I have a case of high vibration which need to conduct field balancing. I would like to share here to discuss.

Some information which you could concern

Operation speed: 5100rpm.

Service: driver for boiler feed water pump.

Turbine was conduct overhaul, replace new bearing, new carbon seal, re-alignment but keep the old rotor. The old rotor was balanced in WS with G2.5 and all parameter was set up in spec.

Inlet pressure: 39barg, 

inlet temp: 370oC.

Outlet: 3barg.

balancing plane: coupling.

Rotor weight: 160kg.

N.V.Đ

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Original Post

Agree, likely unbalance.  Have you checked the rotor design to see what provisions exist for addition of trail weights at the end where the probes are designated 8237 X/Y?

What are the journal diameters and bearing clearances?  I see some evidence of whirl in the full spectrum cascade plot.

HI John, 

You are right about oil whirl but it ís eliminated when coupled. 

The shaft journal is 76.20mm. Clearance: 0.16-0.17mm.

This turbine has no space to input trial weight on the wheel. So, the trial weight should apply on balance plane at coupling. The thing is if we input trial weight in wheel position the position should be at Y probe (0o with XE8237) but if we input on coupling? where you should input and how much weight will apply? 10% of rotor weight would not be enough to make effect.

Your waterfall plot looks like exactly 1/2 X.  Can you verify this - orbits and time base work well for this?  

My first thought would not be oil whirl if this is exactly 1/2 X.  On a steam turbine, I would think RUB.

Also, the polar plots and Bode plots do not look like linear behavior.  This can be a bad sign if you are trying to balance.

 

Thanks Williams,

the cascade is in synchronous type so it could be allittle bit not exactly but orbit shows that it is oil whirl, actually, when we coupled, the eccentricity increase and more load on bearing the oil whirl was eliminated. 

This case actually was done by field balancing. But i just concern two things:

1. The rotor was balanced in balancing machine with G2.5 but it was still unbalanced when installed (speed balance is 500rpm). Is it not enough or balancing is not correct.

2. This rotor is operated under 1st speed, so, at first try i put trial weight at Oo regarding to 8237Y but is not successful. Is this mean that the balancing at coupling should be opposite with on the wheel (this is second time i meet this, 1st time i was for motor of boiler feed water). 

John and Williams are rich experience on this field, could you give your ideal on this,

at the start a friction is observed this is very evident in the graph of the central line and the spectrum supports it with the harmonic medium, this effect may be due to the oil pressure does not lift the rotor during the start,

my question is how does your turbine lubrication system have a prepost system?

really if an imbalance is observed in the rotor but I also observe high vibration at the beginning of the turn, the amplitude at low speed during the ascent and speed deceleration are different, the 1x in the ascent is almost 20um while in the descent it is Less than 20 um, as if it were a thermal effect, but I agree with a lubrication effect, I would like you to share the graph of the bode and the central line during the descent of speed.

 

Greetings and excuse my English.

The lubrication in the case is fine. The problem is unbalance and i finished successfully field balancing, now, is operating normally. 

I give here to discuss about position to input trial weight and what is this different about phase to input trial weight on wheel and coupling hub for machine operated under 1st critical speed?

regards,

 

Firstly, due to under 1st critical speed, I input trial weight at 0o of xe8237y which has highest vibration, as calculation of 10% rotor weight, the trial weight would be 5g but i put 8g to see response of rotor. When started, the vibration and phase almost the same, it strange to me ( this is not first time i do field balancing but first time i see).

so, i try to rotate weight 45o as rotation to see what rotor response but changing not much. Reconsider to cause of high vibration but it looks like the same unbalance. So, i decide to put 15g on 180o and it works with reducing vibration value. Final calculation with split weight at 180o and 210o with total 39g. And vibration is fine now (3 probes under 25um, 1 probe is 43um) but i think we can not do better and accepted this.

this is not a first time i see that the trial weight put at coupling hub with phase different to rule under 1st critical speed. Last year, i do for motor of boiler feed water 1.8MW, the situation is the same.

i know that it could be effect of rotor shape mode but under 1st critical, is this usually happened? Can you share your experience on this?

i do not know how to analyse the rotor model. If able, i hope to hear your sharing.

thanks John,

John, thank you for all the details on the case.

So, if I understood your question correctly, you are asking if the trail weight angle can differ between one axial location (turbine wheel) and another (coupling hub). In other words, you assume that trail weight should have the same angle regardless of its placement on the axial location of the rotor: at the middle or at the ends. Is this your question?

Regards- Ali M. Al-Shurafa

You are right Shurafa. 

