Gas turbine speed monitoring

[DieguitoI]

I’m study a machinery p&id of a GE PGT25 gas turbine. I noticed that there are two separate speed probes, one for gas generator rotor speed and one for power turbine rotor speed. Both probes given high velocity trip; gas generator rotor speed trip is set higher than power turbine rotor speed.
Can you, gas turbine experts, tell me why we need 2 velocity measurements with 2 different trip set point?

 

[crshears]

I don't know about GE PGT25s, but if the two turbines are not mechanically geared or connected together, their speeds can vary independently, hence two separate trips.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[DieguitoI]

Dear, Thank you for your reply. There is only one GT with only one shaft. One section is called Gas Generator the other Power Turbine.

 

[btrueblood]

Manuals available online for the PG25s state they have a "free power turbine", implying a separate shaft downstream of the gas generator (compressor and turbine) which would operate on their own shaft.

 

[crshears]

Thanks, btrueblood; wonder why the OP didn't look that up...

Years ago I operated combustion gas turbine generators like this; the "gas generator" was an aero-derivative Orenda jet engine of the type used in the CF-104 Starfighter; idling speed was 6000 rpm, full power speed was 7500 rpm. [ Incidentally, at idle the fuel consumption [ #2 fuel oil ] was 4 Imperial gallons per minute; at full power it was 18 Igpm. ]

Power turbine speed was 7500 rpm; output was fed to a 25/6 speed reduction gearbox, generator synchronous speed was 1800 rpm. Unit was rated at 7.5 MW, but the outdoor temperature had to be quite cold before this could be achieved.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[DieguitoI]

@btrueblood
I got it, thank you for the explanation.

 

[rotw]

btrueblood,

Manuals available online for the PG25s state they have a "free power turbine", implying a separate shaft downstream of the gas generator (compressor and turbine) which would operate on their own shaft.

Definitely. If I am correct, the Gas Generator is essentially an LM2500; the power turbine is connected 'aerodynamically' to the GG to satisfy mechanical drive applications requirements (for instance, providing variable output speed).

Trip speed instrumentation and control speed instrumentation should be separate I suppose.

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