hi

we have GE frame#6 gas turbine,turbine running at 5163 and generator at 3000. is there any special reason to run turbine at 5163 instead of 3000? in 9E machine turbine and generator both running at 3000. is there any special advantage of run turbine at 5163

 

Gas Turbine speed

The turbine needs air flow at a constant rate for working. the axial compressor just does that. the job of the axial compressor is to maintain a constant air flow at all load conditions. when the load increases the pressure in the combustion chamber also increases and the axial compressor also increases the pressure to maintain the air flow. thus the axial compressor is designed to work (at base load) at a pressure ratio with a given flow. Once the pressure ratio and the air flow of the compressor is known then the no of stages and the rotational speed is determined.

the pressure increase in each stage is determined by blade speed, axial velocity, degree of reaction, the blade angle etc. higher the blade speed and axial velocity then higher the pressure ratio increase per stage. thus i believe that the speed / no of stages is a compromise. by deciding on the speed the no od stages is limited to 17. if you go for a lower velocity then for the same pressure ratio and flow you need to have more stages. thus an optimum value is chosen. for a frame 6 machine it is 5100 rpm with 17 stages so that it produces a peak pressure ratio of 12.2 with 525 tonnes/hr as air flow in iso condition.

my refinery library has a very good book, gas turbine engineering handbook by P boyce. it has a excellent chapter on axial compressor. i read it offhand and most of my info about how design is done comes from it. you can have a look through it. you will understand a lot more if you are a mechanical engineer.

 

For another take on this, taken from one of my prior replies on this subject:

Maximum gas 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. The MS9001 is still larger and has 3000 rpm design speed, which just happens to be the rotational speed of a 2 pole per phase 50 Hz generator.

Another book that I have found useful is "Gas Turbine Power" by George Merrick Dusinberre and John Cambell Lester, International Textbook Company, Library of Congress Catalog Card Number 58-9295.

 

What Process Value laboriously tried to say is: That's the way the machine was designed. "In the old days" it was more common for prime movers to rotate at high speeds to produce the torque required or desired. Nowadays with better materials and better analytical methods (i.e., computers and modeling/simulation software) more things are possible than previously.

Frame 9Es benefited from being designed in an age of better materials and better analytical methods.

 

I also think it in terms of energy. One rotating mass (Turbine) transfers rotational energy to the other rotating mass (Generator). Now assuming that, in practice, no energy transfer process is 100% efficient, the prime mover MUST have higher rotational energy than the driven device (Generator). Now Rotational Kinetic Energy is proportional to square of omega, angular speed. Hence the turbine (Prime mover) MUST have higher angular speed than the driven Generator.

How much higher should it be? Well, that part is for the designers to decide. For a layman understanding, it's just enough to know this much. Experts on this forum surely will be able tell a lot more.

Thanks and Regards,
Ritika

 

Above all explanation is not the exact one that are required......we have Westinghouse gas turbine which has 5418RPM turbine speed and Generator speed of 3000 rpm..but for the steam turbine its the same 3000rpm...also i know many turbine have the same speed as the generator...we need to the point answer why it is that? please share the knowledge...

 

Otised's explanation is on the right lines.

For a given power output, a higher rotating speed allows a smaller turbine footprint (whether its steam or gas). In an industrial context, you will find compressor-drive steam turbines operating at high speeds - these will have a similar physical size and speed to a gas turbine of similar rating. Steam turbines in the electrical industry have "traditionally" been direct drive. Gas turbines originate from the aero industry and use of gearboxes is equally traditional in that industry.

 

CSA gave you the answer!!