By Tom Reid, Vice President of Power Generation Services, ENTRUST Solutions Group
Torsional vibration resonance poses a serious risk to turbines and generators, potentially leading to significant damage and unexpected outages. Resonance with the double-line system excitation frequency (120 Hz) has been linked to critical failures, including blade liberation, retaining ring cracking, exciter fan liberation, and rotor shaft cracks. While this issue is now widely recognized within the industry, concerns about resonance persist.
These concerns often stem from one or more of the following factors:
Field testing is critical in verifying unit behavior when resonance concerns arise from analysis. However, ENTRUST engineers frequently encounter analysis results with significant modeling errors that fall well outside the ±3% tolerance permitted by the 2009 ISO standard (22266-1). Significant errors may leave resonance issues undetected, increasing the risk of failure.
These errors often originate from incorrect assumptions when translating the geometry of turbines and generators into simplified mass and stiffness models. Compounding this, analysis providers may lack access to critical component geometric details, especially when systems have undergone retrofits with parts designed by other companies.
This article outlines two complementary methods for verifying design models: finite element analysis (FEA) and stationary frequency impact testing.
Developing an accurate frequency model is essential for dynamic analysis. Models must account for the rotor’s stiffness and mass characteristics across the frequency range of interest. Abrupt changes in rotor geometry alter torsional stiffness and must be properly quantified for accurate results.
To determine equivalent rotor torsional stiffness, FEA can be employed. Torque is applied to one end of the model while the opposite end is fixed. The two models must produce matching twist angles at various axial positions. Any additional blade or rotor mass not captured by the stiffness model is incorporated as a mass moment of inertia.
Blade rows with frequencies under 150 Hz are modeled as dynamic branch elements, as these modes can couple with rotor frequencies to produce concerning system resonances at or near the double-line excitation frequency.
Stationary frequency impact testing is recommended to ensure the accuracy of analytical models. This method is explicitly acknowledged in the 2009 ISO standard for torsional frequency analysis.
Unlike generator rotors, the stiffness and frequency of unbladed turbine rotors remain constant regardless of rotation. However, when a rotor is bladed, rotating frequencies increase due to centrifugal stiffening of the blades. Despite this, stationary frequency impact testing can be conducted on bladed and unbladed rotors.
The test involves mounting a block on one side of the coupling and striking the rotor tangentially. For torsional tests, the rotor rests on rollers. Moving the accelerometer across axial and radial positions during impact testing generates simplified mode shapes that can be directly compared to analysis model outputs.
A complete multispan analysis integrates calibrated mathematical models for the generator, exciter, jackshafts, spacers, and other turbine elements.
Generator models, like large LP turbine blades, are affected by changes in operational conditions. For example, centrifugal forces on rotor windings alter frequencies with speed. Additionally, temperature influences analysis results, as increased temperatures reduce a rotor’s shear modulus, leading to slightly lower frequencies.
ENTRUST Solutions Group’s experience suggests that temperature effects typically amount to frequency shifts under 1 Hz. These effects are accounted for within the frequency acceptance criteria of the 2009 ISO standard.
Following this systematic calibration process for each turbine enhances the accuracy of full-system analysis models. This approach significantly improves compliance with the 2009 ISO standard and satisfies many insurance carriers’ requirements for field testing.
It’s important to stay ahead with precise engineering, thorough analysis and careful consideration during upgrades or retrofits to turbines and generators. If you’re looking to safeguard your operations against torsional vibration resonance, ENTRUST is here to help.
Contact us today to learn more about how we can support your performance and help you avoid costly turbine and generator failures.
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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.
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