My question is that the phase is changed along rotor even with under 1st critical speed which i thing it should be the same. Maybe, i am wrong, but, phase on wheel and coupling hub is out of phase. I think that it is effected by mode shaft?

is that always as my case or it depend on kind of each rotor? Please share.

This might be difficult for me but I'll try.

The 1x vibration phase angle can be different for the same machine at the same speed (mode) and other conditions if the measurement is taken at qualitatively different axial locations. The key point here is to pass by the nodes which are usually around the bearings. Node is where the mode shape crosses the shaft centerline of a straight shaft. Then the phase changes the sign (180 deg).

To visualize this I've uploaded two illustrations. The first one is a single stage turbine without the coupling. The second is with the coupling. Consider the bearing next to the coupling (DE). You may measure the vibration (amplitude and phase) between the wheel and the bearing. Or you can measure the vibration between the bearing and coupling. Do you see the difference in the direction of shaft defection?

I assume your thoughts about the phase were focused on the case without coupling while you possibly measured the phase between the coupling and bearing.A Rotordynamic Modle of Mode Shape of Single Stage Turbine without a CouplingA Rotordynamic Modle of Mode Shape of Single Stage Turbine with a Coupling

I hope I'm not mixing things up here.

Regards- Ali M. Al-Shurafa

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Photos (2)

Thanks Shurafa,

I also think that due to mode of rotor making phase shift cross bearing. But is it always true under 1 critical speed. As i have ever conducted field balancing for multiple stage pump running at 8000rpm, operating under 1 critical speed, balancing plane at coupling hub but the phase used to balancing is the same at bearing.

My question also is that we is always true to use the rule of high spot and heavy spot in phase when operated under 1st critical speed to input weight for balancing if the balancing plane is between two bearing (with between bearing types). 

Perhaps the rotordynamics of a single stage turbine will be different from a multiple stage pump when the mode shape is studied. The phase difference/swap cross the bearing is not a must in all cases as the bearing clearance and stiffness play a major role here.

Heavy and high spots will coincide under certain (special) conditions. The general case is that there is a lag that we should consider and account for (which could be zero). Phase lag value is a result of many factors. The measurement location (the point discussed in the above post) is one factor. The type of sensor used to measure vibration (displacement, velocity or acceleration) is anothor factor. The gap between the balancing speed and the closest critical speed also has an effect.

 

I'm sure the forum members can give better answer than mine but this is my 2 C.

 

Regards- Ali M. Al-Shurafa 

Hmmm.

I'm not sure there is a generic recommendation for a 90 deg placement.

What I would suggest is to keep a record for the balancing jobs so you recognize the phase lag based on an actual previous job. You may have this lag as a good reference for machines of the same model and working on similar conditions. I guess some people refer to this approach as one shot balancing or something like that. 

 

Regards- Ali M. Al-Shurafa 

Shurafa posted:

Hmmm.

I'm not sure there is a generic recommendation for a 90 deg placement.

What I would suggest is to keep a record for the balancing jobs so you recognize the phase lag based on an actual previous job. You may have this lag as a good reference for machines of the same model and working on similar conditions. I guess some people refer to this approach as one shot balancing or something like that. 

 

Regards- Ali M. Al-Shurafa 

Shurafa is correct with his statement.  Key is understanding your transducers and the effect on the portrayed data.  What you would do with a proximity probe would be different than what you would do using an accelerometer based system.  In a proximity probe based system is the data compensated?  Can you generate a compensated polar plot, which nicely shows the heavy spot?

Don't generalize, understand the dynamics of your machine and understand the instrumentation you have for the data collection.

Ngo Dinh posted:

HI John, 

You are right about oil whirl but it ís eliminated when coupled. 

The shaft journal is 76.20mm. Clearance: 0.16-0.17mm.

This turbine has no space to input trial weight on the wheel. So, the trial weight should apply on balance plane at coupling. The thing is if we input trial weight in wheel position the position should be at Y probe (0o with XE8237) but if we input on coupling? where you should input and how much weight will apply? 10% of rotor weight would not be enough to make effect.

Ngo Dinh posted:

HI John, 

You are right about oil whirl but it ís eliminated when coupled. 

The shaft journal is 76.20mm. Clearance: 0.16-0.17mm.

This turbine has no space to input trial weight on the wheel. So, the trial weight should apply on balance plane at coupling. The thing is if we input trial weight in wheel position the position should be at Y probe (0o with XE8237) but if we input on coupling? where you should input and how much weight will apply? 10% of rotor weight would not be enough to make effect.

Hi Mr Đình,

My turbine has same case as you, could I have your email for more interactive in site balancing.

Thank you for your sharing!

 

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