I am working in a gas turbine (GE) power plant (34.5MW). It is frame 6B, Mark 4 control system, Manufactured in 1988. My question is that why max vibration is increasing while frequency is decreasing (around in 49hz where normal frequency is 50hz). Turbine is running in droop speed control mode. Recently generator of ALSTOM rewinding has done by ALSTOM.

 

Md. Assaduzzaman Bhuiyan,

You have not provided enough information to be of much help.

When did this problem start? After the generator rewinding? Was there other major turbine work while the generator was being re-wound? Were turbine buckets replaced? Was the turbine rotor removed and re-installed?

<b>Precisely <i>WHERE</i> on the <i>unit</i> does the vibration increase?</b> On the generator collector end? On the generator turbine end? On the turbine exhaust end (Turbine Brg. #2)? On the turbine compressor end (Turbine Brg. #1)? On the reduction gear?

Did the unit operate without increased vibration at <b>less than rated frequency</b> before the generator re-wind?

Is the unit operating at Base Load when the frequency decreases to 49 Hz?

Are you aware that when the frequency decreases below 50 Hz that the turbine speed decreases proportionally? And when the frequency increases above 50 Hz that the turbine speed increases proportionally? 1 Hz equals 2% of rated frequency and speed. A turbine rated at 5100 RPM being operated at 49 Hz will be spinning at 4998 RPM.

Frame 6 gas turbines are designed for operation at approximately 5100 RPM (there is a small speed range where the air flow through the axial compressor doesn't vary by much, but axial compressors have very unique and distinctive operating characteristics compared to many other kinds of compressors and other rotating equipment), and the 50 Hz generators they drive are designed to be operated <b>at</b> 3000 RPM. When a GE-design heavy duty gas turbine is built, the rating of the reduction/load gear is taken into account, because it's that rating that determines the actual speed at which the turbine and axial compressor will operate when the generator is being operated <b>at rated frequency.</b> I've personally never seen or heard of a single-shaft Frame 6 GE-design heavy duty gas turbine that operated at anything below 5094 RPM at rated frequency, or more than 5134 RPM at rated frequency (again, the difference depends on the reduction/load gear rating). So, you can see the speed range is very small, as a percentage of speed.

Most rotating equipment with large mass is designed, built and balanced for operation at a particular speed, or for operation in a particular speed range.

Are you aware that most large power producers have two "critical speeds" during acceleration from zero speed to rated speed, where vibration increases for a brief period at each of the two distinct frequencies and then reduces to very low levels? I'm NOT suggesting that the upper critical speed of your machine is anywhere near 5000 RPM (it should be much lower), but I am just attempting to point out that it is a characteristic of many types of machines that they have unique vibration "signatures" that are taken into account when they are designed and built for operation at a particular speed or speed range.

Further, a properly regulated and controlled AC grid (of any size, finite or infinite) is designed to be operated at 50 Hz, not 49 Hz, or 51.4 Hz, or 49.3 Hz, or any other frequency. Exactly 50.0000 Hz? Not likely. 50.00Hz? Again, not likely 50.0 Hz (i.e., plus-or-minus approximately 0.1 Hz), desirable. Sure, the designers understand that grid frequency disturbances do occur, but, it's my learned guess that no one ever anticipated the kinds of grid disturbances that are routinely experienced in some parts of the world today.

Just a personal side note, why rate a unit at 50 Hz and then expect it to operate without any adverse affects at less than, or more than, rated frequency? It's not an automobile that operates at many different speeds over lots of different terrain. Even automobiles can't operate at infinite speeds (RPMS) without experiencing high vibration of some kind or another. A combustion turbine being used to drive a synchronous generator is designed to be started and accelerated to <b>rated</b> speed and operated at <b>rated</b> speed while producing load. It's designed, built, and balanced for rated speed (which is directly proportional to frequency!). And <b>off-frequency</b> operation is not normal.

So, why should anyone be surprised when operation at <b>other</b> than rated speed (frequency) while producing load isn't exactly the same as operation at rated speed while producing load? More and more, I'm asked why combustion turbines don't produce rated power at less than rated speed (frequency). The question really is, "Why can't you make my turbine produce rated power at less than rated speed (frequency) for indeterminate periods of time while protecting my machine and not reducing hot gas path parts life?"

This is a controls forum, and while we do help with many mechanical issues (since we realize that a process is composed of mechanical equipment that requires control and the two--mechanical equipment and control systems--are very closely related) there's not likely much we can do for your issue. If the vibration is excessive and you are worried about potential damage to the unit (generator, reduction/load gear, and/or turbine) you should engage the services of a vibration analyst. Vibration analysis is a part science and part art, meaning that while some vibrations can be easily related to specific stimuli and/or conditions, others are much more subtle and require a lot of experience and practice to understand and resolve--well beyond the ability of most of the contributors to this forum.

Best of luck with your issue.

 

Mr. CSA,

Thank you very much for your kind reply. I have learned more from your valuable reply.

Yes the problem had started after re-winding. And rotor had cleaned by removing it. Vibration increasing in the bearing#1. Unit is operating in speed control mode when it is 49hz.

I understand that turbine is designed to operate at rated speed that’s why vibration (12-13mm/s) is increasing at low frequency (approximately at 49hz) but i have seen that that vibration (7-8mm/s) is not too much varied at high frequency (approximately at 51hz).

 

Md. Assaduzzaman Bhuiyan,

Bearing #1 is the furthest bearing away from the generator, and is also separated from the generator by the reduction/load gear and the load coupling.

It would seem pretty unlikely that some issue with generator rotor balance would affect the bearing furthest away from the generator, but then that's what vibration analysts (the good ones) can discern.

Why would one assume that increasing vibrations when speed decreases would also result in increasing vibrations at the same rate/level when speed increases?

Again, this is a great issue to have a vibration analysis of the entire machine performed. If you do so, please write back to let us know how the problem was resolved.

 

Mr. CSA,

I forgot to mention that vibration in the exciter side also high when low frequency but it is comparatively low than bearing#1 vibration.
I will suggest to my plant manager to call a vibration analysis as you suggested. Off course I will write to you if the problem solved.

Thank you very much to give your valuable time.

 

Did the set operate at 49Hz before the rewinding?

Is the coupling gearbox/generator toothed OR diaphragm?

Was any other work carried out on the package other than the rewinding?

Was a complete realignment of the package carried out after the rewinding?

The posts are very correct about operation of an axial compressor. But 49Hz SHOULD be OK and the upper critical SHOULD be below 49Hz.

Do you have a copy of the vibration plots of the generator rotor from the testing by Alsthom after the rewinding?