damaged by exposure to dirt. A more complex pattern may require more accurate control with the laser source, a smaller laser focus, and higher sampling frequencies. Therefore, a good pattern may include a non-symmetric, non-periodic pattern such that the message of the pattern is changed based on the position and angle. Additionally, a continuous pattern is preferable over a digital pattern often used in two-dimensional bar codes. This allows a distinctive pattern readout variation based on a continuous change in position or angle. Otherwise, the laser could read out points in between digital zero and one values resulting in an unclear message. Information may be encoded in a predetermined pattern 120 so that the position of the rotor blade 14 may be accurately determined.
In an embodiment, several distinct patterns may be used successively on the rotor blade tip to diagnose different blade issues. For example, with one specific pattern it may be easier to diagnose axial shift as opposed to blade bending of the rotor blade 14. Additionally, these distinct patterns may be created by different processes.
In an embodiment, the predetermined pattern may 120 include a two dimensional pattern. An example of a two dimensional pattern may be seen in
In another embodiment, the predetermined pattern 120 may include a three dimensional pattern. The three dimensional pattern may include structures with varying heights on the surface of the rotor blade tip 100. A three dimensional pattern in a pocket 110 of the rotor blade tip 100, for example, could be created by an additive manufacturing process when the rotor blade 14 is manufactured.
To maximize the accuracy of the laser readout, a laser with a wavelength that has a large difference in reflectivity when passing over the predetermined pattern 120 may be used. It may also be advantageous to use a focused laser with a very small beam diameter to differentiate fine pattern differences. The range for the beam diameter may be less than or equal to 0.5 cm in diameter. Preferably, the beam diameter is less than 1 mm in diameter. In order to accommodate this range, for example, a single transverse mode laser may be used.
Referring now to
In accordance with an embodiment, the blade tip 100 includes a predetermined pattern 120 as discussed above. The predetermined pattern 120 may be deployed in a recessed pocket 110 of the blade tip 100.
For example, the laser path, shown as a horizontal line 220 through the center of the predetermined pattern 120 in the illustrated embodiment, would be expected when the blade positioning is correct, i.e., with no axial shift, blade bending or vibrations during operation. If the laser path reads out the encoded information in the laser path shown by horizontal line 210, one could infer that an axial shift has occurred as the read out encoded information would differ from the encoded pattern read out when the blade is correctly positioned. Additionally, from the encoded pattern, which would be unique based on the approach angle and the translation of the laser, one could accurately determine the amount of axial shifting, denoted by distance d in
The method may also be used to characterize the movement of the rotor blade 14 as a blade vibration. As mentioned previously, vibrations of a rotor blade 14 are typically determined using tip timing systems, specifically by marking deviations from a constant time of arrival for each blade. However, using the current methods it is difficult to make an accurate measurement of the time-of-arrival of each rotor blade because of the sensitivity of the sensor to the blade tip distance and noise in the signal. Using the proposed method, deviations of the time of arrival of each blade may be more accurately extracted. The blade monitoring system 20 is configured to record the time of arrival by the sensing the passage of the same pattern of encoded information on multiple passes, where the same pattern of encoded information corresponds to a precise location on the rotor blade 14. From the recording of the time of arrival data, the vibrational movement of the associated blade may be determined.
Using current tip timing systems, typically, either the leading edge or the trailing edge of the rotor blade are used to detect a blade passing event. However, using the proposed system with the predetermined pattern, the tip timing measurement may be refined such that a correlation with a specific encoded message of the predetermined pattern should allow for a more accurate detection of the arrival time than just tracking the two edges of the rotor blade.
Results from this method can be output as a reporting value including an output of the position of the rotor blade 14 which provides an indication of the condition of the blade and operational state. That is, if the rotor blades are vibrating too much, at an undesired eigenfrequency, are bent too much, or approach the outer casing of the turbine, it may be desirable to perform control decisions like unloading. Moreover, a change in operating parameters of the rotor assembly can be implemented based upon this indication of the blade condition and operational state. A variety of operating parameter changes may include, for example, initiating a shutdown, changing the rotor frequency, and reducing the load.
While embodiments of the present disclosure have been disclosed in exemplary forms, it will be apparent to those skilled in the art that many modifications, additions, and deletions can be made therein without departing from the spirit and scope of the invention and its equivalents, as set forth in the following claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2017/044780 | 8/1/2017 | WO | 00 |
