We are running 3 Frame-6B and 2 Frame-9E GE gas turbines, Frame -6B turbine speed is 5100RPM and Frame-9B speed is 3000 RPM. Is is possible to generate electricity in Frame-6 GE gas turbine by reducing the speed from 5100 RPM to 3000 RPM (without reduction gear) by controlling the fuel flow?

 

NO!

The MS6001B gas turbine is designed to operate at 5100 rpm. 3000 rpm is only 58.8% speed, well below the minimum speed for any heavy duty single shaft gas turbine. Even if it were possible, you would never get anywhere close to rated power. Note that GE would not go to the trouble and expense of providing a reduction gear if this were possible.

 

Thank you, I agree with you GE has the purpose for that, I would like to know, If generator side need 3000 RPM,what for speed raised to 5100 RPM? I am eagerly waiting for all of your response...

 

The turbine speed was not raised. Turbine speed is determined by the physical size of the unit. Basically, it is a function of the tip speed of the buckets and compressor blades. It must be kept subsonic. However, higher speed translates to higher airflow and more power. So the turbine designers want the higher shaft speed on the smaller turbines to get more power from the units. GE's MS6001 is actually about the same diameter as the MS5001, hence both have 5100 rpm as their design speeds. (The MS6001 was initially a MS5001 with a third turbine wheel added to get more power.)

Obviously, the other way to get more airflow is to make the turbine larger. And, with a larger diameter of the turbine and compressor wheels, the rotation speed must be reduced to keep the tip speed subsonic. So, the bigger the turbine, the slower the design speed. The MS7001 is larger and has 3600 rpm design speed, which just happens to be the rotational speed of a 2 pole per phase 60 Hz generator.

 

G.Rajesh,

Remember, some of the first combustion (gas) turbines were used as engines for aircraft and speed of rotation wasn't as important as it is for power generation applications. We're talking here about heavy duty gas turbines, which are primarily used for power generation applications but are also used for mechanical drive applications (compressors, pumps, etc.). And heavy duty gas turbines were developed based on the designs of the first combustion turbines.

A synchronous generator (more rightly called an alternator) has a strict relationship with its prime mover and the frequency of its output. That relationship is defined by the formula:

F = (P * N) / 120

where F = Frequency (Hz)
P = Number of poles of rotor
N = Speed of rotor (in RPM)

So, to generate 50 Hz, a two-pole alternator has to spin at 3000 RPM. (This is why alternator and prime mover speed doesn't change when producing power, at least as long as the grid frequency is constant!)

In the day when Frame 5s and Frame 3s and Frame 6s were being developed, the materials in use at the time, the manufacturing processes in use at the time, and the engineering decisions made at the time based on existing combustion turbine designs required that the axial compressors spin at much higher RPM than alternator synchronous speed (3000 Hz in the example above).

That's also true of the turbine section of the machines. To develop torque in the most efficient method possible (given the materials, processes and decisions made during the design and testing), it was necessary for the units (the turbines driving the compressors) to spin at speeds greater than synchronous speed.

So, for a single-shaft gas turbine (a machine with an axial compressor and a turbine coupled directly together to form a common, single shaft) to drive a generator it was necessary to use a reduction gear, or load gear, to reduce the RPM to a value that could be used to drive an alternator to produce usable power.

As otised says, it's would be possible to directly connect the turbine to the alternator, but the power developed would be neglibile at best. Remember, most starting means are still assisting the turbine with acceleration at about 3000 RPM; the turbines are barely self-sustaining at that point.

There is so little air flow through the axial compressor at 3000 RPM that there is very little additional fuel that can be burned. The IGVs must remain closed at 3000 RPM to protect the axial compressor. The compressor bleed valves must be open at 3000 RPM to protect the axial compressor, so, there's just no possibility of increasing the fuel to increase the power produced to have any appreciable power left over to drive the alternator.

Also, if you observe a plot of exhaust temperature as the unit is starting you will see that it is beginning to climb very quickly at about 3000 RPM with the minimal amount of fuel that is being burned during acceleration. Adding more fuel would increase the exhaust temperature (and the internal firing temperature) above allowable limits, damaging hot gas path parts and exhaust components.

Remember also that nearly two of every three horsepower developed by the turbine section of a heavy duty gas turbine goes to drive the axial compressor, meaning that only one horsepower is available to drive the alternator. This is true when the gas turbine is running at rated speed (approximately 5100 RPM for most Frame 5s and Frame 6s), and is not even close to being the case at less than rated speed. Again, the starting means is usually still providing some torque to the shaft at about 3000 RPM, so the turbine is only barely producing enough torque to drive the axial compressor and the inertial load of the alternator, and there is little possibility of increasing the fuel at 3000 RPM.

If this had been possible, someone would have done it already and everyone would have converted their units to eliminate the reduction gears.

As materials improved and manufacturing processes improved and design capabilities improved, it was possible for turbine manufacturers to build turbines that could run efficiently at alternator synchronous speed(s), thereby eliminating the reduction gears. The Frame 9Es at your site are just such machines, developed decades after the first Frame 5s using newer materials and technologies. Frame 6s were built on Frame 5 technology using some similar parts, technologies, and manufacturing and packaging components.

By the way, this question has even been asked and answered before on control.com. Many have had the same thoughts over decades. It should be noted that steam turbines have had the same evolutionary process; early steam turbines also spun at speeds much higher than alternator synchronous speeds and had to employ reduction gears to drive alternators. As materials and technologies developed over time, it because possible to produce steam turbines that would operate efficiently at lower speeds.

 

Thank you very much for your replies. Lot of doubts have been cleared. The way you answered the question is awesome.

 

Dear CSA & All,

I have been reading since it posted though I would like to get more details. Shall you give link about the past discussion?

Thanks in advance.