Fast rotor repair also improves efficiency

When a damaged steam turbine rotor needed repairs, a power generation plant in Indonesia opted for a repair that took four months instead of waiting a year for a new rotor. This repair also achieved an 8% improvement in efficiency. The Indonesian geothermal power plant had been experiencing some issues with one of its 60 MW steam turbines. Despite several attempts to low speed balance the rotor at their premises, it still exhibited high vibration levels.

Initial investigation

During one of these off-site balancing procedures, the plant engineers discovered deep cracks. The initial inspection revealed the rotor had cracks on the radius section between the last stage disk and the gland seal area located on both the governor and the generator sides. There also was considerable evidence of erosion on the blades, the disks, and the balance correction holes.

Repair re-evaluation

The initial plan was to machine out the material until the cracks were removed and rebuild the shaft using submerged arc welding before machining it to nominal dimensions. However, the crack propagation already had a spiral shape through the center of the shaft. This made it impossible to machine out the crack area. The repair proposal involved designing a stub shaft that would be used to join the two pieces together before the shaft was rebuilt to its nominal dimensions.

With the location of the repair being so close to the 6th disk, some additional repairs would be needed to this disk, which would extend the overall time to complete the project. in order to save time, both stage 6 disks would be removed and possibly reinstated at a later date.

Repair expertise

Joining two sections of a turbine rotor requires considerable expertise. This includes computer modeling and finite element analysis (FEA) to ensure the proposed design would withstand the stresses of normal operation. The FEA was carried out at an overspeed of 3600 rpm to ensure centrifugal loading on the disks would not cause any damage to the rotor shaft after the repair. The machine shop then prepared the two rotor shaft sections for the addition of the stub shaft.

In-house precision machining enabled the stub shaft to be shrink-fitted into the prepared connections before the whole joint area was preheated prior to the welding process. The stub shaft was built to a level that would allow it to be machined back to the required dimensions using submerged arc welding equipment. Once the original dimensions had been achieved, a series of non-destructive tests (NDT) helped ensure there were no flaws in the completed rotor assembly.

These processes were repeated to remove the cracks in the thrust end of the rotor, as well, bringing the completed assembly back to finished dimensions. The rotor was dynamically balanced once the machining was complete.

Improving performance

While rotor repairs were being completed, the field service team worked on repairing the diaphragm and improving the casing’s seal. This work would be influential in improving the efficiency of the steam turbine.

Prior to the project, the turbine required 393 tons of steam per hour to produce the 53.4 MW of energy. Static and rotor component repairs to the turbine enabled it to maintain an output of 55.1 MW using only 374 tons per hour of steam.

When the repaired turbine rotor arrived back on site, the field service team carried out installation and commissioning. All the results were well within the original specifications and the generator has remained at full capacity ever since.

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– Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com. See more Control Engineering machine safety stories.

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