Gas turbines have experienced an explosion in growth with the conversion from coal-based power production to natural gas-based power production. Borescope inspections of gas turbines provides a non-destructive method to inspect turbines. This means less downtime for inspections and for repairs and maintenance to address problems identified in a borescope inspection report. Here are six failure modes that may be uncovered during a borescope inspection:
Fouling can have a substantial effect on the operation and efficiency of a gas turbine. Fouling occurs when contaminants accumulate on compressor blades and turbine blades. For example, contaminants, such as oils, water, salt particles, dirt, and dust from dirty gas or fuel delivery pipes can attach to compressor blades and turbine blades. These particles attract other particles to accumulate on surfaces. The weight added to the compressor blades and turbine blades can decrease the turbine’s power output by up to 15%.
Moreover, fouling can be the root cause of other failure modes. For example, fouling can cause blades to become imbalanced, causing vibration or abrasion (both of which are discussed below). Salts and water droplets that accumulate on blades can contribute to corrosion of the blades (also discussed below). Fouling is often noted in borescope inspection reports because visibly dirty blades are so easily identified.
Erosion and Collision
Larger debris passing into the gas turbine can cause severe damage. This debris may consist of large particles that were able to pass through screens and filters, or even pieces broken off of screens, filters, or other blades.
This debris can cause erosion of blades when the debris physically removes material from the blade. Erosion may appear as scraping or scratching in borescope inspection reports.
Debris can also collide with blades causing denting or even cracking. For blades operating at high speed and high temperatures, it is not unreasonable to imagine a small pebble or piece of broken screen causing severe damage when it collides with a compressor blade or turbine blade.
While abrasion seems similar to erosion, abrasion refers to a specific form of degradation. Erosion is removal of material by suspended particles. Abrasion refers to removal of material by a stationary surface. Thus, erosion occurs when debris scrapes material from a turbine blade or compressor blade while abrasion occurs when a fixed support member or casing scrapes material from a turbine blade or compressor blade.
Abrasion usually occurs when wear, fouling, vibration, fatigue, corrosion, or creep deforms blades out of tolerance allowing them to scrape other parts of the gas turbine or compressor. Abrasion appears as scratches or scrapes on the surface of blades in a borescope inspection report.
Galvanic corrosion and hot corrosion are major sources of turbine and compressor failure. Impurities in fuel can interact with all surfaces of the gas turbine and compressor. These impurities will chemically react with the metal surfaces to produce corrosion products. Corrosion may be visible during borescope inspections as either scaling (buildup of corrosion products) or degradation (loss of material) of surfaces. Pitting corrosion, a form of highly localized galvanic corrosion, may appear in borescope inspections as holes or pits in surfaces.
Creep is the product of thermal stress combined with physical stress on materials. These stresses are less than the magnitude needed to cause catastrophic failure. Rather, these stresses cause the material to deform. As noted above, turbine blades and compressor blades suffering from creep may deform out of tolerance and suffer abrasion due to interaction with other parts. In a borescope inspection report, creep may be identified if parts are visibly deformed or show signs of abrasion.
Fatigue is caused by application of cyclical stress below the stress necessary for catastrophic failure. Vibration of compressor blades and turbine blades, for example, can cause fatigue in the blades. This vibration may result from imbalance in the blades caused by fouling, corrosion, creep, abrasion, erosion, or collision. Fatigue causes micro-cracks, which may accelerate corrosion as impurities seep into the material and lead to catastrophic crack growth. Unfortunately, micro-cracks are difficult to spot with a visual inspection, although the signs of crack growth and corrosion would be visible in a borescope inspection.
Whether it takes the form of fouling, erosion, collision, abrasion, corrosion, creep, or fatigue, borescope inspections can often spot signs of degradation before failure.