Why we are not considering stress evaluator & critical speeds in GE gas turbines?

 

There are several reasons--and the thought processes behind them are all changing as we write!

One is that GTs, by nature, experience high thermal stresses during start-up and shutdown--it's a fact. Properly tuned and operated machines should minimize the thermal stresses during starting and stopping. And during loading and unloading, as well. If a combustion turbine could be operated 24x7x365 at Base Load, the hot gas path components would last a lot longer than they do when they are started and stopped repeatedly and subjected to unnecessary trips, and operated on grids with unstable and changing frequency.

If your question is related to why there is so much emphasis placed on these characteristics for steam turbines and not gas turbines, then consider that steam turbines have many more stages and nozzles and that as steam passes through these turbines during start-up from a cold condition it will condense until the metals reach operating temperature and rated speed at minimum rated steam flow. There are many more stages and packings and nozzles and casings and control valves and piping and expansion differentials than on a gas turbine which has only two or three stages on the turbine section.

As for the critical speed issue, there are critical speeds for any piece of rotating equipment but GE gas turbines are generally tuned (balanced) to minimize the effects <b>AND</b> the turbines are not operated in the areas of those critical speeds for any extended period of time. By "operated in" I mean that an operator cannot "pause" or "stop" a GE-design heavy duty gas turbine at any speed below 95% (which is where the two critical speeds would be located). A properly tuned and operated machine should pass through the critical speeds on the way to FSNL (during start-up) and on the way down from FSNL (during shutdown) at a generally smooth and "programmed" rate.

So, the answer to your questions are that the control systems and operating sequences are programmed to take into account the thermal stresses and critical speeds that combustion turbines, by nature, will experience and to minimize the effects of thermal stresses and critical speeds to the extent possible. And, the control systems do it very well--when properly tuned, and when the turbines are properly operated and not subjected to unnecessary tripping and excessive starting and stopping (which is where the majority of thermal stresses and critical speeds are experienced).

Hope this helps!

Now, there are rumors that GE is radically changing the control system philosophy used on their turbines. What this means is anybody's guess at this point. It will undoubtedly be marketed and sold as a performance enhancement (isn't everything?) and it will likely entail some pretty certain shifts in understanding and troubleshooting.

If they properly document what they will be doing, it could be a very good thing. Their documentation has been improving in recent years, but it has a <b>very, Very, VERY LONG WAY to go</b>. And if they are going to be radically changing their philosophy and control schemes, then we have to hope they will also change their documentation appropriately.

And if wishes about GE documentation were six-pence coins, I'd be a multi-millionaire.

 

thanks for your valuable info..

 

>thanks for your valuable info..

GT critical speeds and rotor stress.

well, this is a interesting topic, and i have a few questions of my own here. GE as far as i know does not publish the critical speeds of the turbines. i have asked numerous GE personnel and their answer is around the 2500-3000 rpm mark. even they do not seem to have a clear cut idea. but i guess it is true in a sense as the max vibrations in some machines seems to go high during this speed (but not all, i have seen some frame 6 machines in which there is little or no increase in vibration during acceleration). so is there any document or info regarding critical speeds at all??

regarding rotor thermal stress i think wheel space temp is a good indicator, though no one ever bothers to look into it. what does a sudden increase in the wheel space temp convey. is it that that the wheel space cooling is inadequate or is there anything wrong with the turbine blade or hot gas path. no trip is provided for wheel space so sites just operate even if all the wheel space probes are high. there is little or no documents available about this. its mostly thorough oral communication with BHEL or GE guys decisions are taken.

perhaps CSA or otised can help more in this area.

 

Process Value,

You are correct; GE does not publish critical speed data for their heavy duty gas turbines. Again, GE-design heavy duty gas turbine rotors are balanced to reduce the criticals to the extent possible, AND the Speedtronic prevents control of speed below 95%, which means there should be no danger of continued operation at or around a critical speed (if the start-up and shutdown Control Constants properly tuned). Units equipped with proximity vibration probes will almost always show two distinct areas of increased vibration during starting, particularly. Because of the design and balance of the machines, these peaks can be very low, but they are usually detectable.

Again, a properly configured and -tuned control system should preclude prolonged operation at any speed below 95%, which is where the critical speeds are usually found.

As for the subject of wheelspace T/Cs and what high or low wheelspace temperatures indicate that has been covered many times on control.com. There are some machines that do trip on excessively high wheelspace temperatures; but most only alarm on high wheelspace temperatures. And that's why most sites just ignore them after their first experience with high temp alarms.

And the most common cause of high wheelspace temperatures is improper T/C insertion. And this is usually experienced after a maintenance outage, or after the wheelspace T/Cs have been replaced.

Again, the topic of wheelspace temperature meaning has been covered many times in the past on control.com.

 

thanks for the response