Justification for extending the service life of steam turbines with parts that deviate from the requirements of regulatory documentation
The issue of extending the service life of steam turbines operated at Russian thermal power plants is currently very relevant
A significant portion of turbine equipment has worked out its park resource, which is established for a given type of turbine taking into account operating experience and operating parameters of steam and limits the operating time and the number of starts from cold, hot and not cooled conditions.
When conducting technical diagnostics of the main elements of turbines after long-term operation, sometimes exceeding the park resource by 1.5 - 2 times, defects are often found that cannot be eliminated during routine repairs of steam turbines. In this case, the question arises about the possibility of further operation of the equipment for a limited period up to the factory repair, replacement with new equipment, etc.
The possibility, terms and conditions of operation of turbines with such defects and/or deviations from the requirements of regulatory documentation require justification, which is carried out taking into account data on the current technical condition of the equipment and forecasting its behavior based on calculations and analysis of technical documentation for the entire service life.
Such an integrated approach to solving the problem allows us to issue reasonable recommendations for the temporary operation of turbines with existing defects without reducing their reliability and safety.
The strength department of JSC NPO TsKTI began to deal with the problem of extending the service life of steam turbines in 1970, when the high-temperature elements of steam turbine units exhausted the service life of 100 thousand hours guaranteed by the manufacturers.
Power units are operated at high steam parameters under conditions of frequent and rapid loads and unloading by power, which causes the appearance of defects in the material of parts due to the exhaustion of long-term strength due to creep, low-cycle fatigue, wet-steam corrosion of metal under the action of high stresses in the phase transition zone of steam, as well as warping of high-temperature cylinder bodies and erosion of metal on the surface of parts by wet steam.
The main units that limit the service life of a turbine due to long-term exposure to high stresses and temperatures are high- and medium-pressure rotors, cylinder and valve bodies.
If crack-like defects are detected in a part during inspection, depending on their location and size, based on the requirements of regulatory documents, the defects are either left unchanged, or drilled out, or selected. After sampling the cracks, an assessment of the state of the metal is carried out to decide on the need to weld the sample and the further service life of the part with the existing sample or welding. In this case, various non-destructive testing methods are used (hardness measurements, metallographic examination using replicas, capillary and ultrasonic testing), testing of cut samples and analysis of the stress state of the part with determination of safety margins under static and cyclic loading.
Below are examples from the experience of the Central Design and Technical Institute of solutions to the issue of extending the service life of turbines with detected defects.
Turbine rotors
According to the instructions, inspection of rotors that have worked for over 80 thousand hours is carried out. The rotor inspection program includes ultrasonic and eddy current (magnetic particle) inspection of the boring surface, disk fillets and thermal compensation grooves. The most damaged section of high-temperature rotors is the surface of the axial channel in the first stage zone, where maximum temperatures and stresses from centrifugal forces and significant thermal stresses during start-up and shutdown modes occur.
Table 1 presents the results of the examination, carried out by TsKTI and VTI, of the bores of high-pressure (HP) and medium-pressure (with intermediate steam superheating - ISH) rotors after long-term operation.
Table 1
Results of inspection of steam turbine rotors with defects in boring
| No. | Turbine type | Operating time, thousand hours | Number of starts | Rotor steel grade | Detected defects |
| 1 | RSD K-300-240 LMZ | 81.5 | P2M (25Kh1M1F) |
Point defects, pores. | |
| 2 | RVD K-200-130 | 87.6 | 294 | P2M | 2 cracks: L1 - up to 35 mm long, L2 - up to 22 mm long |
| 3 | RSD K-300-240 HTZ | 96.6 | 237 | EI415 (20Kh3MVF) | 10 cracks from 3 to 30 mm long, see figure 1 |
| 4 | RVD T-100-130 | 100.8 | 150 | P2M | Point defects, cavities, pores |
| 5 | PT-80-130 | 166.2 | 185 | P2M | 7 linear defects from 2 to 30 mm, see Figure 2 |
| 6 | K-50-90 | 185.3 | 1188 | 34KhM | 6 defects up to 60 mm long. |
| 7 | RSD K-300-240 HTZ | 187.3 | EI415 | Crack up to 20 mm, up to 10 mm deep |
|
| 8 | K-50-90-2 | 200.0 | 1200 | 34ХМ | 2 defects up to 5 mm long. |
| 9 | K-55-90-1 | 287.9 | 34ХМ1 | 20 defects from 5 mm long to 40 mm, up to 5 mm deep |
|
| 10 | VT-25-4 | 290.6 | P2 | 24 defects up to 30 mm long, up to 2.5 mm deep | |
| 11 |
9 rotors: |
268-379 | 34ХМ 34ХМ1 | 1-15 defects from 5 to 55 mm long, 1-5 mm deep |
Subsequent honing of the channel surface did not ensure the removal of defects, while new defects were detected. The rotor was not approved for operation.
