Sticky Wickets: Repairing Failed Stator Core Bolts at Thompson Falls

A routine inspection of Unit 7 at PPL Montana’s Thompson Falls facility revealed a broken stator core bolt. Further investigation uncovered a total of 40 cracked bolts. To deal with the problem required a complete replacement of all 200 stator core bolts in this unit, taking into account the resonant frequency of that stator core. The unit has operated without incident since the repair.

Replacement of the 400 stator core bolts for Unit 7 was achieved through the air deflector.
Replacement of the 400 stator core bolts for Unit 7 was achieved through the air deflector.

Understanding the situation

PPL Montana‘s Thompson Falls 50-MW Expansion Unit 7 facility, located on the Clark Fork River of the Columbia River in Thompson Falls, Mt., went online for commercial generation in December 1995. The plant was designed with a single Kaplan turbine-generating unit and, combined with the original powerhouse six units, has a total capacity of 94 MW.

On Sept. 22, 2004, routine inspection performed during annual maintenance of Unit 7 by LeRoy Mock of HydroInsight, a generator consultant inspector, revealed a nut with part of a bolt wedged into the backside of the generator stator coils. Further inspection of this unit’s stator revealed one additional nut with the stud wedged in back of the coils of the generator stator. It is believed that the magnetic attraction caused by the coils pulled the bolt with nut onto the coils.

Paul O’Leary a metallurgist, was brought in the next day to perform an ultrasound inspection of all 200 stator core bolts in this unit to determine if there were any other broken or cracked bolts. O’Leary also took the nut with broken stud back to his office to analyze to help determine the failure mode of the two bolts found loose. The overall length of the bolts on this unit is 66 inches. The ultrasound inspection revealed there were a total of 40 bolts cracked, with 18 cracked at 3 inches down as measured from the top of the bolt (cracked within the threads), 21 at 40 inches down as measured from the top of the bolt (cracked at the tab or flag that was welded onto the bolt) and one cracked at both 3 inches and 40 inches down from the top.

A temporary fix

PPL Montana contacted General Electric (GE) to devise a repair. PPL decided that a temporary repair could be performed that would get the unit back on-line while a permanent repair was planned. It was the only option that could be done rather quickly and that did not require dismantling the machine in the process, thus lengthening the outage.

On Oct. 4, 2004, GE was brought onsite to assist in overseeing the temporary repair, which was performed by outside contractor PMRI. The temporary repair, which was suggested by operator/maintenance plant personnel, was to use an L-shaped bracket that would be welded onto the top of the stator frame skin. The bracket would be used to apply pressure to the lamination core to temporarily take the place of the broken or cracked stator core bolts. Where access permitted, a piece of steel about 5 inches by 2 inches by 0.5 inch (new flag or tab) was welded across the crack at the 40-inch level next to the existing tab.

What caused the problem?

PPL brought O’Leary back in February 2005 to perform another round of ultrasound inspections on all 200 bolts, and he found six additional bolts were cracked, with five at the threads (3 inches down from the top) and one at the tab, which is 40 inches down from the top of the bolt. The bolts were checked again one month later, and none were found to be cracked. In August 2005, the bolts were checked again and three more were found to be cracked, with two at the threads and one at the tab. This yielded a total of 49 bolts, or about 25%, that had cracked or broken, with 25 at the threads, 23 at the tab location and one at both threads and the tab. The unit was returned to service, as the L-shaped bracket for temporary repair was used to provide pressure to the core stack.

Based on the data, O’Leary concluded the following:

– Failure of the bolts was caused by low-stress/high-cycle fatigue due to operational vibrations. The stresses resulting from machine vibrations were concentrated at the root of the first thread (3 inches down from the top) on the bolts.

– Stresses were also concentrated in a change in the rod cross-section resulting from the welded tab (40 inches). The combination of the stress riser effect of the tab and the relatively poor post-weld mechanical properties of the rod material (increased hardness and reduced ductility) resulted in failure at this location.

– Further information from O’Leary and GE yielded that the 40-inch section of the bolts appears to match the resonant frequency (120 Hertz) of the unit and they are failing due to an assumed sideways oscillation of the core bolts. The lower section (26 inches) of the bolts (below the tab) is shorter than the top section and thus has a higher resonant frequency and has not seen failure.

To verify that this was a resonant frequency issue, Stan Bognatz of M&B Engineered Solutions was contacted to come onsite during September 2005 to perform a modal analysis of the stator core studs. To determine resonant frequencies and mode shapes, a calibrated force hammer (load cell) was used to impact the studs specific locations, while a stationary tri-axial accelerometer measured the cross-channel three-dimensional stud responses. Spectra and frequency response functions (FRFs) were then calculated and further analyzed using modal analysis software. The original design indicated several predominant frequencies that would be easily excited by the generator’s electrical field during operation, most notably 120 Hz. Modal analysis further indicated that high cyclic stress would likely be realized due to the mode shapes involved, thus contributing to the observed failure patterns.

Based on the data collected, GE determined that a complete replacement of all core bolts was needed, as all of the bolts were exposed to the same frequency and were going to eventually fail.

A permanent solution

On Oct. 3, 2005, GE began the permanent repair, beginning with removal of the existing bolts.

To make it possible to install the 200 new bolts without dismantling the entire machine, a two-piece core bolt was used that had a sleeve or coupler that allowed the bolt halves to be inserted from within the stator frame air deflector outlets, which had the necessary room, and then assembled.

The coupler was used to facilitate installing the upper and lower portions of the bolts due to clearance issues with installing the lower portion of the bolt into the stator and to provide a flat surface to weld the tab against. A second tab was welded onto the core bolts stud after it was installed, which changed the length of the bolt and therefore the resulting frequency.

Bognatz was brought back onsite in October when some of the core bolts had been installed with the second tab. Testing of the new stud design revealed the lowest modes to be in the 275 to 330 Hz region. Bognatz stated that this should alleviate most of the cyclic stress issues that would arise from the generator field excitation forces. This served as verification that the permanent fix was correct.

On Nov. 3, 2005, the Thompson Falls Unit 7 generator was put back in service, and it has run reliably ever since. GE recommended PPL check the torque of the bolts every year to see if anything has changed, and there has been no changes. While this is a rare occurrence, it is one that could happen at any plant and needs to be watched closely.

– By Gary Peterson, senior hydro engineer, PPL Montana LLC

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