Dear Gents,

We are studying problems in a gas turbine and power turbine from Pratt & Whitney. It is the FT8 PowerPac Model, which is used as a power generation unit. We would like to know for other experience or vibration problems you could have had in this kind of machines. What about some possible failure modes to detect?

Regards,

Allen_Forum

Fantastic posting but, exactly what are the "problems" you say you are studying? We need a lot more information to be able to get involved.

Allen, r u a student? Anyway, to get failure modes for gas turbine and to start with, you may refer to ISO14224, at least for academic purposes. Looking at the P&W user manual under maintenance section may give you clues to what can go wrong.

Sorry, I understand that it is a too general question but anyway we have problems of vibration at 1x rotating speed with higher levels of vibration when the machine is running at 2800 rpm more or less in the starting process; vibration monitor triggers because of the long time in alarm status.

Regards,

Allen_Forum

I have no experience of this particular model of GT but the first thing to do is verify the vibration readings.

a)how many transducers are fitted to this engine
b)what type are they? (accelerometers?)
c)are all transducers giving high readings or just 1 - if so, can you swap transducers from one location to another?

When was the last time the vibration system was calibrated? You probably need a shaker table to check this out.

If the vibration is proved to be real, then the next step is a full borescope inspection to see if you have got any broken or damaged blades (tip rubs or FOD)

You could also check the installation - holding down bolts and alignment of GG to PT to berth.

Are the bleed valves in the correct position at these speeds? Are the IGVs performing to specification?

When did this problem develop? Did the unit experience a hot trip?

Buzz,

Three sensors, one in gas turbine and two in reaction turbine. Only in one direction (45º +/- from the horizontal); they are accelerometers (high temperature certified). I attached you some photos for the sensors. Measures with off-line systems indicate horizontal readings (in similar direction than permanent system) higher than vertical readings, but always 1x in FFT as dominant frequency. Axial measurements indicate 1x equally. Sensors indicate high readings both of them. Critical speed for the rotor is 2100 rpm.

What about flexibility of bolting elements in order to absorb thermal growing for example? Related to some problems coming from the alternator? What do you mean with bleed valves, could them generate 1x vibrations? What do you mean with IGVs?

The problem was developed when the unit of expansion was replaced by other.

Best regards,

Allen_forum

quote:

The problem was developed when the unit of expansion was replaced by other.

Is this some sort of bellows in the hot air/exhaust path? Is the new one identical to the old one - same material/design/dimensions/thermal growth rates?

Based on what you have said so far, I would guess that this is the culprit - different thermal growths are imposing excessive 'static' forces on the GT?

Can you:

a) verify correct alignment of the GT to the berth/exhaust ducting
b) put the old unit back in place
c) check and compare cold dimensions of old and new expansion units?

If the bleed valves and IGVS are not operating properly you will have disrupted air flow (possible surge) through the compressor section

Allen,

IGVs are Inlet Guide Vanes, which channels the gas flow onto rotating blades in a turbine. Are only the vibration on the Reaction Turbine a concern or all 3 sensors are high? You mentioned permanent sensors, are these also indicating high vibration? What are the levels, can you post some vibration profiles, both from permanent and casing monitors?
If it is only the Reaction Turbine sensors that are going into alarm, I'd first make sure that the mounting bracket, accelerometer and output signal cable are not physically contacting the tubing/flex hose behind it.
Let us know.

Cheers...Rajan Muthukrishnan

Rajsha,

It is only the reaction turbine sensors sure. Levels are 6 mm/s more or less in channels 2 and 3, and clearly lower in channel 1 (compression chamber). The frequency is 1x rpm of course.

The sensor for permanent system is in file attached.

Allen

8711064643_Model204300-20UTC-100-200.pdf (164 Kb, 22 downloads)

The frequency of vibration appears to be related to the free-turbine & Generator speed. I assume the engine's low and high stage rotors operate well above generator speed.

You mentioned problem after free-turbine change-out. How did you guys align the units, was the coupling reassembled correctly. Does it have a long Bendix coupling? Was the engine Clocked to the free-turbine. Does the generator also have some vibration. I can't see the free turbine having an imbalance, this would have been found during the test run. You mentioned the problem from start-up so thermal growth should'nt be the problem unless something is bound up but you would have noticed this during the alignment of the free turbine. I am use to FT4's and they were bullet proof. It would take something very wrong to cause the unit to trip.

Allen,

It's been a week since the last exchange of info on your vibration problem on the FT8. What is the present situation? Did you find or identify what was causing the vibration on CH. 2/3?

Regards...Rajan Muthukrishnan