Hello all
Taking vib data on a GE turbine generator set with a single reduction gearbox.
Seismic vib data showed a strong 0.44X peak at the low pressure end bearing location. This is a pressure dam journal bearing.
Got suspicious and took vib data at six different speeds while bringing unit up to full speed. No evidence of 0.44X peak. Then electrically loaded the set and the 0.44X peak comes in strong. This unit operates above first critical.

There was no other equipment running in the backround.
Question: Has anyone else seen or read about strong correlation between electric load and oil whirl? Is it possible to have generator reaction forces go thru gearbox to affect the turbine rotor?

By the way, the bearing was rolled out and the babbit was in like new condition. Pressure dam and critical dimensions/clearances were SAT

Thanks Jim P

Check lubricant and temperatures. Is the oil up to operating temp when you kick in the pwr? And does it go away at temps go up? Or? Any more info.

Is the pressure dam in the right location? No chance of the bearing being in backwards or upside down?

Are you sure the clearance is right? Can you live with the vibration long enough to see if it goes away with time and temperature?

May be alignment between the turbine and gear, more likely internal misalignment in the turbine.

What kind of turbine steam or gas?

It could be possible that changing thermal conditions or changing forces on the turbine rotor are partly to blame. For example, on steam turbines, the opening of a control valve can force a rotor up and unload a bearing making an oil instability more possible. Similarly, thermal growth can cause an alignment problem that could unload a bearing. Is all the insulation in decent enough condition to keep heat off the bearings?

Does 0.44X correspond to the turbine or generator first critical?

I have never heard of electrical load directly affecting the chance of oil instabilities.

A little more info could help. What size t/g? What speeds? Any other peaks in spectral data?

I would do what Sam says and monkey around with oil temperatures and verify that the correct oil is installed in the unit.

Michael Titone

Does electrically energizing the unit bring on the problem? Or only when it is loaded about a certain point? With the gear box, the shaft position is going to change with load. Could there be more clearance in the loaded position than unloaded?

Jon
Spintelligent Labs

Gentlemen
Thanks for all your comments.
I would like to provide more information but for security purposes, I have to keep some details out of the discussion.

I have asked the vendor rep to verify that the bearing is "fool proof" (cannot be put in backward or upper half cannot be flipped 180).

Some additional notes-

The item of concern is the turbine rotor (9980 rpm)

Review of historical data shows this peak seems to "come and go" but not seen prior to several years ago. I need to review some maintenance records to see if inspection PM's performed during that time.

Back in 2000 following rotor replacement, I took Bode plot and noted first critical to be 4700 rpm. I did not notice any subharmonic at that time. The subharmonic (0.44X)is at ~4400 rpm and did not start to show up in spectrum until after 2004. Its possible some changes may have occured over time. I will take another set of startup data next month to confirm freq of first critical.

I should have realized that the 0.44X is not likely coming from generator reaction forces (assuming bearing orientation/fitup is SAT) but a more likely suspect from steam admission forces. I have specified that data collection only be performed at full load (assumes same steam admission forces), but because I dont usually take the data, cant confirm this to be true. Still, it raises some good questions I need to answer.

As far as oil temps are concerned (i.e viscosity sensitivity), I will again check but believe this is fairly well controlled by operating procedures. Still, if current inspections dont lead to smoking gun, I will plan a course of action to acquire data as function of oil temp

Again, thanks for all the insights. I will report back as I learn anything new.

JP

A tilt pad bearing probably solves this. 9980 is pretty fast for a pressure dam bearing.

Having said that if you come down check bearing loads. Your pedestal elevations are probably off. Steam turbines are often sensitive to proper bearing load.

quote:

By the way, the bearing was rolled out and the babbit was in like new condition. Pressure dam and critical dimensions/clearances were SAT

How do you know the design dam height and location? As Steve's questions aimed at, is the dam in the correct location, direction, and height? Has the bearing been changed or taken out prior to the roll out for inspection?

Alignment, hot (steam leaks, thermal insulation) and cold, (Michael Titone additions with regard to this) is important. If the alignment is sufficiently off, then the designed position of the pressure dam with regard to the bearing loading will not be correct. Steam forces can change the bearing loading, also. Gear forces affect the bearing loading in the gear box - what type of bearings are in the gear box? How is the turbine coupled to the gear, flexible coupling?

Just to add, the bearing load might be less. Compare the vibration levels on the other bearings to history. This may give some insight as to where the load is being re-distributed.

JP,

Your first critical is at 0.47 xSS, so you are very fortunate that only Oil Whirl (0.44 xSS) and not Oil Whip is present. Perhaps there is some system damping from shaft seals and turbine glands that keeps this rotor from "taking off". Maybe you can fiddle with oil temperature, bearing clearance, pressure dams, mid-span balance, and shaft alignment to keep rotor response stable. It would be like me trying to get a clear picture on my TV by twiddling with the rabbit-ear antenna (no Cable-TV!). This is certainly a machine that needs a good quality permanent vibration monitoring system.

Walt

I had this problem on a similar unit. I bet if you look at the shaft centerlines you get a big change in shaft position on the Turbine LP bearing when the unit is sync'd. This is due to gear forces, and raises the shaft in the bearing and causes the instability.

In the few steamturbine cases I have seen of bearing instability it is "normal" to be load sensitive, some cases it can be reduced by lowering the load, fiddling with pressure, temperature as described, viscosity and for some reason it seems to be get worse with the age of the turbine, maybe stiffness or clearance get worse by time? If the problem is a design problem that do happen the only long term solution is a "squeeze" bearing where a hi pressure pump supply the faked oil pressure required to get a stable bearing. Not cheap. Olov