A method for measuring ultrasonic sound velocity of in-service gas turbine rotor tie rod bolts

Abstract

The invention discloses a method for measuring the ultrasonic sound velocity of a tie rod bolt of a gas turbine rotor in service, and belongs to the technical field of ultrasonic non-destructive testing. The measuring method adopts a sound velocity measurement system composed of a set of focused ultrasonic probes, a USN60 ultrasonic detector, and a detection tool. Aiming at the problem of accurately measuring the ultrasonic sound velocity of the rotor tie rod bolt under the action of preload, especially the sound velocity of the threaded section to be detected, the true value of the thread pitch of the tie rod bolt after the preload is loaded is calculated based on Hooke's law, the actual refraction angle of the ultrasonic sound beam is calculated by measuring the actual moving distance of the probe on the end face of the tie rod bolt when the highest wave of the two adjacent thread tooth surfaces is reached, and the ultrasonic sound velocity of the tie rod bolt is obtained in combination with Snell's law, so that the ultrasonic sound velocity of the threaded section can be measured quickly and accurately when the actual length of the tie rod bolt is unknown.

Description

Technical Field

[0001] The invention belongs to the field of ultrasonic detection, and in particular relates to a method for measuring the ultrasonic sound velocity of a tie rod bolt of a gas turbine rotor in service. Background Art

[0002] The wheels of each stage of the gas turbine (including the compressor and turbine wheels) are generally fastened together as a whole by circumferential or central tie rod bolts, and transmit the torque generated during the operation of the gas turbine to perform external work. During the assembly process, the rotor tie rod bolts need to be preset with a certain amount of deformation to generate preload. During the operation of the gas turbine, they must also withstand the periodic tensile stress generated by the rotor's own gravity in the axial direction of the tie rod bolts, as well as the impact force of the airflow, vibration stress, etc. The tie rod bolts are always under a high stress level during operation. However, the high stress of the tie rod bolts is mainly borne by the threaded section on the tie rod bolts, especially each tooth of the threaded section is subject to extremely high stress levels, so it is easy to produce cracks in the threaded section of the tie rod bolts, which can damage the structural integrity of the tie rod bolts.

[0003] Ultrasonic testing is generally used to detect the situation of in-service gas turbine rotor tie rod bolts that are not disassembled during the overhaul process. Tie rod bolts are mainly subjected to stress and strain caused by preload in a static state. Due to the large preload, the stress level generated is high (generally 400MPa~700MPa), which will affect the ultrasonic sound velocity of the tie rod bolt itself. In order to accurately locate and quantify the possible defects of the tie rod bolts in ultrasonic testing, and at the same time improve the accuracy of ultrasonic probe selection, it is necessary to measure the ultrasonic sound velocity of the in-service gas turbine rotor tie rod bolts before testing.

[0004] Since the gas turbine rotor tie rod bolts are relatively long (generally 3 to 4 meters in length), and the accurate tie rod bolt length cannot be obtained after the preload force is applied, and the sound velocity measured by the longitudinal wave reflection method is the average sound velocity of the entire tie rod bolt, the commonly used longitudinal wave reflection method is not applicable. Summary of the invention

[0005] The purpose of the present invention is to provide a method for measuring the ultrasonic sound velocity of a tie rod bolt of an in-service gas turbine rotor. In view of the problem that the ultrasonic sound velocity of the tie rod bolt of an in-service gas turbine rotor changes under a pre-tightening force, but the sound velocity cannot be measured by a conventional longitudinal wave reflection method because the actual length of the tie rod bolt cannot be accurately obtained, a transverse wave refraction method is used to accurately measure the sound velocity of the tie rod bolt, especially the sound velocity of the threaded section to be detected.

[0006] The present invention is achieved by adopting the following technical solutions:

[0007] A method for measuring ultrasonic sound velocity of a pull rod bolt of a gas turbine rotor in service comprises the following steps:

[0008] 1) Stably couple the ultrasonic probe to the end face of the tie rod bolt, move the ultrasonic probe back and forth along the diameter direction of the tie rod bolt end face to scan the thread tooth surface to be tested, find the highest echo of each of the two adjacent thread tooth surfaces to be tested, and record the position of the ultrasonic probe at each highest echo. The difference in the position of the ultrasonic probe is the distance Δd between the two sound beam incident points. It should be ensured that the center plane of the ultrasonic beam coincides with the diameter plane of the tie rod bolt during the scanning process;

[0009] 2) Calculate the thread pitch of the in-service rotor tie rod bolts, and calculate and obtain the true value P of the thread pitch of the in-service tie rod bolts through the tie rod bolt preload, thread cross-sectional area, initial thread pitch and elastic modulus;

[0010] 3) Calculate the refraction angle of the ultrasonic beam. Through step 1), find the highest echo of two adjacent threads respectively, and obtain the distance Δd between the incident points of the two beams. Combined with the true value P of the thread pitch of the tie rod bolt, calculate and obtain the refraction angle β of the ultrasonic beam.

