Cracks in the R0 dovetails of rotor forward shafts on 7FAs and 7FBs are said to be possible in about three-quarters of the gas turbines in the combined fleet, according to knowledgeable industry sources. Several cracks in the dovetails of one R0 disc, at the edge of contact between the blades and wheel, were reported in CCJ ONsite last November.
The editors called Mike Hoogsteden, field service manager, Advanced Turbine Support LLC, for an update on what the company’s technicians were finding during their TIL 1907 inspections. As of three months ago, the borescope inspection experts had not found any R0 dovetail cracks. Recently, however, they identified cracking on two units. By the service manager’s count, this brings the industry total to at least seven units with confirmed R0 dovetail cracks. One machine, in service for only about a year, was found to have experienced dovetail cracking after only 57 starts.
Hoogsteden now believes that possibly 10% of the gas turbines considered susceptible to R0 dovetail cracking may be affected. Also, he stressed that some of the cracks confirmed by Advanced Turbine Support initiated in the dovetail downstream of the pressure face of the disc and were hidden from view with the blades in place.
The photos from one unit recently inspected unit confirm this. Fig 1 shows a top-down view of the crack mapped out by ultrasonic inspection of the rotor forward shaft adjacent to the pressure side of the R0 blade (removed). Fig 2 shows the visible-dye liquid penetrant indication obtained by jacking up the bellmouth and removing the R0 blades. Fig 3 is a close-up detail of the crack toward the forward face of the rotor.
The crack inspectors found was approximately 1.5 in. long, propagating from a point about 0.5 in. from the forward face of the rotor. Crack depth, as measured from the rotor OD, was approximately 0.6 in.; but considering the taper of the dovetail, it probably is closer to having an actual depth into the rotor of about 0.35 in.
The OEM’s technical information letter reported cracking that propagated perpendicular to the pressure face allowing “blades-in” visual identification using dye penetrant. The finding by Advanced Turbine Support raises the risk of not performing this inspection as soon as practicable: The disc theoretically could fail before any direct visual indication.
Also important to note is that dovetail cracks are thought to be slow-growing. If they are found, and an immediate shut down of the unit is not required, periodic re-inspections probably will be recommended to monitor crack propagation. The suggested inspection intervals likely would be determined based on engineering and metallurgical evaluations.
Unconfirmed user scuttlebutt is that if replacement of the rotor forward shaft is required, it will put a serious hurt on the pocketbook. Schedule might be an additional problem because replacement forward shafts might not be readily available. Affected owner/operators should consider sharing knowledge through the 7F User Group’s forum. One third-party services provider was said to be working on a possible repair for dovetail cracks, something that apparently has not been attempted previously.