In response to recent industry events and an ongoing requirement to prudently manage an aging hydropower fleet, Hydro Tasmania has introduced integrity assessments of critical turbine components. These assessments help the utility improve its understanding of potential risks within the turbine portfolio.
By Annette Karstensen and Robert Dillon
The development of a prudent asset management strategy for Hydro Tasmania's aging hydroelectric turbine fleet required knowledge building and a fitness-for-service-style approach to be established in order to improve the company's understanding of potential risks within this portfolio. Hydro Tasmania, owned by the government of Tasmia, is Australia's largest producer of renewable energy and owns and operates 30 power stations with a total installed hydroelectric generating capacity of 2,281 MW.
Hydro Tasmania initiated a turbine integrity program in 2008, selecting several machines on which to conduct structural integrity assessments of their higher-risk components, such as head covers and shafts. The selected machines best represented a group of design styles and manufacturing periods within the Hydro Tasmania portfolio. The acquired knowledge from these machines would then be used for similar machines.
Using the knowledge gained through inspection of selected turbines, scenarios were introduced in the calculations to test the sensitivity of typical conditions, such as corrosion pitting, weld cracking and bolt tightness. Extreme conditions were also tested to allow the assessment to provide confidence the results could be used for assessing machines where intrusive inspections were less regularly performed.
The experience gained through the hydro turbine integrity program has allowed Hydro Tasmania to extend structural integrity principles to perform urgent assessment work at its plants in response to findings during planned and unplanned unit outages.
Performing integrity assessments
Quest Integrity Group has used finite element analysis (FEA) and fitness-for-service methods to assess the structural integrity of various hydro power plant components — such as the turbine, alternator and penstock — at hydroelectric facilities in Australia and New Zealand. Many aspects of structural integrity were investigated, including stresses, vibrations, corrosion, susceptibility to fatigue and crack growth calculations, and critical flaw size evaluations.
In some of these cases, metallurgical examinations and material testing have been possible and they have provided valuable knowledge regarding the material properties and defects present in many of the original manufactured welded and cast components.
The following case studies are examples of the structural integrity assessments performed for Hydro Tasmania in the past five years.
Head cover assessments
Assessments were performed on three Francis turbine head covers. The work carried out in this phase has been covered in detail in another publication.1
The assessment included FEA, vibration analysis, fracture mechanics and fatigue assessment of the head cover structures and fasteners. The following issues were of interest:
— Determination of stresses in the bolted and welded joints in the turbine head covers by creating a finite element model of the head cover, including allowance for modelling of the head cover fasteners;
— Determination of the effect of a variation in preload tensions in the fasteners;
— Estimation of the minimum effective bolt area that is subject to corrosion;
— Calculation of critical flaw curves for regions of peak tensile stress; and
— Completion of a modal analysis to determine the optimal locations for mounting accelerometers to take detailed vibration measurements during start-up, rough running, normal operation and shutdown of the units.
Figure 1 shows the calculated von Mises stress distribution in the head cover due to the estimated maximum operating loads. Further refinement of the pressure loading will be made after site testing.
Determination of the effect of a variation in preload tensions in the fasteners was one of the sensitivity scenarios introduced. An important effect of the variation of fastener preload was a reduction in contact area between the bolt flange and the stay ring. Any reduction in contact may cause leakage and expose the fasteners to water, resulting in problems associated with corrosion and aging.
In preparation for the vibration measurement, a dynamic modal analysis was carried out to determine the optimum location for the accelerometers to be placed. It was found that vibration measurement and subsequent analysis did not highlight the risk of service-induced fatigue cracking.
Turbine shaft integrity management
Similarly, integrity assessments of three turbine shafts were performed. The work scope included overall structural assessment to determine potential life-limiting high-stress locations. These locations were:
— Fillets between the shaft and flanges at each end;
— Coupling bolt holes;
— Drive keyway;
— Fillets between bolt head and shank; and
— Coupling bolts at the shear plane.
Figure 2 shows an example of the finite element model, including the details of the upper and lower flanges and the stress distribution in terms of the von Mises stress. It can be seen that, in general, the stress are about 100 MPa. However, around the fillets these stresses increase to about 150 MPa.
Pitting corrosion has been found on the wet areas of the shaft, particularly around the lower flange areas. Pitting is a potential site for crack initiation because the pits themselves are a stress raiser. Therefore, several models were generated, assuming different material loss, which again increases the stress level in the fillet.
Table 1 shows stresses as a function of the local metal loss around the fillet. Based on these stresses shown, the critical crack sizes were calculated. Although the material was not expected to have superior toughness, it was found that in most cases the shaft could tolerate significant material removal without failure.
The coupling bolts tolerance to cracking was also calculated. The bolts are typically highly pre-tensioned, and with a stress concentration of about 2.8 at the radius between the head and shank, the stress would be increased well above the yield stress of the material. Thus this fillet is very susceptible to crack initiation and fracture.
Optimizing repair options of cracked valve
Once an original turbine main inlet valve was dismantled, significant cracking was discovered in the door and body halves. Cracks in the weld close to the bearings were of particular concern due to their individual size (e.g. 300 mm in length in a 400 mm-wide flange and 70 mm in height in a 90 mm-thick section). Full repair could result in significant distortion of the body half-joint flange and trunnion bush bore areas.
A stress analysis showed the stresses in the areas of the cracks were very low for normal operating conditions. Rather than repairing the entire cracked section, the possibility of executing a partial repair was considered to help manage distortion and residual stress effects. Only part of the crack would be excavated for re-welding, leaving the remaining part of the crack and the original land area as a buried flaw.
It is important to note that the weld in the current state had been subjected to post-weld heat treatment, while the repair weld would have a significant amount of residual stress.
A parametric study was performed for different combinations of buried flaw heights and weld heights to determine the minimum weld height required to meet the fitness-for-service criteria. Starting with a 40 mm-high crack and a 30 mm-high weld repair, the weld height was gradually decreased until eventually a combination of crack and weld reached the critical envelope of the failure assessment diagram (see Figure 3).
Combining FEA modeling, critical flaw size calculations, fatigue crack growth calculations, vibration analysis and, in some cases, metallurgical examinations provided the knowledge required by Hydro Tasmania to understand and evaluate potential risks within its entire hydro turbine portfolio, assisting in the prioritization of capital expenditures. In addition, the structural integrity principles have been extended to allow the utility to perform urgent assessment work as well as tailor repair solutions in response to findings during hydropower plant outages.
1. Karstensen, Annette, and Robert Dillon, "Integrity of Head Cover and Head Cover Fasteners," Proceedings of 3rd Hydro Power Conference, New Zealand, 2011.
Annette Karstensen is Asia-Pacific structural integrity manager with Quest Integrity Group. Robert Dillon is principal engineer - turbines with Hydro Tasmania.