[0011] 4) Calculate the shear wave velocity of the in-service rotor tie rod bolts. By using Snell's law, combined with the incident angle of the ultrasonic beam and the longitudinal wave velocity in the incident medium, the shear wave velocity v of the in-service rotor tie rod bolts is calculated and obtained. s :

[0012]

[0013] Where α is the incident angle of the ultrasonic beam, v m is the longitudinal wave speed in the incident medium;

[0014] The actual value P of the thread pitch of the tie rod bolt in service and the shear wave speed v of the tie rod bolt in service rotor s Substituting into formula (3) we can get:

[0015]

[0016] 5) Calculate the longitudinal wave velocity v of the in-service tie rod bolt l ;

[0017] Due to the longitudinal-transverse sound velocity ratio The longitudinal wave velocity v of the in-service tie rod bolt can be obtained l :

[0018]

[0019] Where μ is the Poisson’s ratio of the tie rod bolt material.

[0020] A further improvement of the present invention is that in step 1), the incident angle of the ultrasonic probe is such that the refracted sound beam generated in the stay bolt is vertically incident on the threaded surface of the stay bolt.

[0021] A further improvement of the present invention is that in step 1), the ultrasonic probe is a focused ultrasonic probe with a smaller sound beam diameter, so as to improve the ultrasonic reflectivity and the tooth surface positioning accuracy.

[0022] A further improvement of the present invention is that in the step 1), the matching inspection tooling is processed to cooperate with the end face of the tie rod bolt to ensure that the incident plane of the ultrasonic sound beam always coincides with the diameter surface of the tie rod bolt.

[0023] A further improvement of the present invention is that in step 2), the calculation formula of the actual value P of the thread pitch of the tie rod bolt in service is as follows:

[0024]

[0025] Where F is the preload of the tie bolt, D is the minor diameter of the tie bolt thread, E is the elastic modulus of the tie bolt, and P0 is the initial pitch of the tie bolt thread.

[0026] A further improvement of the present invention is that in step 3), the calculation formula of the refraction angle β of the ultrasonic beam is as follows:

[0027]

[0028] Where Δd is the distance between the sound beam incident points when the highest echo occurs on the tooth surfaces of two adjacent threads.

[0029] A further improvement of the present invention is that in step 3), in order to improve the measurement accuracy of the distance Δd between the incident points of the two sound beams, a vernier caliper measurement function is integrated on the detection tooling.

[0030] A further improvement of the present invention is that in steps 4) and 5), in order to improve the accuracy of ultrasonic sound velocity measurement of the tie rod bolt, multiple groups of tooth surface echoes are measured, and multiple sound velocity values ​​are calculated and averaged to eliminate random measurement errors.

[0031] The present invention has at least the following beneficial technical effects:

[0032] In order to solve the problem of accurately measuring the ultrasonic sound velocity of a rotor tie rod bolt under the action of a preload, especially the sound velocity of a threaded section to be detected, the present invention proposes a method for measuring the ultrasonic sound velocity of a rotor tie rod bolt of an in-service gas turbine. The method calculates the true value of the thread pitch of the tie rod bolt after the preload is loaded based on Hooke's law, calculates the actual refraction angle of the ultrasonic sound beam by measuring the actual moving distance of the probe on the end face of the tie rod bolt when the highest wave of two adjacent thread tooth surfaces occurs, and obtains the ultrasonic sound velocity of the tie rod bolt in combination with Snell's law, thereby realizing rapid and accurate measurement of the ultrasonic sound velocity of the threaded section when the actual length of the tie rod bolt is unknown. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1A schematic diagram of an ultrasonic velocity measurement system for a pull rod bolt of a gas turbine rotor in service according to the present invention;

[0034] Figure 2 The present invention is a schematic diagram of a method for measuring the ultrasonic sound velocity of a tie rod bolt of a gas turbine rotor in service.

[0035] Description of reference numerals:

[0036] 1- Focused ultrasonic probe, 2- USN60 ultrasonic detector, 3- Detection tooling, 4- Pull rod bolt.

