Rotor life and death - I

November 19 2013 - Drew Robb

The 7EA User Group conference this year featured a rotor lifecycle roundtable where vendors and consultants answered questions regarding a TIL by GE indicating that 200,000 hours or 5000 starts were the end-of-life limits for the 7EA.

Group leader Pat Myers moderated and speakers included Ashok Koul of Life Prediction Technologies, Mike Burton and Paul Tucker of First-TBS, Richard Curtis of ETA Technologies, Chad Garner of PSM, Greg Snyder of Dresser-Rand TTS, David Taylor of Masaood John Brown and Ted Papageorgiou of TurboCare. 

One user asked why an hours limit should exist if you do a thorough inspection on an aging machine that has reached the OEM-mandated limit yet you find nothing wrong. Panelist responses were many and varied. Koul said that each inspection technique has a detection limit and so they were each prone to missing miss certain types of damage or crack sizes. One must assume that a crack exists at the fracture critical locations in the rotor assembly and compute an inspection interval for future engine overhauls based on analysis guidelines provided in MIL-STD-1783B of USAF.

Most agreed that you didn’t necessarily have to shut down the machine once the TIL limits were passed. “I disagree that you have to replace rotor at OEM end of life,” said Tucker. “It is ultimately an end user choice, but if you find nothing, in our view, it is good to go. However, you should continue to inspect critical parts and follow up with eddy current inspections.”

Curtis added that for base-loaded machines, the life of the rim of the turbine wheels is an important factor in overall turbine life, since these areas see the highest temperatures. In addition to inspection, modeling (engineering analysis) should be used to uncover any undue risk before the next major inspection.

Papageorgiou said user decision on whether to go over the OEM limit or not depended on the appetite for risk the company has. The rotor is going to reach the useful life limit and fatigue, creep, or lead to a forced outage situation sooner or later, but the question is when. While the OEM is pushing for a complete rotor replacement, users can change out specific components and that might be as good as having a new rotor in most cases.

Another user wondered if onsite inspections could be done before sending out a rotor for repair. Curtis explained that there is only a limited amount of inspection you can perform on a stacked rotor, since critical features such as bolt holes or wheel/spacer bores cannot be accessed.  A customized engineering and risk analysis can still be performed by inputting ambient conditions, compressor discharge temperature and other operating conditions into a model to conduct a site-specific analysis. But in reality, de-stacking the rotor is required in order to really tell how the machine is doing at what the OEM defines as end of life.

Some vendors claimed that they could come into a plant, remove the rotor, ship it to their repair facility, conduct testing and have it reinstalled within 30 days. One vendor said that was an adventurous target due to how long destacking can sometimes take. The type of inspection can also be a factor. A basic rotor inspection can be done rapidly. However, a lifetime assessment and lifetime extension is more complex: you have to know the history of the rotor, how many trips, hot starts, cold starts, and so on. Such an inspection should be planned and prepared months in advance in order to minimize the risk of delays. 

And when it comes to lifetime extension, more advanced NDT inspection is called for in order to help the user come to a conclusion as to how to progress with the rotor. Damage found during these inspections might be addressed with something as simple as blending a flaw or as complex as replacing specific disks, spacers and bolting.

“These three types of rotor inspections get progressively more complex,” said Papageorgiou. “If you have several units, it is a good idea to obtain a spare to make the logistics simpler.”

A user from deep inside the Amazon basin in Brazil said it wasn’t feasible to ship his turbines to repair facilities. So he wondered about the value of second-hand turbine rotors as a spare.  The advice from Curtis was to look for spare rotors or components from a base loaded (hours based, low starts) machine if you are a peaking (starts-based) application, and vice versa, look for low hour parts from a peaking operation if you run base load.  

(Part 2 in next edition)