As the hot end component of the gas turbine’s internal flow path, low-pressure turbine blades are impacted by airflow for a long time and are affected by the combined effects of centrifugal load, aerodynamic load, and thermal load, which require high blade strength. A gas turbine was rebuilt from a retired aerospace engine. The life of the gas turbine was tested through the power station chief test. During the test, a low-pressure turbine blade fracture occurred. After analyzing, testing, and calculating the fault, it was determined that the main cause of blade fracture was high-cycle fatigue caused by blade resonance. Measures were taken to quickly pass through the dangerous speed range where multi-order resonance is prone to occur when the gas turbine operates for a long time.
Blade fracture failure has always been one of the technical bottlenecks that plague the reliable operation of gas turbines and aero engines. It is difficult to study the fracture of turbine blades because of many aspects including temperature load, aerodynamic load, mechanical connection, and machining.
The turbine rotor blades of gas turbines work in a high-temperature, high-pressure, and high-speed airflow environment. The loads acting on the working blades mainly include the following types: centrifugal load, aerodynamic load, temperature load, and various vibration loads. Common failure modes of turbine blades include fatigue-related fracture failure (including high-cycle fatigue, low-cycle fatigue, thermal fatigue, creep/fatigue, high-low cycle composite fatigue, etc.), creep failure, damage by foreign objects, and high-temperature damage, manufacturing process, and material defects, etc.[1].
Rotor blade fracture failure has the highest probability and is also the most harmful. Often one blade breaks and damages other blades, causing the entire engine to become inoperable and affecting safe operation. In addition to the instantaneous overload fracture of blades caused by the impact of foreign objects, most of them are different types of fatigue fracture failures caused by various reasons.
Blade fatigue fractures mainly include fatigue fractures caused by centrifugal force superimposed on bending stress, fatigue fractures caused by flutter, torsional resonance, and bending vibration, as well as fatigue fractures caused by high-temperature fatigue, fretting fatigue, and corrosion damage caused by environmental media and contact conditions. Due to the complexity of the working environment of blades, the actual fatigue fracture of blades is often not one of the above modes, but the superposition of two or even more modes, that is, “composite” fatigue fracture failure caused by composite reasons [2].
High cycle fatigue, also known as high cycle fatigue or stress fatigue, refers to fatigue with low cyclic stress levels, no plastic strain, and the high number of cycles (generally above 104 to 105). High-cycle fatigue is generally caused by periodic high-frequency small loads. During cyclic loading, the material only produces elastic deformation. The smaller the load, the longer the life of the structure. That is, fatigue damage will only occur when the number of cycles is sufficient.
Turbine blades will produce high-frequency vibrations due to weak aerodynamic disturbances or system vibrations during operation. Under normal circumstances, through reasonable structural design and process control, the blades can be prevented from working in the resonance state for a long time and the occurrence of high-cycle fatigue damage can be avoided.
Low-cycle fatigue, also known as low-cycle fatigue or strain fatigue, refers to fatigue with high cyclic stress levels, plastic strain playing a dominant role, and low cycle times (generally below 104 to 105). Generally, periodic low-frequency large loads can easily lead to low-cycle fatigue. The material will undergo plastic deformation during cyclic loading. The greater the load, the shorter the life of the material or structure. That is, when the number of cycles is small, fatigue damage will occur.
A gas turbine is modified from a retired aircraft engine and is mainly used for civil power generation or mechanical drive. During the power station operation assessment, a sudden increase in power turbine vibration caused an emergency shutdown of the gas turbine. After removing the gas generator of the gas turbine on site, it was found that one of the low-pressure turbine blades was broken, and the remaining blades were damaged to varying degrees. This article provides an in-depth analysis of the low-pressure turbine blade fracture failure of this gas turbine and puts forward opinions and suggestions on how to effectively avoid such failures and ensure the reliable operation of this type of gas turbine.
1 Failure cause analysis
1.1 Review blade design and processing
After review, it was found that the low-pressure turbine blades of a certain gas turbine have the same structure as that of a certain engine, but the thermal load, centrifugal load, and aerodynamic load of the blades are all lower than those of a certain engine, and the blade calculation strength reserve is sufficient.