The appearance of cracks on the surface of the rotor axial channel causes great concern during operation, since EI415 steel has reduced crack resistance, and the critical defect depth can be as little as 8 mm. Metallurgical defects are found on approximately 20% of rotors made of EI415 steel, which indicates reduced reliability of the rotors after long-term operation, including due to the risk of brittle fracture during start-ups.
The K-200-130 turbine RVD after 87.6 thousand hours of operation had a 22 mm long and more than 3 mm deep defect on the surface of the axial channel. If necessary, to remove defects, general turning of the axial channel is allowed, increasing the nominal diameter of the channel according to the manufacturer's drawing by no more than 10%. This section of the rotor channel was machined to a depth of 3 mm. The rotor was approved for temporary operation.
The figure shows a diagram of the location of defective zones on the surface of the axial channel of the PT-80-130 turbine rotor after 166.2 thousand hours of operation. Visual inspection revealed: 27 defects 3-10 mm long, 11 defects 12-15 mm long, 2 defects 20 and 30 mm long. The total number of extended unacceptable defects (over 3 mm long) was 40. A calculation was made of the possible increase in the depth of defects during further operation for 14 thousand hours with an assessment of the resistance of the HPS metal to brittle fracture in the presence of a defect.
Considering the low level of stresses and temperatures on the boring surface (505 - 519 °C) in the rated power mode and the high resistance of R2M steel to long-term loading and brittle fracture, the rotor was allowed to continue operation for 14 thousand hours until the total operating time was 180.2 thousand hours. It was recommended to perform repeated visual inspection and sampling of all defects on the boring surface during the next repair of the unit.
Creep of high-temperature HP and MP rotors of steam turbines occurs mainly in the area of the first stages, where the maximum temperature of the rotor metal occurs and high stresses from the centrifugal forces of the rotor disks and blades act. As a result, there is a gradual increase in the rotor outer diameter in the zone of the first and sometimes subsequent stages, while the diameter of the axial channel increases.
For more than 20 years, the employees of the Central Design and Technical Institute have measured changes in the axial channels of HP and MP rotors of various types of turbines. Based on the measurements, the values of residual deformations were calculated, which make it possible to assess the condition of the rotor metal and predict the creep rate of steel [6]. On the surface of the RVSD of the combined cylinder of the K-160-130 KhTZ turbine (EI415 steel), in the area of the front end seal, after 138 thousand hours of operation with 524 starts, deep cracks were found at the bottom of two thermal compensation grooves. The cracks were removed by turning.
Strength calculations were performed to confirm the operability of the rotor after turning the grooves. Table 2 shows the results of calculations of stresses in grooves during hot start.
Table 2
| Groove | Original geometry | Geometry after turning | ||||
| Depth, mm | Fillet radius, mm | Maximum stress at start, kgf/mm2 | Depth, mm | Fillet radius, mm | Maximum starting stress, kgf/mm2 | |
| A | 9.0 | 1.5 | 10.4 | 27.8 | 8.0 | 14.0 |
| B | 9.0 | 1.5 | 13.3 | 20.0 | 8.0 | 19.4 |
Based on the calculations of long-term and cyclic strength, the service life was extended by 52 thousand hours to a total operating time of 190 thousand hours.
Similar damage in the thermal grooves was found on 6 grooves of the RSD dummy of the K-200-130 turbine, which had operated for 126 thousand hours with 507 starts (cracks up to 1.8 mm deep), and in the thermal grooves of the front end seal of the RSD of the K-210-130-3 turbine after 122 thousand hours of operation with 521 starts.
On the thrust disks RND of PT-50-130, T-50-130 and T-100-130 turbines after long-term operation in the steam phase transition zone often develop cracks on the surfaces of the relief holes, longitudinal keyway, rim in the area of rivet joints, hub part and web. Figure 4a shows cracks on the surface of the keyway of the disk of the 22nd stage of the PT-50-130-4 TMZ turbine. The possibility of further operation of RND with such defects is decided on the basis of strength calculations. The rotor was allowed for further operation for 25 thousand hours. In the presence of deep samples, it is sometimes necessary to cut off the disk web.