[0037] Figure 2 Symbols: F-tightening force of tie rod bolt, D-minor diameter of tie rod bolt thread, E-elastic modulus of tie rod bolt, P0-initial pitch of tie rod bolt thread, P-true value of pitch of tie rod bolt thread, Δd-distance between the incident points of the sound beam when the highest echo is generated between two adjacent threads, β-refraction angle of ultrasonic beam, α-incident angle of ultrasonic beam, v m -Longitudinal wave speed in the incident medium, μ -Poisson’s ratio of the tie rod bolt material. DETAILED DESCRIPTION

[0038] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to be able to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features described in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0039] In the description of the present invention, it is necessary to understand that the orientation or positional relationship indicated by the terms "tension rod bolt end face", "front and back", "tension rod bolt diameter face", etc. is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0040] like Figure 1 and Figure 2 As shown, the ultrasonic velocity measurement device for the in-service gas turbine rotor tie rod bolt used in the present invention comprises a focused ultrasonic probe 1, a USN60 ultrasonic detector 2, a detection tool 3 and a tie rod bolt 4. The measurement object in this embodiment is the in-service gas turbine rotor tie rod bolt. The actual length of the tie rod bolt is an unknown quantity. In order to eliminate the influence of the rotor's own gravity on the elongation of the tie rod bolt, the ultrasonic velocity of the tie rod bolt at the 3 o'clock or 9 o'clock position of the rotor is measured. The method adopts the following steps:

[0041] (1) The focused ultrasonic probe 1 is mounted on the detection fixture 3, and the detection fixture 3 is fixed on the end face of the tie rod bolt 4, so that the focused ultrasonic probe 1 is stably coupled with the end face of the tie rod bolt 4, and the focused ultrasonic probe 1 is moved forward and backward along the diameter direction of the end face of the tie rod bolt 4 to scan the thread tooth surface to be detected, and the highest echo of each of the two adjacent thread tooth surfaces to be detected is found by the USN60 ultrasonic detector 2, and the position of the focused ultrasonic probe 1 when each of the highest echoes is recorded, and the probe position difference is the distance Δd between the two sound beam incident points. In this embodiment, the incident angle of the focused ultrasonic probe 1 is 33°, the incident medium, i.e., the sound-transmitting wedge, is organic glass, and the center frequency is 5MHz. The detection fixture 3 can make the focused ultrasonic probe 1 slide forward and backward strictly along the diameter direction of the tie rod bolt for scanning, and is integrated with a vernier caliper measurement function to accurately record the position of the focused ultrasonic probe 1 when each of the two adjacent thread tooth surfaces to be detected has the highest echo.

[0042] (2) Calculate the thread pitch of the in-service rotor tie rod bolts. The actual value P of the thread pitch of the in-service tie rod bolts is calculated and obtained by using the tie rod bolt preload, thread cross-sectional area, initial thread pitch, elastic modulus and other parameters:

[0043]

[0044] Where F is the preload of the tie bolt, D is the minor diameter of the tie bolt thread, E is the elastic modulus of the tie bolt, and P0 is the initial pitch of the tie bolt thread.

[0045] In this embodiment, a certain type of F-class gas turbine is taken as an example. The preload force F of the tie rod bolt of this type of gas turbine is 3000 kN, the thread diameter D of the tie rod bolt is 60 mm, and the elastic modulus E of the tie rod bolt is 2×10 11 Pa, the initial pitch of the tie rod bolt thread is 3 mm. Substituting into formula (1), the actual value of the tie rod bolt thread pitch P = 3.01 mm.

[0046] (3) Calculate the refraction angle of the ultrasonic beam. Through step (1), find the highest echo of two adjacent threads respectively, and obtain the distance Δd between the incident points of the two beams. Combined with the true value P of the thread pitch of the tie rod bolt, calculate and obtain the refraction angle β of the ultrasonic beam:

[0047]

[0048] Where Δd is the distance between the sound beam incident points when the highest echo occurs on the tooth surfaces of two adjacent threads.

[0049] In this embodiment, the distance Δd between the incident points of the highest echo sound beams of two adjacent thread tooth surfaces is 2.56 mm. Substituting it into formula (2), it can be calculated that the actual refraction angle of the ultrasonic sound beam β=40.38°.

[0050] (4) Calculate the transverse wave velocity of the in-service rotor tie rod bolts. By using Snell's law, combined with the incident angle of the ultrasonic beam and the longitudinal wave velocity in the incident medium, the transverse wave velocity v of the in-service rotor tie rod bolts is calculated and obtained. s :

[0051]

[0052] Where α is the incident angle of the ultrasonic beam, v m is the longitudinal wave speed in the incident medium.

[0053] Substituting formula (1) and (2) into formula (3), we can obtain:

[0054]

[0055] In this embodiment, the incident angle α of the ultrasonic beam is 33°, and the longitudinal wave speed v in the incident medium is m is 2730m / s. Substituting it into formula (3), we can get the transverse wave velocity v of the in-service rotor tie rod bolts. s =3247m / s.

[0056] (5) Calculate the longitudinal wave velocity v of the in-service tie rod bolt l .

[0057] Due to the longitudinal-transverse sound velocity ratio The longitudinal wave velocity v of the in-service tie rod bolt can be obtained l :

[0058]

[0059] Where μ is the Poisson’s ratio of the tie rod bolt material.