In terms of processing, the blades of a retired engine were repaired according to the engine repair requirements before being installed on the machine. No processing defects affecting the service life of the blades were found after the blades were repaired.
1.2 Analysis of collision between blade and casing
Since a certain gas turbine casing, blades, rotor, and stator radial clearance inherit the structure of a certain engine, the gas turbine has been tested in the factory and verified by the power station. The vibration of the whole machine is good during starting and power-up states, and the vibration of the front and rear fulcrums does not exceed the design requirements. , no obvious signs of wear and tear were found before the failure occurred. At the same time, due to the sudden increase in vibration and vibration sound of the faulty gas turbine (hereinafter referred to as the faulty engine) when the power is increased, it can be ruled out that the blades are broken due to friction between the blades and the casing.
1.3 Physical and chemical analysis of faulty parts
Because of the fracture failure of low-pressure turbine blades, further test analysis and calculation are required to determine whether there is a fatigue source or damage from foreign objects in the fracture and conduct fracture analysis and energy spectrum analysis of the faulty blade.
1.3.1 Fracture analysis
The fracture surface of the faulty blade showed a blue oxidation color, and the macroscopic morphology of the fracture surface is shown in Figure 1. The fracture showed fatigue fracture characteristics, with radiating ridges visible in the source area. Fatigue started from the leaf basin side. The lateral morphology of the source area is shown in Figure 2, and no damage morphology was found.
The fractured blade was placed into a scanning electron microscope for microscopic observation. The morphology of the fracture source area is shown in Figure 3. The direction of fatigue expansion is shown by the arrow. Fatigue starts from the surface of the leaf basin. The starting area shows a 45° small facet morphology. No metallurgical defects are found in the fatigue starting area. The morphology of the fatigue strips in the 1# and 2# areas of the fracture interface in Figure 3 is shown in Figures 4 and 5.
From the macroscopic and microscopic morphological characteristics of the fracture surface, the following conclusions can be drawn:
(1) From a macroscopic view, no damage morphology is found on the side of the source area, indicating that the blade is not broken due to the shear force on the side.
(2) From the microscopic view, no metallurgical defects were found in the fatigue initiation zone, indicating that metallurgical defects are not the cause of blade fracture.
(3) From a microscopic view, fatigue starts from the surface of the leaf basin. The starting area shows a 45° small facet morphology. The fatigue strip morphology can be seen in the 1# and 2# areas of the fracture, which has an interface with other morphologies, consistent with High-cycle fatigue fracture characteristics of blades.
1.3.2 Energy Spectrum Analysis
Energy spectrum analysis was performed on the metal residues, rings, and lumps collected at the outlet of a certain gas turbine power turbine. The percentage of chemical composition is shown in Table 1.
The chemical composition of the ring in the table is similar to that of GH708. The gas generator casing, low-pressure turbine guide insert block, turbine support, and other materials are all made of ХН62ВΜЮΤ-ВД, and the domestic alternative material is GH708; the chemical composition of the lump is similar to GH3030, and the power A large number of parts of the turbine are made of GH3030; the residue is identified as a mixture and the specific metal grade cannot be determined.
Energy spectrum analysis was conducted on the surface of the damaged blade, and no new metallic elements were found.
Through energy spectrum analysis, it can be concluded that the residue discharged after the gas turbine blade is broken is the part that damaged nearby parts after the blade was broken. No proof of damage to the blade by foreign objects in the gas turbine was found.
1.4 Vibration characteristic testing and calculation
1.4.1 Natural frequency measurement
Due to the limitation of the test equipment capacity, this experimental study only tests and determines the natural static frequency values of the first bend (first order), second bend (second order), and first twist (third order) of the low-pressure turbine working blades, and other frequency values are not tested. Elaborate in detail.
This test uses the resonance method to measure the natural frequency of the blade. The test system connection diagram is shown in Figure 6.
