One key characteristic of turbine blade quality involves the internal cooling passages formed in each blade so that air can pass through. Because the hundreds of blades used in each engine are subjected to high operating temperatures, a defect in any of the cooling features can cause overheating of the blade, ultimately resulting in blade failure and an unplanned engine shutdown. Failure of any sort requires tear down and repair, accompanied by out-of-service delays and costs.
Alstom's turbine blades are produced by casting the blade around a ceramic core. This core is etched away with acid leaving the blade hollow. This industry-wide process presents potential problems:
- The ceramic core can easily crack
- The core may leave debris that could block the cooling passage
- A slightly misplaced core could leave uneven wall thickness around the blade.
Alstom wanted to improve the quality of its turbine blades and increase the level of defect detection in its blades during manufacturing. The company had been using various types of nondestructive testing, including infrared pyrometry. But certain types of anomalies-such as casting flash and malformed cooling features-were not being detected until after the blade had been installed into an engine. With each blade costing hundreds of dollars, Alstom realized it would be more effective to dynamically measure and inspect its blades' external and internal properties in greater detail before they reached the engine.
To develop a solution, Alstom worked with the Bales Scientific subsidiary of Computerized Thermal Imaging Inc. (Lake Oswego, OR). The result was the development of a custom NDT system based on thermal imaging, which the vendor shipped to Alstom late last year. Known as the Automated Thermal Nondestructive Testing System, the system is designed for installation in a factory environment. It fits into a 6 feet wide by 6 feet high by 3 feet deep space, requiring only nominal power and compressed air connections to operate.
This is hot To operate the system, a worker places the component to be inspected into a clamping fixture that holds it in the correct position. The operator initiates the inspection, and the system subjects the blade to a series of thermal stresses, while capturing the blade's thermal response via a high-performance infrared imager. The captured images are processed and analyzed by the system's automatic defect recognition software. The system then alerts the operator if the blade passed or failed.
Automated Thermal NDT provides various advantages for Alstom. In particular, the use of thermal stress with the blade's response monitored by an infrared imager provides a dynamic view of the properties of the blade as it would be stressed in the operating environment inside the turbine. More traditional NDT methods provide only a static view of the blade's properties. Because the prime characteristic of the blade's operation in the turbine is how well it performs thermally, having an inspection method that shows this response directly is valuable.
Alstom has been pleased with the system. "The entire inspection procedure cycle time takes us a mere 60 seconds per blade," says Anthony W. Davis, principal design engineer at Alstom. "We have already inspected thousands of blades using the Automated Thermal NDT System. This has saved hundreds of hours in inspection time and enabled us to build engines with blades we know are good, hence avoiding the cost of replacing defective blades and a repeat engine test."
Computerized Thermal Imaging Inc.