Blade crack and blade damage detection is an important component of monitoring the health of turbomachinery.
Cracked blades eventually lead to blade failures, with shrapnel running through the turbomachine and damaging other blades in downstream stages. Results can be catastrophic and lead to extensive repair.
Cracks in blades have two effects:
- Blade deflection: a cracked blade will lean more than other blades around it, so its time of arrival is later than other blades. As the crack propagates, the blade will arrive later and later.
- Blade frequency: a cracked blade will also have a different resonance frequency than a healthy blade.
Blade damage, such as a distorted or broken blade, lowers the efficiency of the turbomachine. It is important to detect and to repair damaged blades in order to restore the original performance of the turbomachine.
Damaged blades can behave similarly to cracked blades. A distorted blade can deflect more or less than undamaged blades around it. It can also exhibit a different resonance frequency. Or the blade can be partly or completely missing, failing to generate any returned signal to the optical probe. A damaged blade can also twist differently under aerodynamic loads than undamaged blades around it.
Using FOCISTM to detect cracks and damage in blades
The behaviors above can be captured with the blade tip timing module of our FOCISTM optical system to detect cracks and damages in blades.
A cracked blade will show up as an anomaly in the blade stack plot compared to other blades, it will typically arrive later than other blades. A cracked blade can also be detected by looking at historical data of the same rotor, because it will have a different lean over time as the blade propagates.
FOCISTM can also detect a cracked blade by looking at resonance frequencies. This can be done either by finding discrepancies between the measured blade behavior and the model prediction, or by looking at shifts of resonance frequency over time.
Image Above: Blade Resonance Frequency Measurement With FOCIS
Similar to cracked blades, damaged blades will appear as anomalies in the stack plot or with different resonance frequencies.
FOCISTM software also has the ability to detect and flag missing blades.