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vogel

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Reply with quote  #1 
Once again I'm trying to wrap my head around an instability case. It's somehow similar to others that I've seen in the past.

Case 1: HP ST + Gbx + Gen + LP ST

The HP steam turbine shows an instability during startup after reaching a certain steam pressure level. Once full load is reaxhed, the instability will reduce slowly to a minimum in a couple of hours (thermal stabilization?). However, after some time operating at full-load and low vibrations, the instability may trigger again suddenly tripping the unit.

Case 2: Again a steam turbine generator.

It's possible to see a subsynchronous component on the steam turbine waterfalls at different loads and on different startups. Vibration is low so no one is worried, then one day the subsynchronous component rises suddenly and trips the unit.

Case 3: Steam turbine + generator.

The generator DE bearing shows a subsynchronous component after reaching full load. Amplitude is low but some hours after startup it increases dramatically and trips the unit.

It was not possible to correlate the sudden triggers of instability with any process variables, axial or shaft centerline position ...

What may trigger suddenly an instability that is present but ''silent''?

I can't post any case specific plots but I may answer questions if more information is needed.
John from PA

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Reply with quote  #2 
Probably best to start with some "basics" by my friend Richard Thomas, then proceed from there with additional questions. See https://www.turbomachinerymag.com/back-to-basics-fluid-induced-instability-i-e-oil-whirloil-whip/.
ruben

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Reply with quote  #3 
Una vez más, intento abarcar un caso de inestabilidad. De alguna manera es similar a otros que he visto en el pasado.

Caso 1: HP ST + Gbx + Gen + LP ST

La turbina de vapor HP muestra una inestabilidad durante el arranque después de alcanzar un determinado nivel de presión de vapor. Una vez que se carga la carga completa, la inestabilidad se reducirá lentamente a un mínimo en un par de horas (¿estabilización térmica?). Sin embargo, después de un tiempo operando a plena carga y con bajas vibraciones, la inestabilidad puede dispararse de nuevo y disparar la unidad.

Caso 2: nuevamente un generador de turbina de vapor.

Es posible ver un componente subsíncrono en las cascadas de la turbina de vapor a diferentes cargas y en diferentes arranques. La vibración es baja, por lo que nadie está preocupado, entonces, un día, el componente subsíncrono sube repentinamente y dispara la unidad.

Caso 3: turbina de vapor + generador.

El cojinete DE del generador muestra un componente subsíncrono después de alcanzar la carga completa. La amplitud es baja, pero algunas horas después del arranque aumenta dramáticamente y dispara la unidad.

No fue posible correlacionar los disparadores repentinos de la inestabilidad con ninguna variable del proceso, la posición de la línea central axial o del eje ...

¿Qué puede desencadenar repentinamente una inestabilidad que está presente pero '' silenciosa ''?

No puedo publicar tramas específicas de cada caso pero puedo responder preguntas si se necesita más información.
[/CITAR]

For the Case 3


Do you Know the criticals speeds in your steam turbine? look for a critical speed between 0,42x to 0,48x. If there is any critical speed over this frecuency margin perhaps your turbine instability may be an OIL WHIP. 

What´s the viscosity of your journal lubrication oil in this turbine? 
OLi

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Reply with quote  #4 
They are not the same design are they? Of the very few cases I have been in contact with some or maybe most are actually design "features" so one bearing do not have enough load and get instability so either you spend a lot of money and induce a fake load by oil pumps and squeeze bearing or you on purpose treat it rude and create some load by making a bad alignment...... Or they are old and bearings getting worn, clearings increase and there you go trip on vibration. So what could trigger? I guess the list is long and depend on design, condenser temp, pressure, oil temp, pressure and most common load. If you are quick and reduce load when the vibration suddenly increase you may avoid tripping, in some cases. I guess others can add to the list.....
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vogel

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Reply with quote  #5 
Thanks for your answers.

John, thanks for that link, it looks like a summary of what one can find in Bently books and trainings. Always valuable, although sometimes it defies reality. In two of the three cases the subsynchronous frequency was below 0,4X and in the other one it was above 0,5X. Only in one of the cases the orbits were fully circular.
Anyway I'm not so much interested in the diagnostics of instability, in all three cases the instability was confirmed by the OEM and/or a vibration specialist (Bently and/or others), I only had a collateral involvement to it. So I'm quite sure that those three cases were an instability. What I fail to understand is what mechanism makes that a turbine or generator incresase a stable/controlled subsync component vibration from 10 to 150 um in a matter of seconds when it has been operating at full or stable load for some hours.

Ruben, Do you really think that there's the need to google-translate my whole post to Spanish and paste it on your post? It makes communication more difficult. If a user can't understand a part of my post in English they can use google translate themselves. I'm not a native English speaker and I think neither is Oli, but we don't feel the need to make this thread illegible by posting a whole long post in another language.