[0060] In this embodiment, the rod bolt material is a high temperature alloy, and the Poisson's ratio μ is 0.3. Substituting into formula (5), it can be calculated that the longitudinal wave velocity v of the rod bolt in service is l =6075m / s.

[0061] Although the present invention has been described in detail above with general descriptions and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements may be made thereto based on the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention all fall within the scope of protection claimed by the present invention.

Claims

1. A method for measuring ultrasonic sound velocity of in-service gas turbine rotor tie rod bolts, characterized in that: The following steps are involved: 1) Stably couple the ultrasonic probe to the end face of the tie rod bolt, move the ultrasonic probe back and forth along the diameter direction of the tie rod bolt end face to scan the thread tooth surface to be tested, find the highest echo of each of the two adjacent thread tooth surfaces to be tested, and record the position of the ultrasonic probe at each highest echo. The difference in the position of the ultrasonic probe is the distance Δd between the two sound beam incident points. It should be ensured that the center plane of the ultrasonic beam coincides with the diameter plane of the tie rod bolt during the scanning process; 2) Calculate the thread pitch of the in-service rotor tie rod bolts, and calculate and obtain the true value P of the thread pitch of the in-service tie rod bolts through the tie rod bolt preload, thread cross-sectional area, initial thread pitch and elastic modulus; 3) Calculate the refraction angle of the ultrasonic beam. Through step 1), find the highest echo of two adjacent threads respectively, and obtain the distance Δd between the incident points of the two beams. Combined with the true value P of the thread pitch of the tie rod bolt, calculate and obtain the refraction angle β of the ultrasonic beam. 4) Calculate the shear wave velocity of the in-service rotor tie rod bolts. By using Snell's law, combined with the incident angle of the ultrasonic beam and the longitudinal wave velocity in the incident medium, the shear wave velocity v of the in-service rotor tie rod bolts is calculated and obtained. s : Where α is the incident angle of the ultrasonic beam, v m is the longitudinal wave speed in the incident medium; The actual value P of the thread pitch of the tie rod bolt in service and the shear wave speed v of the tie rod bolt in service rotor s Substituting into formula (3) we can get: 5) Calculate the longitudinal wave velocity v of the in-service tie rod bolt l ; Due to the longitudinal-transverse sound velocity ratio The longitudinal wave velocity v of the in-service tie rod bolt can be obtained l : Where μ is the Poisson’s ratio of the tie rod bolt material.

2. The method for measuring ultrasonic sound velocity of in-service gas turbine rotor tie rod bolts according to claim 1, characterized in that: In the step 1), the incident angle of the ultrasonic probe is such that the refracted sound beam generated in the stay bolt is vertically incident on the thread surface of the stay bolt.

3. The method for measuring ultrasonic sound velocity of in-service gas turbine rotor tie rod bolts according to claim 1, characterized in that: In the step 1), the ultrasonic probe is a focused ultrasonic probe with a smaller sound beam diameter, so as to improve the ultrasonic reflectivity and the tooth surface positioning accuracy.

4. The method for measuring ultrasonic sound velocity of in-service gas turbine rotor tie rod bolts according to claim 1, characterized in that: In the step 1), the matching inspection tooling is processed to cooperate with the end face of the tie rod bolt to ensure that the incident plane of the ultrasonic beam always coincides with the diameter surface of the tie rod bolt.

5. The method for measuring ultrasonic sound velocity of in-service gas turbine rotor tie rod bolts according to claim 1, characterized in that: In step 2), the calculation formula of the actual value P of the thread pitch of the tie rod bolt in service is as follows: Where F is the preload of the tie bolt, D is the minor diameter of the tie bolt thread, E is the elastic modulus of the tie bolt, and P0 is the initial pitch of the tie bolt thread.

6. The method for measuring ultrasonic sound velocity of in-service gas turbine rotor tie rod bolts according to claim 1, characterized in that: In step 3), the calculation formula of the refraction angle β of the ultrasonic beam is as follows: Where Δd is the distance between the sound beam incident points when the highest echo occurs on the tooth surfaces of two adjacent threads.

7. The method for measuring ultrasonic sound velocity of in-service gas turbine rotor tie rod bolts according to claim 1, characterized in that: In the step 3), in order to improve the measurement accuracy of the distance Δd between the incident points of the two sound beams, a vernier caliper measurement function is integrated on the detection tooling.

8. The method for measuring ultrasonic sound velocity of in-service gas turbine rotor tie rod bolts according to claim 1, characterized in that: In the steps 4) and 5), in order to improve the accuracy of ultrasonic sound velocity measurement of the tie rod bolt, multiple groups of tooth surface echoes are measured, and multiple sound velocity values ​​are calculated and averaged to eliminate random measurement errors.

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