PROCESS EQUIPMENT UPDATE
EVALUATING THE ROOT CAUSE For stress corrosion cracking to occur, three conditions must occur: the alloy is susceptible to stress corrosion cracking; the stress intensity factor is above the threshold value; a corrosive environment is present. In this case, the higher tensile
strength levels made the alloy more susceptible to corrosion and the location of the high stress region within the disk corresponded perfectly with the crack initiation locations found during the MPI. Finally, the presence of chlorine at the fracture site indicated a corrosive atmosphere, which led to the cause of the failure being confi rmed as stress corrosion cracking.
MITIGATING ACTIONS Kirill Grebinnyk, engineering analysis group lead for Sulzer, explains further: “To avoid stress corrosion cracking it is necessary to only remove one of the prerequisite conditions. Modern steam turbine components use the latest alloys as well as diff erent blade root designs. In this case, redesigning the blade root to reduce the peak stress levels would not be feasible because the ball root design is very compact and does not al- low for much improvement in the stress profi le.“T e more obvious solution is to address the presence of corrosive agents in the steam. Conducting a complete analysis of the steam being used in this machine will allow the chemicals to be identifi ed and subsequently the quality of the steam can be improved.”
Finite element analysis of a blade identifi ed
maximum stress levels at the operating speed
CASE STUDY NUMBER TWO: LEARNING FROM OTHERS A similar failure of a steam turbine’s row six disk’s root section was investigated using very similar processes. Apart from some minor variations in material quality, the major mechanical property that did not meet the required specifi cation was the measured impact value, which was considerably below specifi cations. Further investigation using a SEM
showed that the entire fracture surface exhibited intergranular failure mode.
T is was confi rmed after assessment of a polished section inspected using optical microscopy that also indicated branched cracking. In addition, EDS analysis found heavy oxide on the fracture surface. All of these fi ndings confi rmed that the failure had been caused by stress corrosion cracking.
After chemically etching a polished sample of the fracture surface, it was clear that microstructure did not show fully tempered martensite. Combined with the low impact values, this indicated that the forging had not been properly heat-treated and this may have accelerated the crack propagation in the disk.
A range of investigation techniques, including optical microscopy, are required to determine the failure mode
10
www.engineerlive.com
ROOT CAUSE AND PREVENTION Getting to the root cause of a failure within a steam turbine can take considerable eff ort and may require a suite of technical evaluations to be carried out. However, this time will be well spent as the performance and reliability of the turbine will be assured and this information may also be useful for similar pieces of equipment. Using experience and expertise, it is possible to identify a cause and recommend remedial actions that will prevent further incidents. Passing this information onto others who operate in a similar fi eld will help them to understand the importance of regular inspections and regular maintenance.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68
