Steam turbines (see here for more) are continuously in operation under demanding conditions - high temperature, high pressure, and for extensive periods, often weeks or months. Small imperfections can turn into major issues that result in forced outages and costly repair costs. Non-destructive testing (NDT) allows the plant team to inspect for defects, plan maintenance, and keep some megawatts available for the electrical grid.
What Is Non-Destructive Testing?
As the name suggests, NDT is a group of inspection techniques that evaluate material integrity and performance without cutting, scraping, or disassembling parts beyond normal access. In the turbine environment, technicians incorporate the use of physical principles - sound, magnetism, capillary action, and light - to expose defects otherwise hidden to the human eye.
NDT techniques such as ultrasonic testing (UT), eddy current testing, and magnetic particle testing provide measurable data, which over time can be trended. NDT used in combination with borescopes inspection during outages provides the plant with a clear understanding of rotor and casing component health while components remain in the operating condition.
Why Steam Turbine Parts Are Vulnerable
Turbines operate under high temperature, high pressure, and cyclic stress. Moisture carryover and/or chemistry upsets lead to faster corrosion and pitting. Blade rows are subjected to vibration and aerodynamic loading that can induce stress at the root, shroud, or leading edge, thus resulting in fatigue. Rotors undergo thermal transients during starts and trips, leading to low-cycle fatigue failure.
Over many cycles, these loads can transform small, hidden manufacturing marks into potential failures. This is the reason structured inspection cycles (where feasible, including phased array ultrasonic testing in the inspection cycle) is a foundational aspect of all reliability programs.
Common Flaws NDT Helps Detect
Tiger defects don't "happen" on their own. They tend to grow quietly between outages. Before we move to outlining the most common aspects of having a problem with the turbine, it is important to note that all NDT methods are specific to defect and are limited to defect type and location, and as such all-NDT methods serve distinct purposes.
Surface methodology will find open to surface flaws while volumetric methodology will evaluate the metal surface and provide information take several layers below the surface creating multi-layered protections against failure.
- Fatigue cracks at blade roots, tenons and dovetails
- Stress corrosion cracking found in disks, keys and shrink fit applications
- Erosion and pitting can be found on stationary nozzles, and rotating airfoils
- Creep damage and/or micro-voids in the high temperature rotating stages.
- Fretting wear can be found at the shroud ties, and lashing points
Surface-breaking defects are often revealed by dye penetrant inspections, while subsurface cracks and inclusions are better captured with UT or phased array techniques. Eddy current testing excels on conductive alloys and complex geometries, especially in blade attachments and seal areas.
How Early Detection Prevents Downtime
Finding a formative crack at the blade root during planned outage is always a much better state of occurring than finding the crack from a rub, trip or liberation event. Early detection gives engineering a chance to first assess not only the size of the flaw, but also how fast it will grow, in context of service stress intensity- early detection allows reasonable engineering decision making. In many situations, the monitored defects can be mitigated by prescribed blend repairs, relieving stress to maintain the service condition vs full rotor event repairs, again, based on early detection.
Additionally, NDT findings will also provide for life assessment to be discussed as part of risk-based inspection plan discussions around if a unit can run to the next planned outage or an interim borescope inspection. Even the more reliable indications, assist for plants to mitigate, if not skirt, the secondary damages and repair costs associated with forced outage.
The Long-Term Benefits of Inspections
NDT is not a check box; it is a reliability strategy, compounding returns; within inspections cycles- the trend information identifies what of the stages and components have the most risk and what to direct precision maintainable to a reliability plan while trying to manage crises.
- It will increase unit reliability and availability by directing our repairs on what we can sustain
- It will reduce life-cycle costs by preventing secondary damage and part liberation.
- It will improve safety performance from avoiding unplanned interventions and hot work.
- It would provide more accurate modeling for documentation for insurance, regulators and OEM warranty recoveries
- It will extend component life cycles through optimally run/inspect/replace cycles.
NDT, once it is paired with condition-based maintenance and the third leg of diligent outage planning will help provide forward plant alignment between operations, engineering and finance. Plants that "institutionalize" ultrasonic testing (UT - https://www.youtube.com/watch?v=Xm43oDOHcFQ), magnetic pile testing, eddy current testing, phased array ultrasonic testing, and borescopes will build defensibility around operational reliability and generate online when it matters the most.