To verify whether the natural frequency of the blade has changed, the low-pressure turbine blades of the faulty engine and the prototype engine were selected for natural frequency comparison testing (the two blade structures only differ in the matching angle of the blade crown). Among them, the first-order frequency range of the faulty low-pressure turbine blade is 541~558 Hz, the second-order frequency range is 1160~1183 Hz, and the third-order frequency range is 2167~2245 Hz. The first-order frequency range of the prototype engine’s low-pressure turbine blade is 528-555 Hz, the second-order frequency range is 1 155-1 183 Hz, and the third-order frequency range is 2 135-2 238 Hz.
Through comparison, it was found that the first, second, and third-order natural frequencies of the low-pressure turbine blades of the faulty engine and the prototype engine were the same.
1.4.2 Natural frequency calculation
The ANSYS structural analysis program was used to calculate and analyze the vibration characteristics of a certain gas turbine low-pressure turbine rotor blade. The influence of centrifugal load and temperature on the material is taken into account when calculating the dynamic frequency, and the vibration frequencies and Campbell diagrams of each order are given. The finite element model used is established according to the nominal size [3]. Blade material performance data are shown in Table 2 [4].
The various rays emitted from point 0 in the Campbell resonance diagram (Figure 7) are frequency doubling lines, as well as natural frequency lines, which represent the changes in the natural frequency of each order of the blade with the rotation speed. The intersection of the natural frequency line and the frequency line represents the resonance situation. Due to the manufacturing error of the blade, the natural frequency will be dispersed, which is manifested as a natural frequency band. Therefore, resonance does not only occur at one point but may occur over a range. Within this range, blades may resonate. All vibration frequencies of the blades should not be within 10% of the known excitation frequency.
A certain gas turbine combustion chamber has 28 nozzles, 42 high-pressure guide vanes, and 11 sets of low-pressure guide vanes (a total of 28 pieces, the prototype engine has 33 pieces, with 5 pieces removed), and the load-bearing casing has 11 support plates. There are 5 times, 11 times, 28 times, and 42 times the excitation frequency. In addition, considering that there may be ovality in the processing of the casing, there may be 2 times to 6 times the frequency of the excitation frequency, so the main focus is on 2 times the frequency to 6 times the frequency, 5 times the frequency, 11 times the frequency, 28 times the frequency, 42 times the frequency Doubling and other excitation frequencies are analyzed.
The gas turbine low-pressure rotor is set to 3 066 r/min, 8 767 r/min, 9 497 r/min, 9 993 r/min (as shown in Figure 7), which is close to the 1st to 8th order natural frequencies and 4,11,28,33,42 frequency multiplication caused by resonance speed, among which the 1st to 3rd order natural frequency resonance has the greatest impact on the blade. It can be seen from Figure 7 that at 3066 r/min, the first-order natural frequency and 11 times the frequency, the second-order natural frequency and the 28 times the resonance frequency margin are all less than 10%, which requires the gas turbine low-pressure rotor to pass quickly, which cannot Stay.
The low-pressure rotor of a gas turbine has been working in the speed range of 7,000 to 8000 r/min for a long time. As shown in Figure 7, relatively dense natural frequency resonances of different orders occur:
(1) The first-order 5-fold frequency corresponds to the number of blades removed from the low-pressure turbine guide blades.
(2) Second-order 11 times frequency, corresponding to the number of support plates.
(3) The fourth-order 28-multiple frequency corresponds to the number of nozzles and the remaining number of low-pressure turbine guide blades.
(4) Sixth order 42 frequency multiplication, corresponding to the high number of guide blades.
1.5 Fatigue strength test
1.5.1 Test methods
The test adopts the vibration table resonance method. The test system consists of an electromagnetic vibration table, a stress measurement system, an amplitude measurement system, etc.
Install the fixture equipped with the test blade on the electromagnetic vibration table. Adjust the vibration table so that the excitation frequency of the vibration table is consistent with the natural frequency of the first bend of the blade, and the blade resonates. Measure the stress distribution on the blade surface through strain gauges at the first-order bending frequency to determine the maximum stress point (see Figure 8). The stress distribution on the blade surface is measured at the first-order bending frequency, and the maximum stress point is determined to be the No. 1 strain gauge, located at the blade root R.
