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Tech Note: January 2017

Solving a Sub-Synchronous Rotor Vibration for a Critical Steam Turbine in a Seemingly Small, but Actually, a Large Application

In many applications such as refineries, pulp and paper mills, and mines, a 23 MW steam turbine-generator may not seem like a “big deal”. 

As occurred in this project, the turbines, generators, and other equipment that experience high amplitude rotor vibrations are seemingly unsolvable problems, and it is almost a certainty that TRI will be able to provide an effective and workable solution that can be implemented in a short time period.

TRI Align-A-Pad® Bearings have made very significant contributions to a number of these seemingly smaller applications.  One of the latest success stories is of a 23 MW extraction steam turbine-generator that was uprated over 15 years ago, and since then, has had a continued history of unacceptably high amplitude rotor vibrations when certain steam valves with high steam flow were opened. The consequence was that the steam flow through the turbine had to be limited to avoid these high amplitude rotor vibrations, and this resulted in the maximum sustainable MW generation being only around 15 MW.  This situation represented a considerable financial loss for years to the company for this reason:  The steam pressure was dropped down through a pressure reduction valve rather than going through the turbine to recover the energy in the form of electrical generation, a loss of 8 MW of generation that had to be purchased from the outside.  In this circumstance, there were times when the steam flow to the header and on to the using components was limited, as well.

“23 MW extraction steam turbine-generator that was uprated over 15 years ago, and since then, has had a continued history of unacceptably high amplitude rotor vibrations”

Many engineers and companies, including the OEM, have tried different solutions to this vexing problem without success for over a decade.  One company proposed to perform an extensive fluid flow and rotor dynamic study that would take many months to complete.  If this path had been taken, an opportunity to use an upcoming planned maintenance outage to install their (future) proposed solutions would be missed.

TRI has solved so many of these sub-synchronous rotor vibration problems that are caused by steam forces acting on the turbine rotor, usually on much larger machines, that when TRI was approached for a solution, TRI was quite confident of being able to solve this problem in “one shot”, and to do so in the rapidly approaching outage.  TRI proposed to perform a very short engineering analysis of the steam forces acting on the rotor at high steam flow conditions, and then to design and manufacture three TRI Align-A-Pad® Tilting Pad Bearings (both turbine bearings and the outboard generator bearing) to suppress the rotor vibrations. They would be installed in the plant’s approaching outage. Plant management agreed to TRI’s proposal.

When the outage occurred, the existing bearings were shipped to TRI to take outside “envelope” measurements for installation, the new TRI Bearings were finished and installed, and then the unit was started.  The unit went right up to 23 MW with full steam flow.  There was no hesitation, and it has been functioning with satisfactory rotor vibration amplitudes and bearing metal temperatures since this start-up.

Typically for three bearing systems, only bearing #2 needs to be upgraded to a suitably designed tilting pad bearing. Due to the severity of the vibration conditions in this case, all three bearings were redesigned to assure maximum control of the rotor vibrations.

Please contact TRI for a review of your circumstances and a recommendation for moving forward toward a solution. 

“TRI plans to participate in the upcoming EPRI Turbine-Generator Users Group Conference and Exhibition on January 16 and 17, 2017, being held in the Peabody Hotel in Memphis, TN.

We look forward to meeting attendees and discussing any rotor vibration and bearing damage issues that you have, as well as other subjects related to TRI Products and Services.