By Tom Reid, Vice President of Power Generation Services, ENTRUST Solutions Group
Gas turbine maintenance intervals are determined by hours, starts, or a combination of both. The latter is often referred to as equivalent operating hours (EOH). The increasing integration of renewable energy sources into generation portfolios has meant changes in dispatch, and many traditionally base-loaded assets are being forced to load following an on-off cycle, as seen in many gas turbines and combined-cycle arrangements across the country.
With such shifts in operational patterns comes a shift in the failure modes that manifest and the inspection techniques required to effectively diagnose these respective modes.
Given the competitive marketplace, understanding applicable failure modes and inspection techniques is valuable to effectively balance the fine line between prematurely scrapping parts and running hardware beyond safe conditions.
When a unit starts and stops, it is exposed to significant cyclic stresses in addition to large thermal transients in the high-temperature sections of the engine. This can lead to thermal, mechanical fatigue or low-cycle fatigue cracking. After cracks begin, they continue to propagate with each new cycle. If not addressed in time, the liberation of a rotating blade can lead to substantial forced outage time and repair costs.
For cycling units, sustaining damage at the interface or contact surfaces is common. Damage occurs from the repetitive relative movement between surfaces or due to increased rotor deflection through critical speeds. Some examples of these surfaces include rotor-to-blade root interfaces; tip contact faces on adjacent shrouded blades, and compressor or turbine blade tips.
In addition, cycling has been shown to increase coating spallation rates for coated parts, leading to premature oxidation of the hardware.
The impact of cycling is not limited to a single section of the gas turbine. ENTRUST has been involved in root cause failure analyses ultimately attributed to cycling in the compressor, combustion, and hot sections of gas turbines.
It is also important to understand base load failure modes. Base-loaded machines are mainly limited by failure modes that result from prolonged operation at elevated temperatures. These failure modes include creeping, coating/surface oxidation damage, and embrittlement.
Creep damage is very difficult to detect non-destructively. As a result, hot-section rotating blades often have conservative life guidelines. This design philosophy is understandable given the scatter in material properties, difficulty detecting creep, and the severity of a blade failure.
The risk for failure modes requiring extended time at high temperatures, such as creep, is lower in gas turbines exhibiting cycle-dominated maintenance intervals.
Understanding the applicable failure modes and how they manifest is key to effective inspections. This holds true for broad condition inspections, such as in-situ borescope inspections, as well as detailed inspections completed during major outages.
Before the overhaul, and as a routine maintenance practice, users should complete borescope inspections at recommended intervals. Although not a precise indicator of part condition, signs of major damage such as large crack indications, excessive wear or oxidation, foreign object damage, tip rubbing and missing coating should be explored. It is important to document the damage that occurs over time to track the progression of known conditions.
Users should conduct cycling-targeted, non-destructive testing (NDT) during major overhauls, particularly on rotating hardware. Depending on the material, multiple NDT techniques can identify cracks. These include liquid penetrants, magnetic particles, and eddy current inspections.
Users should complete pre- and post-repair inspections, especially if weld repair is required. In addition, they should inspect the integrity of the coating and determine if a new coating is required. Always review repair inspection reports for non-conformances to understand the condition of your hardware prior to reuse.
With inspection results in hand, it must then be determined if it is safe to continue to use hardware. Recalling the difference in failure modes between base-loaded and cycling machines and the fact that many parts are life-limited by time at temperature failure modes.
When coupled with the results of the cycling targeted failure mode inspections, knowledge of the accumulated operating hours of the hardware enables educated decisions concerning part reuse. Given the cost of replacement hardware, significant monetary benefits can be realized using this strategy.
Contact one of ENTRUST’s engineering experts to find out more about understanding applicable failure modes and inspection techniques for your gas turbines.
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Tom has spent the entirety of his 15-year career in the power generation industry.
In his current role as Vice President of Power Generation for ENTRUST, Tom oversees a team of approximately 100 engineers, whose expertise covers power plant equipment, modeling, and testing.
Prior to ENTRUST, Tom held turbine design and repair roles at General Electric. Tom is a graduate of GE’s Edison Engineering Development Program and holds 7 U.S. patents. He holds an BSME degree from Virginia Tech, an MSME degree from Georgia Tech, and is a registered professional engineer in the state of Delaware.
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