Cylinder and valve bodies
The most stressed parts of the steam turbine stator are the bodies of the stop and control valves and the steam inlet zones of the cylinder bodies, as they are operated at maximum temperatures and pressures of the supply steam. The bodies are made of cast heat-resistant steels 20KhMFL and 15Kh1M1FL.
Due to the casting technology, these steels have a significant number of defects, especially on the outer surface of the casting, as well as in places of sharp shape changes: transitions from the wall to the flanges, branch zones, changes in wall thickness at the attachment points of the collars and diaphragms.
During long-term operation, due to the exhaustion of long-term strength and the accumulation of fatigue defects, surface cracks appear. During major repairs, these cracks are drilled along the edges or removed, depending on the depth. The deepest metal cuts are welded.
A significant number of deep cracks, porosity, crack networks, etc. occur in the valve bodies in areas of stress concentration, repair welds and welds. In each case, a decision is made to repair or replace the damaged valves.
On one of the K-200-130 turbines, the left and right HPC stop valves were dismantled after 210 and 230 thousand hours of operation due to intense cracking.
In 2003, a through crack was discovered in the HPC stop valve body of the K-200-130 turbine after 110 thousand hours of operation with 275 starts, developing from the inner surface, 75 mm long with a wall thickness of 45 mm. The crack was selected and welded.
During the inspection of the PT-50-130-4 TMZ turbine after 378 thousand hours of operation with 474 starts, a crack was found on the inner surface of the stop valve housing (SVHP) of the HPC. The crack was selected and welded.
The valve is made of 15Kh1M1FL KP30 steel. The housing was calculated at the elastic stage taking into account the sample without welding and with a welded sample. It was shown that the stresses in the sample without welding due to stress concentration reach 9.0 kgf/mm2. In the wall of the housing with a welded sample, the stresses do not exceed 4.5 kgf/mm2. The service life of the valve was extended by 25 thousand hours.
Defects in the form of cracks are often observed in the steam inlet zone on the wall of the HPC housings. The figure shows a crack in the K-200-130 turbine HPC casing cover. The casing was in operation for 267 thousand hours with 350 starts. The crack was drilled. The casing was allowed for further operation for 35 thousand hours.
Cracks may appear on the flange connector of the lower half of the cylinder, which extend to the holes for attaching the studs. Such defects are subject to sampling and welding.
The figure shows the location of 46 metal samples on the inner surface of the lower half of the K-200-130 turbine HPC casing after 42 thousand hours of operation at a live steam temperature of 565 ° C and 110 thousand hours at a temperature of 540 ° C. The total number of starts is 614.
The sizes of the samples ranged from 80-10-5 to 2000-40-45 mm. All samples were welded, and the body was subsequently annealed in a furnace to relieve residual stresses. After annealing, due to warping of the body, the sealing belts of the horizontal connector were scraped to ensure a tight fit of the flanges when tightening the studs. The service life was extended by 48 thousand hours to a total of 200 thousand hours.
Steam bypass pipes
VD and SD steam bypass pipes operate under high temperature conditions. The most stressed areas are bends, the reliability of which determines the service life of the pipeline as a whole. The overwhelming majority of damage occurs on bends of steam pipelines made of 12Kh1MF steel. Damage to steam pipeline bends made of 15Kh1M1F steel occurs much less frequently.
During repairs of the K-200-130 LMZ turbine, measurements were taken of the hardness, ovality and thickness of the HPC and MPC bends made of 15Kh1M1F steel (?273x32) after 250 and 270 thousand hours of operation. The figure shows the change in hardness of the examined bends (the permissible minimum hardness value is 156 HB). The results of the surveys help determine which bends need to be replaced during the current or next repairs.
CONCLUSIONS
The presence of defects and deviations from the requirements of regulatory documentation in the elements of steam turbines that have exhausted their fleet life is not always an obstacle to their further operation.
In each specific case, it is necessary to carry out a set of works, including technical diagnostics of the turbine, analysis of operating conditions, calculations of the strength and service life of the main elements. Based on the above, a decision is made on the possibility, timing and conditions of further operation of the turbine.