For Gas Turbine, when we start, Starting Motor Starts and remains at low speed around 650 RPM. How does it maintain the speed? I see there are 2 SOV, low speed and high speed, How it works? Also when GT reaches certain speed we do have enough power from gas firing to run turbine, so when motor stops around 2300 RPM, does it disengage? what happens actually when we start and when motor stops at 2300 RPM.

 

Well, have you been out to the Accessory Compartment to look at the Diesel Engine starting motor, the torque converter that it's directly connected to, and the jaw clutch between output of the torque converter and input to the Accessory Gear/turbine shaft?

You say you "see" there are two solenoids, so I presume you are looking at the Starting Means Piping Schematic drawing (P&ID). The solenoids direct the flow of hydraulic oil (from the Diesel, not the turbine) to a hydraulic ram that opens and closes the diesel fuel rack. There should be some notes on the drawing to indicate which direction causes an increase or decrease in speed.

Since the starting means is coupled to a torque converter, which is really like a hydraulic pump (the input shaft is driven by the Diesel) driving a hydraulic turbine (the output shaft drives the turbine shaft) the input can be variable. The Diesel warms up for a minute, usually, and because it's not putting out very much torque, the turbine shaft won't break away from rest (zero speed).

When the solenoids direct the flow of diesel hydraulic fluid to increase the speed and torque, the jaw clutch also starts to turn. It usually takes both the torque from the starting motor *AND* the assist from the torque converter to overcome the inertia of the turbine shaft when it's at rest to start it turning.

As the shaft starts to rotate, the Diesel speed is increasing and the turbine shaft speed is increasing. The unit will complete its purge cycle, and some will decelerate down to fire at a lower speed and some will just initiate firing.

When fuel is flowing and flame is established, the turbine starts to put out more torque, but it's not enough to be self-sustaining. Some Diesels have another solenoid to increase speed and torque a little more to assist with acceleration.

When the turbine shaft speed exceeds the torque converter output shaft speed the jaw clutch will open and a limit switch will eventually shut the diesel down. (Many go back to idle speed for a minute or two to cool down, then are shut off. Some are just shut off.)

And, yes, the torque converter output shaft speed can be higher than the diesel speed (input shaft speed). Because you're next question is likely to be, how can the diesel only be running at 2300 RPM and the turbine shaft is about 3000 RPM when the jaw clutch opens to shut down the diesel.

That's about it!

Learn to understand the Piping Schematic drawings (P&IDs), no matter how difficult it seems in the beginning. And read the Notes on the drawings, they usually have some very good information (VERY good). If you want to really understand how the "turbine" operates, it's necessary to understand the Piping Schematics (P&IDs). Full stop.

Of course, they don't explain everything, but they do explain a lot. If you have questions, that's what we're here for!

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