Short- vs long-term validation

Over the last few decades, gas turbine (GT) technology has evolved in pursuit of greater efficiency and increased flexibility. Such objectives impose higher operating temperatures, larger pressure ratios and faster ramp rates. Tougher operating conditions impose increased stresses on components, adversely affecting their durability. In addition, longer operating intervals between scheduled inspections raise the risk of component deterioration and failure as well as potential secondary damage. Detection of potential issues, therefore, is the key to enhancing the reliability of new or modified GTs prior to going commercial. This verification process can be divided into short- and long-term validation. Short-term validation allows characterization of the compressor, its surge and rotating stall behavior, as well as the evaluation of GT behavior during off-frequency operating conditions. It also allows discovery of unexpected issues that might arise during the first few hours of operation, such as vibration, rubbing and high temperatures. Positive results from short-term validation represent a green light for the initiation of a long-term validation program where the time and temperature effects on durability, reliability and performance can be evaluated.

Once the generator breaker is closed and the GT is dispatching power, the RPM of the GT-generator train is determined by the frequency of the grid. This is the condition the plant will run during its future life and is the appropriate venue for detecting and correcting long-term issues or failures prior to commercial deployment.

Component lifecycle

It is important to differentiate the various factors impacting the GT component lifecycle. Low cycle fatigue, erosion, corrosion and fretting are among time and start-stop dependent phenomena, while creep and aging of Thermal Barrier Coatings (TBCs) are among those phenomena that depend on time and temperature.

These effects are difficult to estimate based on a few hundred hours of operation. They require validation at least one or two orders of magnitude greater, and it is the primary motivation for some members of the insurance community to use 8,000 actual operating hours to classify the technology proven as compared to prototypical. Long-term effects are important for the reliable and durable operation of the equipment. They are commonly mentioned in Root Cause Analysis (RCA) reports on component failures and must therefore be detected and corrected prior to commercial introduction. On the other hand, off-the-grid, shorter term tests allow gas turbine-generator operation at rotating speeds different from synchronous speeds for off-frequency verification. However, the range of RPMs is typically restricted by blade frequency excitation concerns.

 

You can read the remainder of this article in the July/August issue of Turbomachinery International.