GT 9FA increased proximity readings at TE of generator

[npower0073]

Dear all. On site we have a 9FA DLN +2 gas turbin coupled with a hydogen cooled 270MW generator. After the end of a combustion inspection outage we saw the following : When GT goes from 140 MW to base load 260MW then we see increase readings on the proximity of bearing 1 (TE) of generator. When the load decreases we saw decrease of the rpoximity reading. THe seismic sensors have increase as well but far below limits. We suspect possible air leaks from turbine that lead to possible thermal expansion of turbine and change of mechanical load of the generator. Let me know your opinion. Nothing has changed as far it concerns hydrogen purity, bearing metal and oil temperatures, ambient air etc.

 

[ScottyUK]

Does the field current change significantly when you flex the load? A vibration signature which varies with field current is one sign of a shorted turn on the rotor.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[npower0073]

The field current is steady, the fluctuation of vibrations has to do with change (increase) of active power (MW).

 

[byrdj]

IF you hold power (MWs)constaint, but vary excitation (VARs), does the vibration change. again looking for a possible bar or end turn problem

 

[npower0073]

During the change of load we have not seen any change on the excitation current.

 

[ScottyUK]

I'm wondering about magnetic asymmetry in the stator. Has the machine had any repair work done on it? What did the last RSO results indicate - anything unusual? Any sign of heating in the stator iron?

What frequency is the increased vibration at? 1x, 2x, something else?



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[npower0073]

The last RSO showed perfect condition. The actual problems we had were cross fire tubes hot air leaks inside turbine compartment. These leaks led to thermal expansion of the turbine and change of place of the generator. The vibrations were characterized as subsynchronous. Gap increases during the alarms period but the 1x and 2x harmonics do not increase. Gap 91y (Gen TE brg proximity) increased and Gap 101x (Gen CE proximity)also increased.

 

[byrdj]

Thermal expansion?

could this include elevation changes for the journal bearings? this would dependant upon the design of the bearing support, whether the bearing is pedistal or centerline. elevation changes will cause vibration problems. an unloaded journal, with a flexiable rotor will easly go unstable and sub synchronse vibration increase, usually very excessive. However there should be at least very small changes in the bearing metal and oil drain temps from either past operation or as MWs are changed. the adjancent (accros coupling) bearing should have a slight increase in temp just prior to a bearing going unstable. and the unstable bearing show a slight decreas just before vibration increases. Once vibration gets excessive, the temp changes may not be prevalant.

Is it possible to provide cooling to the journal support that is hot, maybe a fan (I've even used water spray on steam turbine's concrete pedistal) or something to see if that reduces vibration.

 

[npower0073]

byrdj, now we have concentrated on the load compartment enclosure wit the cooling fans capacity and the damper backdraft orientation. Although, it is a 9F turbine, where the loead shaft compartment is fillowed by air intake and then by turbine. I will give you the final data of troubleshooting.

 

[abeltio]

Do you have remote monitoring of the unit? What does the OEM say?

saludos.
a.

 

[npower0073]

Last week we made a repalcement of the piping ofcross fire tubes , all the flanges on these tubes were in very bad wuality. After these works the comaprtment temperatures dropped from 180 to 115 degrees Celsius.

 

[irishal]

We also have 9FA and had lots of cross fire tube leaks. Did the replacement of your cross fire tube gaskets solve your vibration problem?

 

[npower0073]

The repalcement of cross fire tubes did not solve the problem of vibrations.

 

[Romefu12]

npower0073
There is an issue with the 7FA units coupled to the 7FH2 Hydrogen Cooled Generators that the 9FA units may also be susceptible. The issue is that of the alignment elevation of the Generator. On the 7FA / 7FH2 configuration, the Generators were being lowered to account for an increase in thermal growth to eliminate the GT Compressor Bearing from unloading.
It is also common for the 9FA units to take a longer time to thermally soak and correct for rotor bow after an extended downtime when the rotor is not on turning gear, thus the vibration you are seeing may correct itself after some run time.

Kind Regards
Romefu12

 

[NCTHAI]

Increase in vibration while increasing load must be investigated properly. Increse in both types of sensors means there is something.

Hotgas leak to Gen is rare possiblity as turbine and Gen are normally on opposite site on 9FA machine.

Check for instrumentation integrity. Are these increase in vibration only on Gen or also on turbine bearings (compressor end and turbine end)also?

Is there any issue with seal oil supply? PResumable unti should have hyd cooled gen. All seal oil hoses are ok? Any leak can be seen? any significant change in cooling water water flow to gen coolers?

All cooling fans are working fine??

Read the vibration signature using bentely nevada equipments or any other external signal analyser. Which pick is high? 1X, 2X??

Having only gen vib increase is strange and may early indication of wdg fault. Is there any change (increase) in field wdg temp? Or Stator wdg chnge??

Most importantly, check intergrity of earthing brush. Is all connections healthy? Is there any double earthing?