Volume 3 nr 1 / 2011 - Academia Oamenilor de Stiinta din Romania

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48 Mircea O. Popoviciu, Ilare Bordeasu, Liviu Marsavinashaft of a Pelton turbine. The runner was overhang disposed and the shaft wasworking in small corrosion conditions (permanent dense fog but also droplets oreven small water jets washed continuously the shaft). When the fluctuating stresseshave great values, a tendency of multiple crack inceptions appear. For the shaft indiscussion, in the same cross section there were simultaneously, without beingobserved, three fatigue cracks disposed approximately at 120° (see Figure 1). Afterthe extension of these cracks, the shaft was fractured and the aggregate sufferedsevere damages.Fig. 1. Pelton turbine shaft fatigue fracture.While for Bulb turbines, the shaft is placed into a case, well sealed off and heatedby the electric generator, it is considered that the fluctuating stresses take place inthe absence of corrosion, the material has “fatigue limit” and fatigue failure hasvery reduced probabilities. Our observations show that this opinion is not inconformity with the facts and for bulb turbines it appear simultaneously bothcorrosion and variable stresses and the material do not have a fatigue limit, failurebeing possible.2. Turbine parameters, geometry and shaft manufacturing procedureDuring the year 2008 we examined some turbines shafts, with the service andconstructive parameters presented in Tables 1 and 2. For some aggregates, therewere effected refurbishing works (HA3- 2006 + 2007; HA4-2005 and HA5-2008).During the refurbishing operations, the shaft was completely replaced. Next, wepresent the principal differences between the initial shafts and those refurbished.Those consist exclusively into a new manufacturing procedure.Table 1. Bulb turbine parametersParameter Symbol ValueNet Head H 7,8 mWater discharge Q 475 m 3 /sEffective power P 32,5 MWTurbine and Generator rotation sped n T 62,5 rpmRunner diameter D 1 7,5 mPosition of the runner weight against the shaft flange1650 mmNumber of blades Z 4eight of rotating subsystem (without oil) G rotor 99,6 ToPosition of the runner weight against the shaft flange1650 mmCopyright © Editura Academiei Oamenilor de Știință din România, 2011Watermark Protected
Analytical Evaluation of Crack Propagation for Bulb Hydraulic Turbines Shafts 49Initial shaft (Russian license). The shaft is constituted from three parts assembledthrough welding. The two half-finished parts placed near the electric generator aremanufactured through forging. The half-finished flange for coupling is manufacturedthrough casting. The machine working at final dimensions is done after weldingfollowed by heat treatment and cheek-out. After that, the connecting zones betweenthe flange and the main cylindrical part (zones where occurred cracks during theoperation) were bring-up to the final shape through rough turning (Ra = 20 m).Table 2. Data regarding shaft geometryDataShaft lengthShaft diameterFlange diameterShaft weightValue7572 mm1200 mm1700/2298 mm51.170 kgTable 4. Running hours for the refurbished turbines(hours)HA3 HA4 HA5Total R 143.647 136.397 157.2792008 16.257 22.915 7.417Total R - total running hours till the refurbishingTable 3. Running hours for not refurbished turbines (hours/state)HA1 HA2 HA6 HA7 HA8 HA9 HA10177.560 178.594 158.336 165.563 165.804 108.109 93.908F F F N N F NN –Not controlled, F – cracksRefurbished shaft. The half-finished shaft is manufactured entirely by forging.The employed material, the final dimensions and the machine working operationsare identical with those for the initial shaft.3. Shaft material and corrosion resistanceIn Tables 5 and 6 there are presented characteristics of the employed material.From the chemical composition (Table 5) it results, that the material is lightmanganese alloyed steel. The low proportion of chromium and nickel (especiallychromium) is inadequate to confer a good resistance to corrosion, inclusive intercrystalline corrosion. Taking into account the influence of manganese on thecrystalline grains, we appreciate that the principal alloying element determines arough structure of the flange material, especially for casted pieces (great and nonuniformdimension of the grains). Therefore, the corrosion resistance of the materialis relatively low.Table 5. Shaft material mechanical characteristics Table 6. Shaft material chemical compositionCharacteristic ValueChemical Element ProportionR p0,2 255.05 N/mm 2 Mn 1…1,3 %R m 470,88 N/mm 2C0,16…0,22%Si0,60…0.80%Cr, Ni, Cu < 0,3%S, P
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Page 5 and 6: 3FOREWORDBased on a rich scientific
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Volume 3 nr 1 / 2011 - Academia Oamenilor de Stiinta din Romania

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