Inspecting turbine blades’ internal structure

Aircraft turbines produce high output by highly compressing air in the combustion chamber. Within the engine, the high-pressure turbine sits behind the combustion tube, subjecting it to great stress from exposure to the pressure generated by combustion energy and temperatures higher than 1,832°F (1,000°C).

Hollow cooling structures in high- pressure turbine blades and nozzle guide-vanes are manufactured using the lost wax process, with molten metal poured into a mold made by a wax model of the component. A heat-proof core in the model enables manufacturers to create parts that have hollow structures. However, a foreign object or processing defect in the hollow structure can reduce heat dissipation, potentially resulting in engine failure. Manufacturers inspect the inner part of the turbine blade’s hollow structure during manufacturing and engine maintenance to prevent such flaws.

Inspection challenges

Mini borescopes with a diameter of 3mm or less were used to inspect the hollow structures inside turbine blades for residues, scratches, and cracks. A rod lens in the mini borescope provided good-quality images but suffered from drawbacks that slowed inspection, including:

• Rigid, easily damaged, long, thin lens on insertion tube
• Small images difficult to see without a monitor
• A camera unbalances the scope, making it difficult to use
• Non-articulated insertion tube; can’t look around inside hollow structures
• Insufficient illumination from weak light guide

Thin videoscope advantages

Unlike a mini borescope, thin fiberscopes and videoscopes are flexible and offer articulation. A fiberscope is made of a bundle of glass fibers, while videoscopes have a built-in image sensor. Both have a 2.4mm OD insertion tube and articulation at the distal end.

Fiberscopes are cost-effective, although images acquired through the glass-fiber bundle have a mesh-like appearance that can make it difficult for users to detect tiny defects.

Videoscopes acquire images via a sensor providing bright, clear images with no mesh-like appearance. By electrically amplifying image signals, videoscopes enable users to observe deep inside a hollow structure with a wide field of view (FOV) and sufficient brightness, even with limited light intensity. This makes it easier for users to detect residues, scratches, and other defects.

Durability

The Olympus IPLEX TX videoscope’s insertion tube has a protective resin layer and 2.4mm OD. The insertion tube’s multilayered structure prevents resin surface damage from irregularly shaped parts, improving durability.

An ordinary videoscope needs to be entirely replaced when its insertion tube gets damaged, but a detachable insertion tube allows quick replacement on the IPLEX videoscope.

Summary

Inspecting hollow structures inside turbine blades presents significant challenges that can be overcome by using a videoscope with a small-diameter insertion tube. With durability, high image quality, and articulation, inspectors can easily maneuver inside these narrow structures, viewing, recording, and archiving images.

Olympus Industrial Solutions