Rotor life and death - II

November 24 2013 - Drew Robb

This is the continuation of the article Rotor life and death - I

 

Several users questioned whether the TIL was a commercially driven document. The consensus from the panelists was that it might be 50% commercial, but they also noted that the OEM has a legal responsibility to warn users on lifecycle issues. Tucker said that these limits were far from new. “The OEM named these as the limits for the 7EA long before the TIL was published,” he said.

The OEM, of course, is using a worst case scenario with these design limits. So by no means will all users be suffering from heavy damage at that point. In fact, the latest TIL allows for a life extension of around 50,000 hours. But it is up to the OEM to make fleet-wide recommendations which by their nature will be conservative. After all, there have been some failures out there and warranty claims. So the OEM is putting out TILs as there is risk.

Tucker stated that there have not been any failures on the 7EA relative to the issues covered in the end-of-life TIL. That doesn’t mean there haven’t been damaged rotors, though. Koul said that second and third stage rotors in the assembly are most damage prone.

How should users work with insurance? Several panelists made it clear that it is possible to talk to the insurance companies. Far from only wanting to follow OEM guidelines rigidly by insisting a rotor be replaced when it reaches the end of life limits, many insurers advocate thorough inspections, analysis and modeling and taking mitigation measures to extend the unit’s life while evaluating risk appropriately.

If you have a large number of the same units that have the same operating profile and the same ambient conditions, is it possible to only inspect a few of them and make informed decisions across the fleet concerning their lifecycle? The panelists agreed that you have to inspect them all. Factors such as material variability and OEM quality control practices mean that there will be variability among components and machines.

A murmur of approval could be heard when one user asked if the panelists had ever recommended going beyond the OEM specified end of life. Koul said yes, but that you had to back that up with MIL-STD-1783B-based analysis and a strict inspection regiment that specifies what parts to look at and when to use what kind of inspection technique. A fleet of non-7EA turbines, for example, had been repeatedly inspected after the 100,000 hours OEM design life limits were exceeded and were still running at 300,000 hours. Cracked discs had been found at overhaul and replaced during that time.

Curtis emphasized that end of life decisions are done at the user’s risk. Inspection and repair firms can provide plenty of data so that decisions are more informed. “The increase in risk in going beyond the TIL limits does not warrant buying a new rotor based on our numbers,” said Curtis. “But the risk is your own.”

Finally, a user asked that if you destack and inspect at 200,000 hours and decide to continue running the machine, what do you do once you get to 50,000 more hours? Koul said you have to destack again if you want to know for sure. Otherwise, there is high risk of missing a sizeable crack.

(To read the rest of this article, read the Turbomachinery International Handbook due out in December)