Fast repair needed for a power plant steam turbine
The repair of a steam turbine, which had a cracked rotor, required some innovative and quick thinking to ensure a quick repair time for a power plant in the Philippines.
In this case, the steam turbine rotor consisted of a high-intermediate pressure (HIP) section connected to the low pressure (LP) rotor. Together, these powered the 150 MW generator.
The power plant engineers in the Philippines were studying elevated vibration readings on one of the steam turbines. Further investigation revealed a severe crack in the coupling of the HIP rotor. In addition, there was damage to the coupling bolts’ holes of both HIP and LP rotors, which together suggested a possible alignment problem.
The power plant was faced with two options: to replace the cracked rotor with a new rotor or find a specialist with the expertise to complete an effective repair of the rotor.
A new rotor would take at least a year to arrive; engaging a specialist maintenance provider could see the turbine back in action much more quickly. Fortunately, the engineers could not only repair the coupling but also improve the rotor alignment and reduce stress levels within the coupling. After further inspection, engineers learned the coupling was no longer operational.
The engineers used reverse engineering techniques to recreate a 3-dimensional drawing of the part and apply stress analysis for flawless design. The coupling, which connects the HIP rotor to the LP rotor, must be manufactured at very tight tolerances to ensure perfect alignment of the complete rotor shaft.
The process of repairing the coupling involves gradually building up material using an automated submerged-arc welding process. Once complete, the residual stresses caused by the welding process were removed by post weld heat treatment. Once that was done, repairing the coupling could begin. The exact dimensions of the original coupling were established using a large precision lathe. These were completed along with the holes in the LP rotor coupling flange that also needed to be repaired.
The calculations for the alignment were made along with a simulation of the rotor alignment. Both took thermal growth of the rotor train during operation into consideration. The original alignment had been suspected as one of the contributors to the damage to the rotor. Field engineers implemented new calculations and made adjustments to the bearing pedestals.
The two rotors were dynamically balanced in house for optimum performance once they were installed. Dynamic balancing is done at relatively low speeds. In most cases, some minor adjustments are required once the rotor is installed and operating at higher speeds.
In all, the workshop operations were completed within eight weeks, after which the rotors were carefully packaged and airfreighted back to the Philippines to minimize the overall project duration. The protective shipping containers were slightly modified by to ensure they would fit into the aircraft, preventing any delays in the process.