Improving Inspection of Turbine Blades
By Danny Shacham
Chief Technical Officer
Nextec Laser Metrology
Quality is a given today—no company can afford to find defects at the end of its line where parts end up as scrap or having to be reprocessed. However, operating successfully in a global economy requires delivering quality while controlling costs and boosting productivity.
This can be difficult when it comes to the quality control of complex free-form compressor and turbine blades, where manufacturers have been challenged to find inspection methods that can meet production rates. Blades are designed to generate the maximum power and efficiency at the minimum weight and cost. Any variation in structure or shape from the original design can significantly impact the efficiency of the turbine or compressor operation.
Although blades in energy applications are typically not held to the same high standards as in the aerospace industry, confirmation that components meet specification is still needed. One-hundred-percent inspection is most likely not required, but the larger the sampling size, the more reliability there is with process stability.
To check a decent sampling of blades requires that an inspection system deliver both speed and precision. With free-form blade shapes and the special form geometry of the leading and trailing edges, this cannot be accomplished with traditional mechanical probe solutions, which must remain in contact with the blade surface and are limited in the amount of useful data they can collect. The dynamic range of a mechanical probe is basically zero, making the scanning pattern, alignment routine and programming of the scan path more complicated and time consuming. Blade alignment is particularly challenging and requires a trial and error approach unless high-precision and very expensive mechanical jigs are employed.
To overcome these deficiencies, the introduction of 3-D laser technology has significantly improved the inspection of blades. This solution works through combination of a 3-D laser-based scanning sensor, advanced laser and vision technologies, state-of-the-art motion controller and special software for blade alignment, blade measurement, blade analysis and reporting. The laser spot size enables measurement of very fine geometry details—especially leading-edge and trailing-edge profiles. The laser’s large dynamic range and high accuracy makes it easy to set up measurement paths and collect hundreds of points quickly and accurately.
Any variation in structure or shape from the original design can
impact the efficiency of the turbine or compressor operation.
A 3-D laser scanning system quickly compares actual blade dimensions and airfoil cross-section profiles to the original CAD model, revealing relevant deviations within the manufacturing process. By monitoring the forging, casting and machining process and measuring the parts, it can swiftly be discovered if there is deviation. In a machining process, closed-loop control can be accomplished using the SPC results of the measurements so that if deviations are growing due to cutting tool deterioration, the tool offset can be adjusted accordingly. This helps to maintain both process and component integrity.
Within the context of power plant efficiency, the ability to quickly access blade geometry can have significant impact on the performance, and repair cost in a maintenance environment. Blades are one of the most highly-stressed components in a turbine or compressor. Their weight and height make them subject to tremendous centrifugal forces. Additionally, the harsh environmental conditions of temperature and pressure after long service hours can result in corrosion, erosion, distortion and loss of the original component dimensions.
MRO blades tend to deviate significantly from the original CAD model, making touch probe scanning problematic as it is based upon CAD model free-form perpendicularity. Deterioration usually affects the blades in the following ways: blade edge, affecting chord length; blade thickness variation; and blade twist—the variation in twist from root to tip. Similar to the production floor, the MRO blade industry is continuously seeking higher throughput and yield, high-accuracy inspection, higher blade measurement speed and minimized requirements from the operator.
Blade inspection has been a difficult, expensive, slow venture. Traditional measurement has relied upon expensive precision fixtures, tailored jigs and templates, or mechanical touch trigger and scanning probes. New automated 3-D laser technologies can both improve inspection rates—measuring at five times the rate of traditional technologies—and allow a higher sampling rate without impacting manufacturing efficiency.
This article first appeared in Manufacturing Engineering's 2012 Energy Yearbook. For past yearbooks from ME Media, click here.