Good day house
Please I need an enlightenment on what are the causes for a gas turbine too enter ratchet mode after a sudden system collapse (grid collapse) what are the possible causes and solutions for instrumentation and control engineer should focus on

Thanks for your various response.

 

Loss of AC power, can failsafe to ratchet mode. Depending on your controls configuration.

 

@Deprince,

You failed to mention what machine you are enquiring about. Some machines have DC ratchet motors, and some machines do not.

In addition to the ratchet motor (or some pump producing hydraulic pressure for the ratchet operation) there needs to be a source of Lube Oil for the machines bearings. Most GE-design heavy duty gas turbines equipped with ratchet mechanisms for cooldown (and starting) have AC motor-driven auxiliary Lube Oil pumps--which don't operate when there is no AC mains source for them. Most GE-design machines with DC motor-driven ratchet mechanisms have always used DC motor-driven Emergency Lube Oil pumps and most of them are configured to start the DC Emergency Lube Oil pump every three minutes when the ratchet mechanism operates to provide Lube Oil to the bearings during the ratchet operation. (The Lube Oil also provides a small amount of cooling during the period when the AC motor-driven Auxiliary Lube Oil pump may not be operating to provide cooling to the bearings.)

But, we really don't know anything at all about the machine and its auxiliaries and configuration based on the information provided. So, there's not too much we can say in response to your "question."

The subject of what to do when the machine can't operate the ratchet mechanism after a trip or shutdown from load has been covered MANY times before on Control.com. Yes; it's very likely the axial compressor rotor will sag or bow because of it's weight and length and distance between bearings--BUT, the metal temperatures are only in the elastic range of the metals, and as the machine cools the rotor will begin to return to a more normal straight condition. The important thing is NOT TO PANIC and try to use manual rotor turning mechanisms when the rotor is hot and has bowed or sagged because that can cause serious damage to stationary and rotating axial compressor blades which can cost a LOT of money to repair--not to mention the lost generation while the machine is being repaired. After about 24 hours the rotor, under normal circumstances, will return to almost perfectly straight as it cools. The REALLY IMPORTANT thing is to try to keep some kind of Lube Oil flowing to the bearings because the bearing material will start to deform if it gets above approximately 300 deg F, and if the shaft is warm (such as in the exhaust section of the machine) the heat from the shaft will cause the bearing material to deform and even melt to some extent. So, Lube Oil flow on a regular basis until the AC mains power for the Auxiliary Lube Oil pump is critical. And that will happen when the machine ratchets on a regular basis using the DC Emergency Lube Oil pump motor. And, if it's not automatically happening then someone needs to be stationed to periodically operate the DC Emergency Lube Oil Pump motor to provide some Lube Oil flow to the bearings for cooling while AC power is being restored to the site AND the axial compressor rotor is cooling.

Once AC mains power has been restored and approximately 24 hours has passed then and only then can the machine be STARTed in CRANK mode to run the machine shaft at low speed to help straighten the axial compressor rotor even more. Usually after about 20-30 minutes or so of CRANKing the Master Selector can be changed to FIRE mode to establish a flame in the machine at low speed to help straighten the shaft even more at low speed. (The speed will usually increase a couple of hundred RPM during FIREing; that's normal.) After about 15 minutes of FIREd operation the Master Selector can be changed to AUTO mode and the machine should accelerated to FSNL (Full Speed-No Load). The vibration levels during acceleration may be a little higher than normal but should not be high enough to cause the machine to trip. Run the machine for about 30 minutes or so at FSNL and then the machine can be synchronized or shut down to await the next loaded operation. This procedure will help restore the axial compressor rotor to a fully straight condition without any severe damage to the axial compressor rotor (UNLESS misguided attempts to rotate the shaft as it was cooling using hydraulic jacks, straps or manual turning mechanism were used and actually caused serious damage to the machine.)

This is the recommended method of GE for their machines. Hundreds of machines have had problems ratcheting for one reason or another and by following this procedure almost ALL of them have returned to normal service after the cause for an inability to ratchet was investigated and resolved.

Next time you write, please provide a LOT more information--basic stuff, as we are not sitting or standing next to you and can't see or know what you are seeing or know. We can provide some really useful--and concise--information IF we have good information to base our responses on. Help us to help you.

 

@Deprince,

GE-design heavy duty gas turbines with ratchet mechanisms for cooldown and starting are designed to automatically start ratcheting ANY time the machine has been above a preset RPM level (usually around 10-20% of rated machine speed) even if there is no AC mains power--as long as the ratchet pump uses a DC motor and there is a Emergency Lube Oil pump (usually DC motor-driven, also). An operator and operations supervisors and engineers should know this and be watching to see that this happens.

Of course, as mentioned above, the DC pump motors won't run continuously when there is no AC mains power available, but the will cycle as mentioned above approximately every three minutes as the ratchet mechanism is sequenced.