Combustion Turbine Services
SIB-96-173
Structural Integrity Associates (SI) provides innovative consulting services in several areas related to large combustion turbine engines used for electrical power generation. These services include:
Failure Root Cause Determination
A significant rate of forced outages has been experienced in the operation of large combustion turbines with simple cycle outputs above 100MW reflecting a number of design and operational problems. SI has assisted utilities in the root cause determination of several accidents in which the extent of damage has ranged from individual blade loss to catastrophic destruction of the entire unit. In addition to field and laboratory evaluation of failed components, the reconstruction of a failure involves a variety of analytical computations. As part of any evaluation, SI conducts metallurgical and mechanical property evaluation, thermal and stress analysis, dynamic analysis, and fracture mechanics analysis.
SI engineers have conducted a wide range of failure root cause analyses, including the verification of blade loads and stresses at the time of failure, the estimation of dynamic bearing loads and rotor clearances following blade loss, and the thermal stress and fracture mechanics analysis of hot gas path liners.
Turbine Blading Condition Assessment
 One of the most complex issues in the maintenance of combustion turbines is the remaining life of high temperature blades and vanes. These parts are usually protected by proprietary coating systems which can be breached by cracking or oxidation, allowing rapid attack of the airfoils, as shown below. The life of a coating is dominated by local surface temperature, since temperature governs the diffusion rate of elements into and out of the various coating layers. Cyclic thermal strain may crack the outer layers, hastening oxidation or structural failure of the airfoil.
As part of the condition assessment, SI conducts both destructive metallurgical and nondestructive eddy current evaluation of service-operated blade sets to estimate surface temperature and remaining life. The eddy current method allows on-the-rotor testing of the integrity of some common coating systems. The eddy current measurement simultaneously provides an estimate of the remaining aluminide concentration in the protective layer of the coating. In addition, condition assessment includes a correlation between eddy current measurements and microstructural analysis for blades from several different engines.
Life Prediction
Component cracking observations often require an independent analysis of the useful lifetime of a modified design, material substitution, or a repair or refurbishment process. SI routinely conducts lifetime evaluations based upon condition assessment and industry-accepted software programs. Analyses are performed for all of the known failure mechanisms of combustion turbine components: including corrosion fatigue, low-cycle fatigue, creep-fatigue interaction and special cases of high-cycle (vibratory) fatigue involving multiple periods of transient or steady-state high vibration amplitude, including prediction of excitation and response.
SI has performed several such evaluations in recent months. A new weld repair design was analyzed for a hot gas path casing that had exhibited recurrent cracking. SI calculate the transient temperature and thermal stress distributions from the gas and cooling air flows and predicted the rate of crack propagation from an assumed weld defect. Several iterations were performed until the crack growth rate was reduced to a practical minimum. In another study, the effect of a material change and attachment reconfiguration of a last stage turbine blade on high-cycle vibratory fatigue lifetime was estimated from fracture mechanics analysis. An investigation was undertaken to determine the cause of intergranular cracking in turbine disks in order to establish the appropriate creep and fatigue crack growth algorithm for remaining life prediction. This investigation included the time and cycle-dependent effects of surface residual stress.
Development of a Standardized Life Assessment Guideline with analytical methods for predicting remaining life for CT first stage nozzle vanes and blades is supported 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.
If you would like more information regarding SI's capabilities in failure root cause determination, turbine blading condition assessment, life prediction of combustion turbines, or need assistance in any other area of combustion turbines, please contact SI. |