Oli, very valuable comments. These three machines are different designs. I agree with you on the design point: on another ocassion I monitored a gas turbine model in several combined cycle power plants and all of them were showing traditional oil whirl instabiility during runup/coastdown, that was a design "feature" as you mention, same waterfalls and orbits across all sites.
These are quite new machines with recent overhauls by the OEM so clearances should be ok. I checked all the parameters you mention and there isn't a change in any of them when the instability triggers. I know there must be, but there was huge work to find them and no success (in one of the cases someone run several "big data" algorithms to correlate all parameters to the instability and nothing was found).
In all cases the instability triggers and trips in less than five seconds or so, really very little time to decrease load or even reach the HMI.
ruben

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Reply with quote  #6 
Quote:
Originally Posted by vogel
Thanks for your answers.

John, thanks for that link, it looks like a summary of what one can find in Bently books and trainings. Always valuable, although sometimes it defies reality. In two of the three cases the subsynchronous frequency was below 0,4X and in the other one it was above 0,5X. Only in one of the cases the orbits were fully circular.
Anyway I'm not so much interested in the diagnostics of instability, in all three cases the instability was confirmed by the OEM and/or a vibration specialist (Bently and/or others), I only had a collateral involvement to it. So I'm quite sure that those three cases were an instability. What I fail to understand is what mechanism makes that a turbine or generator incresase a stable/controlled subsync component vibration from 10 to 150 um in a matter of seconds when it has been operating at full or stable load for some hours.

Ruben, Do you really think that there's the need to google-translate my whole post to Spanish and paste it on your post? It makes communication more difficult. If a user can't understand a part of my post in English they can use google translate themselves. I'm not a native English speaker and I think neither is Oli, but we don't feel the need to make this thread illegible by posting a whole long post in another language.

Oli, very valuable comments. These three machines are different designs. I agree with you on the design point: on another ocassion I monitored a gas turbine model in several combined cycle power plants and all of them were showing traditional oil whirl instabiility during runup/coastdown, that was a design "feature" as you mention, same waterfalls and orbits across all sites.
These are quite new machines with recent overhauls by the OEM so clearances should be ok. I checked all the parameters you mention and there isn't a change in any of them when the instability triggers. I know there must be, but there was huge work to find them and no success (in one of the cases someone run several "big data" algorithms to correlate all parameters to the instability and nothing was found).
In all cases the instability triggers and trips in less than five seconds or so, really very little time to decrease load or even reach the HMI.



Sorry Goggle traslate me in automatic...but it´s ok now. I hope you can read in English now.

ruben

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Reply with quote  #7 

Do you know the critical speeds in your steam turbine? Look for a critical speed between 0.42x to 0.48x. If there is any critical speed over this frequency range, the instability of your turbine may be an OIL WHIPS.
What is the viscosity of your daily lubrication oil in this turbine?
Study the possibility of increasing the viscosity slightly to avoid the probability that the WHIRLs oil coincides with a resonance and becomes an OIL WHIPs.
 I send you a link with a page of oils with viscosity calculators mixing different oils. Obviously ask your oil manufacturer first if you can mix two oils of different viscosities, those of your brand, without degenerating and even achieving a slight increase in viscosity.

http://www.widman.biz

ruben

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Reply with quote  #8 
One of the main reasons for the appearance of the OILWHIRL and the OILWHIP are changes in the equipment load. Normally they usually happen when the equipment works very low of load. If it happens to you with high load you can try to lower the load and see if it disappears after a while. The second solution is the one that I already mentioned to vary the viscosity of the oil or to vary its working temperature so that the first one varies. It is advisable to lower the temperature so that the viscosity increases, this will have a considerable influence on the internal process of instability.
Shoveldr

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Reply with quote  #9 

I've had two instances where I saw instabilities that looked like oil whip or whirl, but were actually instabilities from other sources.

One was actually due to seal gases.  The compressor had vibration when they first started it up but it "fixed" itself.  They seals were rubbing and once they opened it up the seal gases were causing the instability in the bearing.

The other was a misaligned steam valve in a certain load range they were putting too much steam into the turbine, causing the instability.

OLi

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Reply with quote  #10 
0.42x to 0.48x are really statistically defined numbers. I had a case with several hundred small fans with a specific problem and they all were within those freq. limits when they misbehaved. Changing bearing load one way or another and they were stable in the range of several thousand pcs. So for problems creating frequencies outside that range I would look for other reasons. Normally when things produce unexpected frequencies I look for resonances and what are exciting them, normally touching/rubbing or flow connected.
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vogel

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Reply with quote  #11 
Quote:
Originally Posted by Shoveldr

I've had two instances where I saw instabilities that looked like oil whip or whirl, but were actually instabilities from other sources.



I agree, actually at least in one of the cases mentioned above the source of the instalbility was the steam flow at the turbine seals. 

Any more thoughts on what may trigger an instability? For instance, on page 10 of the following document, it is possible to see a sudden rincrease of vibration at a frequency that is always there in the waterfall: https://www.researchgate.net/profile/Paolo_Pennacchi/publication/257526645_Analysis_of_the_Instability_Phenomena_Caused_by_Steam_in_High-Pressure_Turbines/links/55e9997508ae65b6389b0085/Analysis-of-the-Instability-Phenomena-Caused-by-Steam-in-High-Pressure-Turbines.pdf 

What could be a cause for it?


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