By Tom Reid, Vice President of Power Generation Services, ENTRUST Solutions Group

The potential failure of generator rotor fan vanes and blower blades has become a critical focus within the electric power generation industry. When a fan component liberates, it can cause catastrophic damage to both rotor and stator components, sometimes leading to widespread operational and financial consequences.

Industry awareness of this significant risk has improved in recent years, shifting inspection practices from simple visual checks to comprehensive non-destructive examination (NDE). Visual inspections alone are insufficient, as they cannot reliably detect small cracks. Generator rotor fans and blowers are highly stressed components, requiring detailed scrutiny of design, material quality, fabrication methods, and NDE.

Failure Mechanisms

Generator rotor fans and blowers are exposed to high steady stresses during operation as well as fatigue stresses that amplify over time. Blade stress levels vary depending on the design and location of the fan or blower.

• Axial Blower Blades: The highest stress points are typically found at the base of the blade or in the blade root attachment to the blower hub.
• Radial Flow Fans: The critical stress areas are usually located at the blade’s attachment to the side shroud or fan ring.

These areas demand targeted NDE during every major outage. A study conducted by Electric Power Research Institute (EPRI) examined over 25 case studies of axial and radial fan failures within fossil and nuclear power plants. 

These case studies highlighted considerable differences in fan geometry across manufacturers.

Design Considerations:

• Fan designs must prioritize optimized geometry for attachments and fasteners to minimize stress concentrations.
• Larger fans often require increased dovetail sizes to distribute centrifugal loads efficiently.
• Small design changes, such as refining root contours, can vastly improve component reliability and lifespan.

Additional Failure Points:

• Cracks can develop at the fan wheel inner bore, where it is typically shrunk onto the generator rotor. Like retaining rings, these interfacing surfaces are subject to significant stresses and require periodic inspections.

Common Failure Mechanisms Include:

• Low Cycle Fatigue (LCF)
• High Cycle Fatigue (HCF)
• Brittle Fracture
• Corrosion
• Erosion

Installation Best Practices

Proper installation practices are vital to reduce the risk of failure in generator fan assemblies. For axial blower designs, bolting often secures individual blades to the hub. These bolts must be tightened to precise torque specifications and secured using locking mechanisms or staking procedures. 

Failures due to loosened hardware have been documented, but can be prevented by following these best practices:

• Locking Devices: Do not reuse locking hardware, including lock washers, tab washers, or locking strips.
• Torque Specifications: Verify with the OEM before an outage that the correct torque values and techniques are applied.

For axial blower blades with threaded or smooth radial attachments, blade angles are often adjustable. Incorrect blade angles can adversely affect airflow, potentially causing blade flutter and HCF failures. 

To avoid these issues during assembly, adhere to the following:

• Use the correct angle measurement method and verify tolerances.
• Ensure blade base angle settings are within the manufacturer’s specifications.
• Properly torque and stake set screws used for locking angular positions.

For both axial and radial designs, blower hubs are secured to generator rotors through bolting or a shrink-fit method. 

During assembly, confirm the following:

• Measure rotor forging and hub diameters to check that the interference fit is within tolerance.
• Ensure proper heating methods and metal temperature targets are used during assembly.

Case Study

A generator fan design with welded blades and an inner hub was analyzed following a major failure. The hub featured blades welded to it, with the assembly shrunk onto the generator rotor shaft end. Over time, one of the blades liberated from the hub, causing severe damage to the generator (Figure 1). 

Failure Analysis

The highest stress in this design corresponded to the weld attachment areas between the blades and the hub. Subsequent NDE identified additional indications (cracks) at the weld-to-hub interface (Figure 2).

Solution

To eliminate these vulnerabilities, the design was modified. A new fan hub and blade assembly were manufactured from a single forged component, successfully removing the weld interface as a potential failure point.

Summary

Generator fan and blower assemblies play a critical role in ensuring reliable turbine operation. However, these components are exposed to significant stress, making them prone to fatigue and other failure mechanisms if not properly designed, inspected, and maintained.

Key Takeaways for Operators:

1. Use NDE techniques to assess all critical fan assembly areas during each major outage.
2. Prioritize optimized attachment designs to reduce stress concentrations and extend component life.
3. Adhere to best practices during installation, including accurate torque application, interference fit verification, and proper blade angle settings.
4. Where possible, replace outdated welded blade designs with forged assemblies to eliminate high-stress weld points.

With the right design and operational practices, as well as thorough inspections, operators can mitigate the risk of catastrophic fan failures while ensuring long-term turbine reliability. 

Contact our team of experts at ENTRUST to find out how we can protect you against the failure of your generator rotor fan vanes and blower blades today. 

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Tom has spent the entirety of his 15-year career in the power generation industry. 

In his current role as Vice President of Power Generation for ENTRUST, Tom oversees a team of approximately 100 engineers, whose expertise covers power plant equipment, modeling, and testing. 

Prior to ENTRUST, Tom held turbine design and repair roles at General Electric. Tom is a graduate of GE’s Edison Engineering Development Program and holds 7 U.S. patents. He holds an BSME degree from Virginia Tech, an MSME degree from Georgia Tech, and is a registered professional engineer in the state of Delaware.