Combustion Turbine Services - Related SI Project Experience
SIB-97-181
NDE Methods Development
Under the sponsorship of a group of six utilities, an eddy current method and procedure were developed for field evaluation of the remaining life of coated GE turbine blades, specifically GE's GT-29PLUS duplex coating on columnar cast GTD-111 buckets for the Frame 7FA and Frame 6B first turbine stage. The method consisted of measuring the slope of the variation of the magnitude of impedance with frequency above and below a balance point and correlating the slopes with the thicknesses of specific layers of the coating. The outermost layer consists of a protective, aluminum rich phase, while the innermost layer is formed by the interdiffusion between the plasma sprayed GT-29 coating and the substrate alloy. The width of the interdiffusion layer provides the equivalent time-temperature exposure of the surface, while the width of the protective outer layer indicates the degradation of the coating. This project is expected to be continued into a field application and verification phase.
Development of nondestructive condition assessment methods for CT blading is supported in part by the Combustion Turbine & Combined Cycle Users' Organization (CTC2), managed by J. A. Jones Applied Research Company, located in Charlotte, NC. This group sponsors a wide range of development and engineering application activities for the operators of industrial combustion turbines. The short-term projects are aimed at reducing O&M costs and improving reliability. For further information on the CTC2 please visit their web site at http://www.ctc2.org.
An ultrasonic method was applied to the detection of internal cooling cavity surface intergranular oxidation and cracking. A highly focused shear beam directed from the airfoil surface interacts with the grain boundaries intersecting a cooling hole or serpentine cooling passage. By examining the shape of the ultrasonic signal it is possible to differentiate cracked from uncracked surfaces. The signal to noise ratio is inadequate in some cases because of the scattering of the beam from intervening grain boundaries, and signal averaging must be performed to reduce the "clutter" from coarse grained areas. This work has been funded by Dow Chemical Company and a foreign utility, that has at present extended the development of the clutter reduction algorithm for inspection of advanced turbine blades. It is expected that the ultrasonic method will be applicable to columnar cast nickel base superalloys.
Failure Root Cause Investigations
SI has investigated the causes of damage in ABB, Siemens, General Electric and Westinghouse combustion turbines. Four of these incidents are currently in litigation, and the details are confidential. Four involve fatigue failure of turbine blades with consequential damage, which in one case was catastrophic. Three failures reflect design deficiencies, and one involved cold straightening of a GE Frame 7F second stage bucket. Investigations have included metallurgical and structural analysis, both dynamic and transient stresses, and the fracture mechanics analysis of crack growth and criticality. One case required the transient dynamic analysis of the rotor assembly and its supports under impact loading.
Several investigations of component damage have been performed at plant sites, including cracking of last stage blades and bearing failures in ABB GT11N2 engines at East Kentucky Power Cooperative and Kentucky Utilities, hot gas casing distortion and cracking in Siemens V84.2 and V64.3 engines (three sites), and turbine vane and bucket cracking in a GE Frame 7FA engine at Florida Power & Light.
Design Review and Life Prediction Analysis
SI is collaborating with BWD Turbines, Ltd., in reverse engineering of hot section turbine blades with optimized coatings and casting processes for specific operating cycles. The cost effectiveness of repair procedures is incorporated in the optimization. Assessments have been made of the mechanisms of degradation and failure and the relative service lives of competing material systems in thermal / mechanical fatigue, oxidation and corrosion. First stage blades of the Siemens V84.2 and GE Frame 7F engines are being evaluated.
One of the most effective means of extending the life of advanced turbine airfoils is the application of thermal barrier coatings. However, the ceramic component of these coatings is subject to spalling after repeated cycles of exposureto high temperature. SI is working with Optimal Corporation to develop a software code for predicting the lifetime of the coating under a research grant from the National Institute of Standards and Technology.
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