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Shrink-fit Turbine Coupling – a revolution and a revelation 18/07/2015 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 1 The Damaged LP Front Coupling DAMAGED LP FRONT COUPLING FLANGE 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 2 The Damaged LP Front Coupling COUPLING MACHINED UPTO THIS POINT RUBBING MARKS ON LP FRONT COUPLING 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 3 What caused it? AUXILIARY BEARING IN PEDESTAL NO.2 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 4 The cause of the damage Apparently, the cause of the damage can be attributed to a drop in the voltage in the 220V Battery System. When AC Supply failed and the DC tried to take over, this drop in the voltage resulted in start and immediate stop of the DC Oil Pumps, there by creating a temporary oil starvation to the bearings. When this incident occurred, the Unit was at 50MW load and was slated to touch full load in a few hours. This resulted in the machine coasting down to “Zero” Speed in under 4 minutes. When this happened, the babbit material from all the bearings melted and the rotor rested on the bearing shell. 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 5 The cause of the damage (Contd.) During the process of coasting down, as the babbit metal melted, the gap between the LP Front Coupling and the Auxiliary Bearing in Pedestal No.2 went on reducing. This gap is usually kept at 5mm . The minimum thickness of the babbit material at the point where the rotor rests on the bearings is usually 8mm. From this it can be deduced that, as the babbit metal melted down 5mm, the LP Coupling came in direct contact with the Auxiliary coupling. 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 6 The consequence This kind of a rubbing and damage to the coupling was unprecedented in the history of BHEL. As can be seen from the slides, 12 round spots corresponding to the 12 nos. of Coupling Bolts, were formed. These spots were formed in the area of the nut, as, there is always an annular space between the nut and the coupling and the thickness of the coupling flange in this area is less. The LP Rotor was then sent to the works of BHEL Hyderabad for repair. 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 7 The consequence (Contd.) The hardness of the LP Coupling Flange in all these areas exceeded 450BHN (it is normal to have the hardness between 250-300BHN) and any amount of heat-treatment and skin cutting did not yield results. It was calculated that a cut of up to 4mm on the coupling flange would be safe operationally. This too proved futile. Besides, the damage to the coupling, there were quite a few other damages, but none of this kind. Every other damage was repairable. Since the damage to the LP Front Coupling was unprecedented, the repair procedure too had to be unprecedented. 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 8 The Shrink-fit LP Front Coupling 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 9 The repair and Commissioning The normal LP Rotor Coupling is an integral part of the Rotor and is machined from the Rotor Forging itself. Discarding the entire LP Rotor and manufacturing a new one, not only meant a huge loss of money, it also meant a 5 month delay as the minimum time required for manufacturing a new LP Rotor is 4-5 months. Thus was born, the idea of shrink-fit coupling. As the hardness of the coupling flange showed no signs of reduction, the damaged portion of the coupling was machined right upto the base. A new coupling hub with an interference of 2.0mm was fitted to the shaft. This coupling, as per the calculations of BHEL Hyderabad, could generate a power of approx. 250MW. Since the MCR of the machine was 125MW, this was within a safe range. 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 10 The repair and Commissioning Being the 1st such instance of a Turbine Coupling for machines of higher rating having been shrink-fitted, it was greeted with the usual skepticism associated with such a change. As is common in such cases, when the rotor was received at Site, it was inspected in every which way in which it could be rejected. After repair, the LP Rotor was received at site on 14.03.2010 from Hyderabad works. It took 14 days for site to carryout the following activities before putting the machine on Barring Gear: a. Placement of LP Rotor and checking flow-path clearances. Since there was minor distortion to LP Guide Blade Carriers, which too were sent to Hyderabad for repairs, the clearances of LP Diffuser too got affected. This required grinding of the diffuser halves. This alone took 4 days. 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 11 The repair and Commissioning … Contd. b. Alignment, Coupling and Swing Check before release for Reaming and Honing of HP/LP Coupling. c. Reaming, Honing, fitting of new coupling bolts (Since new coupling bolts were required to be installed, this necessitated machining & balancing), CRO and Swing Check. d. Alignment of LP-Generator Rotors, coupling and CRO. (These coupling bolts & nuts were not changed; hence, there was no need of Reaming & Honing). However, a machine cut on the LP Rear Coupling face was made at the works of BHEL Hyderabad which resulted in higher CRO. This was corrected by scrapping the coupling face. 2 days were lost in this activity. e. Box-up of LP Turbine f. Box-up of Generator 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 12 The repair and Commissioning … Contd. g. Alignment of Generator & Exciter and Swing Check h. Box-up of Exciter. i. Preparation of all keys. j. Box-up of all pedestals for flushing through pedestals. k. Normalisation of bearings and Box-up of pedestals after oil flushing The machine was put on Barring Gear on 29.03.2010, and synchronised on 02.04.2010. It was observed that the vibrations were slightly higher than before but well within limits, prompting one to believe that this was probably because of the shrink-fit coupling. Shrink-fit or otherwise, a rotor subjected to such high stresses as a result of the instantaneous stop, would even otherwise have vibrations on the higher side. 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 13 Conclusion This is a revolution because, this is the 1st time that BHEL has undertaken a repair job of this type. This is a revelation because, it was not known until then that Turbine Couplings too (like BFPs & CEPs) can be shrink-fitted. The gap between the Coupling and the Auxiliary Bearing provided in the pedestals should be kept at a minimum of 10mm whereby, even in the event of complete melting down of the babbit material, the coupling would remain unaffected. A safe and sound DC System should be ensured before rolling the machine. 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 14 18/07/2015 Exploring Heights ..................... Setting Standards .................... Marching Towards Excellence Power Sector Western Region 15