Engineering Services for Large Steam Turbines
SIB-96-144

Structural Integrity Associates (SI) offers consulting, field evaluation and engineering analysis services for nuclear and fossil steam turbine generators of any manufacture. These services include the following major items taken from SI's recent project list.

Consulting
Failure root cause analysis -- SI engineers have investigated catastrophic failures of rotors, shrunk-on disks, retaining rings and generator windings to identify the initiating events and the underlying design, manufacturing or operational factors. Many incidents of damage or fracture of components are routinely encountered, such as blade corrosion fatigue failures, blade attachment stress corrosion cracking, blade and shaft seal erosion, casing and valve body thermal fatigue, fretting fatigue of blade roots and generator teeth, bore low-cycle fatigue cracking, and stator vane vibratory fatigue. SI conducts metallurgical and mechanical property evaluation, thermal and stress analysis, dynamic analysis and fracture mechanics analysis.

Repair and refurbishment assessment -- SI is advising utilities on the most cost effective procedures for repair of creep or corrosion damaged rotors, usually involving weld repair. Projects include transferring technology of weld repair to a large utility and overseeing the replacement of stress-corrosion cracked rotor rims with weldments of martenistic stainless steel for another. Erosion and corrosion resistant coatings for gland seals and high pressure inlet stages have been evaluated. Analysis of the effects of grind-out of cracks and of the benefits of shot peening on stress-corrosion cracking has been carried out, as has the decision whether to excavate thermal fatigue cracks in casings.

Mechanical and aerothermal performance upgrade assessment -- Opportunities for improvement in efficiency as well as extending the intervals between maintenance outages are evaluated by SI and a team of highly experienced consultants and subcontractors. Services include planning and oversight of outage activities, independent verification of the manufacturer's design analysis and witnessing performance testing, as well as assistance with the selection and installation of monitoring instrumentation.

Field Services
Condition assessment -- SI has developed its own equipment for special applications to nondestructive evaluation of turbine generator components. A combination of novel ultrasonic transducers and advanced signal processing using proprietary software and instrumentation has proved to be effective in detecting and sizing defects in generator retaining rings, disk bores and rims and thick sections of valves, casings, shafts and seals. Rapid crack detection is provided by multiple parameter time of flight diffraction, and high resolution sizing and characterization is obtained with SI's dynamically Focused Array Transducer System (FATS), with waveform analysis carried out with the TestPro system. Time of flight diffraction has been widely accepted for retaining ring inspection. FATS has been applied to tangential-entry blade attachments and to bores and keyways of shrunk-on low pressure disks. Other special capabilities include an automated ultrasonic system for sizing tenon cracking and coating thickness and integrity evaluation by multifrequency eddy current imaging.

Dynamic testing and monitoring -- Instrumentation is provided for measuring torsional rotor system vibrations, blade mode shapes and frequencies, and coupled nodal responses of stator vane and bladed disk assemblies. SI installs strain gage and piezoelectric vibration monitors; other team members install optical and anemometric monitoring instrumentation.

Corrosion and steam chemistry monitoring -- SI has access to full capabilities for analysis of water chemistry and electrochemical evaluation of coatings and dissimilar alloys.

Engineering Analysis
Remaining life prediction -- SI routinely conducts lifetime evaluations based upon condition assessment and industry accepted software programs. Analyses are performed for all the known mechanisms of failure of turbine generator components, including corrosion fatigue, stress-corrosion cracking, fretting 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. Life prediction is available on a near real-time basis through on-line monitoring of operating parameters using EPRI's FatiguePro or Creep-FatiguePro software systems developed by SI.

Dynamic excitation and response -- Finite element calculations are conducted for rotor, retaining ring, blading and stator vane mode shapes, frequencies, and amplitudes.

For additonal information on engineering ser-vices for large steam turbines, please contact SI.