When New is not Necessarily Good: Problems in Steam Turbine Vibration

  • Steam Turbine Vibration

When you pay top dollar for a piece of brand new machinery, you expect it to be top-of-the-line, and to operate and such. At Cascade MVS, we get calls from customers all the time that demonstrate that this is not always the case. We ran in to one such instance earlier this year when we were contacted by a major U.S. Gas Production Company for help dealing with a vibration problem regarding a new Steam Turbine driven fan. The fan in question was of a unique design and was one of only a dozen made worldwide.

The Challenge
Cascade MVS was contracted by a major Gas Production Company following continuous problems with a Steam Turbine Driven Fan. To ensure better reliability and to slow down the Turbine without effecting fan speed, the company had exchanged coupling and installed a new Gearbox, with a different gear ratio. Following the change the unit, which should have operated up to approximately 5300 RPM, exhibited high instability and unacceptable vibration above 4000 – 4200 RPM. After a number of unsuccessful operational parameter measurements and procedures, the company sought help from Cascade.

The Cascade MVS Solution
After initial contact and teleconference with the customer, Cascade MVS set out to review all machine records following the original turbine failure, and to supply technical drawings on:

  • Steam Turbine Rotor
  • Steam Turbine Bearings
  • Coupling
  • Gearbox

From above information Cascade MVS was able to build a rotor-dynamic model consisting of the Steam Turbine rotor, coupling and Gearbox pinion. Appropriate bearing damping and stiffness were calculated and applied for each bearing location. Engineering drawings of replacement bearings were then made, and measurements were obtained to build a rotor model.

The goal of the model was the following:

a. Determine lateral dynamic behavior of the combined rotor with existing bearings.
b. Design optimized pressure dam bearing to ensure minimum vibration, maximum possible stability, sufficient damping, and move any critical out of the rotor running speed range.

After applying bearing stability calculations to the rotor model, the bearing modeling and calculations indicated possible instability. The next step in the rotor modeling was to attempt to stabilize the bearings as well as to achieve unit operation through full operational speed range up to 5500 RPM. We achieved this by optimizing both Steam Turbine Bearings by changing the geometry, type, or clearances of the bearings.

Resulting rotor model with optimized bearing clearances and geometry:

Steam Turbine Vibration

Final Result
The First Critical speed was moved above minimum required 5500 RPM, allowing full speed range operation as requested by the customer.

For more detailed information on how we reached our final result, including shaft dimensions used and plot presentations, download our Steam Turbine Vibration PDF.

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