Under first-bend resonance, adjust the excitation force of the vibration table, read the vibration amplitude and stress value of the blade tip under different excitation forces, and determine the corresponding relationship between amplitude and stress.
According to the vibration stress value of the maximum stress point of the blade, the vibration table excitation energy is adjusted so that the inspected blade reaches the required stress level. During the fatigue test, the amplitude is controlled to reach 80% of the test stress level. As the starting time, recording is started, and then finely adjusted to the test stress level. During the test, the vibration amplitude of the blade is strictly monitored to keep it stable. The extracted test pieces are subjected to fatigue assessment.
1.5.2 Test piece
Select 6 low-pressure turbine blades each from the faulty engine and the prototype engine for fatigue strength testing (select the qualified ones from the faulty engine, and select the qualified blades with a certain remaining life from the prototype engine. Compared with the blades from the faulty engine, the blade crown The matching angle is different, but the rest of the structure is the same), and the differences in fatigue performance of the two blades are assessed.
1.5.3 Test results
A total of two batches of fatigue strength tests of low-pressure turbine blades were conducted. The first batch of tests was conducted on faulty machine blades, and the second batch of tests was conducted on selected prototype blades that passed inspection. The fatigue test results of the two batches of blades are shown in Table 3.
After completing the test, perform a fluorescent inspection on the leaves in Table 3. The inspection results are as follows:
(1) The first batch of faulty machine blades with serial numbers 1, 4, and 6 failed to pass the fatigue test assessment, and their corresponding crack locations were close to the location of the failed and broken blades.
(2) In the second batch of prototype engine blades, blades No. 3, 5, and 6 failed to pass the fatigue test assessment. Blade No. 2 was not included in the statistics (cracks appeared at the vibration clamping position). No cracks were found in No. 4. Blades No. 1 and Serial No. 3. The crack positions of blades No. 5 and No. 6 are close to the fracture position of the failed blade (the crack position of blade No. 3 of the prototype engine after the fatigue test is shown in Figure 9).
Through the fatigue test results in Table 3, it is found that the life of the low-pressure turbine blade of the faulty machine is affected by the remaining life of the damaged blade and the retired blade. The fatigue life deviation is large. The life of individual blades is only 1/8 of the qualified blade. If the remaining faulty blades are installed again, there are major safety risks.
By comparing the crack location of the fatigue fracture blade in Figure 1 and Figure 9, it is believed that the fracture location in the fatigue test is close to the fracture location of the failed blade, so there is a factor of high cycle fatigue.
Since the low-pressure turbine blades of the prototype engine selected for fatigue testing have been used for a certain period, the fatigue life of individual blades is less than half of the normal life and is only used for experimental research.
1.6 Analysis of Gas Turbine Operating Conditions
During the debugging and operation of the power station, the gas turbine experienced multiple supporting system failures, causing the gas turbine to shut down multiple times. In addition, due to the limitation of low natural gas supply pressure, gas turbines often operate at low power (5 to 8 MW), resulting in the low-pressure rotor speed of gas turbines often operating in the range of 7 000 to 8 000 r/min, which is required in the natural frequency calculation. There are multiple orders of natural frequency resonance in this speed range, and it is impossible to avoid the resonant speed quickly. Therefore, the fracture of low-pressure turbine rotor blades is mainly affected by blade resonance factors, and the impact of low-cycle fatigue on blade life cannot be ruled out.
2 Conclusion
Through the physical and chemical analysis, vibration calculation, and fatigue test of a faulty low-pressure turbine blade of a gas turbine, it can be proved that the high-cycle fatigue of the blade causes the blade to break and damage other parts of the gas turbine. Gas turbines operate between 7,000 and 8,000 r/min for a long time, and the existence of multi-order resonance is the main cause of blade high-cycle fatigue. In addition, frequent starts and stops of gas turbines will produce high-cycle and low-cycle fatigue, which will accelerate blade fatigue fracture.
3 suggestions
The gas turbine does not stay at low power (7 000 ~ 8 000 r/min) during the rotation process, and can continue to increase the speed to the rated power state. The structure of the low-pressure turbine blades is optimized to meet the fatigue test verification qualification standards, and the optimized blades are inspected for installation.
