Root Cause Failure Investigation
SIB-96-156

In recent years, Structural Integrity Associates (SI) has conducted a number of root cause failure investigations of electric generating and transmission equipment. These investigations have benefitted from SI's many years of experience and expertise in the relevant disciplines of stress analysis, fracture mechanics, metallurgical failure analysis, load monitoring and testing, fatigue and corrosionrelated damage mechanisms, and nondestructive examination (NDE). SI is also experienced in utility equipment and operating environments. SI can assemble teams of highly specialized experts in a wide assortment of disciplines to identify the underlying cause of material degradation or structural failure problem and, more often than not, devise creative solutions to correct or mitigate the problem.

The following case studies highlight some of SI's recent failure investigations, and the benefits that customers have derived from them:

Collapse of High Voltage Transmission Towers - In 1995, a utility client suffered the collapse of approximately thirty high-voltage transmission towers during a wind storm. SI provided an independent assessment of the cause of the failures for this utility. The SI evaluation team included an experienced metallurgist, who surveyed the field damage and collected samples for further investigation, and civil engineers, who developed a model of the towers to predict collapse loads under various combinations of wind and conductor loads. The model predicted that the wind speeds experienced in the storm were sufficient to cause the towers to collapse, and identified weak links in the system which initiated the failures. The model also demonstrated that the towers were adequate for their design wind velocity, but that this velocity was exceeded in the storm. Recommendations were provided to enhance the towers' capacity for future wind storms.

Vibration - Induced Failure of Generator Retaining Ring - In 1993, SI assisted another utility client in the investigation of a generator retaining ring failure, caused by high-cycle torsional fretting fatigue. SI assisted with field testing and analyses, which isolated the cause of the vibration as electrical disturbances from electric-arc furnaces at a nearby steel mill. SI's evaluation helped avoid an unnecessary, premature shutdown of a second unit at the plant, confirmed the adequacy of the new ring design, and identified the source of the electrical disturbances.

To quote the plant manager . . . "Your expertise through all phases of the project helped us successfully manage the situation with both our generators, and led to an understanding of the failure that turned out to be truly unique in the industry. Your efforts saved us from spending unnecessary funds chasing recommended solutions that had little technical merit."

Thermal Fatigue Failure of a Nuclear Plant RCS Drain Line - During a recent refueling outage, a leak developing in the reactor coolant system (RCS) drain line of a pressurized water reactor (PWR). SI helped diagnose the problem: high-cycle thermal fatigue associated with unstable turbulent penetration of hot water from the RCS loop into the drain line. A through-wall crack formed which was caused by intermittent surges of hot water from the RCS loop creating heating and cooling cycles in the uninsulated drain line. On the basis of SI's evaluation, the repair was limited to replacing the segment of piping containing the crack. Similar lines in other loops were deemed acceptable for continued service. To eliminate the problem, the entire drain line (as well as the two similar lines) were insulated from the nozzle to the first isolation valve. Data from thermocouples installed during the restart supported the above failure hypothesis and the effectiveness of insulating the lines as a corrective measure.

Steam and Combustion Turbine Catastrophic Rotor Failures - SI has conducted root cause failure investigations of two steam turbines in cogeneration installations. One involved catastrophic overspeed of a low pressure rotor resulting from the disintegration of a steam strainer and wedging of sections of the screen in both the control and emergency stop valves, preventing their full closure. Another investigation was conducted into the cause of multiple fretting fatigue cracking of control stage blades in a high-pressure unit. Both assembly tolerances and low power operation were contributing factors.

SI also investigated the cause of fatigue cracking and multiple last stage blade separation for two large industrial gas turbines. Contributing factors in both instances were high stress concentrations and high levels of stochastic excitation from the exhaust gas. Consequential blade loss from impact and the integrity of the bearings were investigated.

Fatigue Cracking in Wind Turbine Transmission Housings - For an independent power producer, SI investigated the cause of a severe cracking problem in a large number of cast ductile iron transmission housings of power generating wind turbines. SI took the lead role in an exhaustive failure investigation, which included finite element stress and fracture mechanics analyses, fatigue crack growth testing, comparative fractography of both field fractures and fatigue test samples, and instrumentation of turbines in the field with strain gauges. The work resulted in a concise understanding of the applied loads and causes of the cracking, and a clear definition of allowable load limits and corrective measures to ensure trouble-free future operation.

Boiler Main Steam Line Rupture - When a fossil plant main steam line erupted in 1996, SI mobilized a fast-response team of highly qualified personnel to perform post-failure NDE of all similar locations in the plant. SI's team worked long hours, using state-of-the-art ultrasonic testing techniques (Time of Flight Diffraction - TOFD and the Focused Array Transducer System - FATS) to provide this client with timely input on other systems requiring replacement concurrent with repair of the failure.

Failure of Generator H2 Cooling Fan - When catastrophic failure of a hydrogen ventilation fan occurred in an overseas generator, SI performed an independent review and critique of the vendor's failure analysis. SI engineers visited the plant site, met with cognizant metallurgists and vibration testing personnel, and performed an independent fracture mechanics analysis. SI's review concluded that the failure was caused by high-cycle fatigue due to non-synchronous excitation of fan natural frequencies, and was not associated with fan manufacturing defects (as originally suspected). SI's fracture mechanics analysis also demonstrated that it was safe to operate the plant's sister unit for a short period of time, for testing to isolate the source of the excitation.

When cracking or structural failures occur in electrical generating equipment, few companies are as experienced, dedicated, and responsive as SI in identifying the root cause of such problems and identifying practical, cost-effective, and often-times creative, solutions. If you would like more information regarding SI's capabilities in root cause failure analysis, please contact SI.