Calculating part-load output to base-load (ISO)
Calculating part-load output to base-load (ISO)
(OP)
I'm trying to use correction curves to correct gas turbine performance to ISO conditions. As these correction curves are developed for base load, i dunno if its advisable to use them for part-load.
I have some power output datapoints at part-load. Is there a method to convert this part-load power output to base-load (ISO) or to use these correction curves for these part-load data points?
I have some power output datapoints at part-load. Is there a method to convert this part-load power output to base-load (ISO) or to use these correction curves for these part-load data points?





RE: Calculating part-load output to base-load (ISO)
Keep in mind there are different "ISO" conditions for gas turbines. Land based units generally use 1000' altitude/59 Degrees F. Marine based gas turbine ISO is 0' Altitude/100 Degrees F.
j79guy
RE: Calculating part-load output to base-load (ISO)
Also note that units, which vary the position of the Inlet Guide (Variable Stator) Vanes at lower loads, must be corrected for guide vane position.
If you have all the correction curves, you should be able to correct back to ISO conditions.
RE: Calculating part-load output to base-load (ISO)
I've been running my test cell now for nine years, and use 1000'/59 Degrees F. as "ISO". GE has audited/signed off on my cell, and customers seem to be happy with our products.
j79guy
RE: Calculating part-load output to base-load (ISO)
let me move forward with the discussion. The turbine under discussion is a PM6101 FA type GE gas turbine (Nat gas fired). I have gone through the GE documentation, infact already built a calculation model based on it. ISO conidtions for this model are:
Tambient=10°C
Pambient=1013.26 mmH2O
There are no correction curves available in the present documentation for inlet guide vanes. (My little experience with gas turbine monitoring, still had this important factor in my head).
After correcting the gas turbine test results to ISO conditions, i came across few results which i couldnt understand. The energy consumption (heat-rate KJ/KWh) curves for different temperatures (calculated from ISO conidtions) show that at part-loads the heat-rate is better at higher temperatures. Which is contrary to the normal understanding (atleast which i have) that heat rate increases with temperature rise. At base load it is well the case that heat rate increases with increasing temperatures.
That's why i thought it might be result of missing part-load corrections. Now it ends up into two important questions:
1- Do the correction curves determined at base-load stand true for part-load conditions?
2- Is it possible to get lower heat rates at part-loads with raised inlet temperatures? If it's the case then its better to preheat the inlet air in part-load situations and cool at base-loads.
Unfortunately i cant access my calculation model now, otherwise i would have attached few graphs to show the heat-rate curves.
waiting for your input guys!!
RE: Calculating part-load output to base-load (ISO)
Pambient=1013.26 mbara
RE: Calculating part-load output to base-load (ISO)
1- Yes.
2- Yes, but not on all turbine units.
In studying the heat rates for some of the LM class machines, there is a slight "sine wave" in the curves, especially pronounced at the lower outputs/speeds that is effected by T. Ambient.
You will find that this heat rate "bump" is only present in a narrow temperature range, where if slightly hotter or colder outside, will no longer be observed, or less pronounced. However, I do not think it practical to introduce inlet air heating, to take advantage of this slight heat rate gain, unless in your particular application, you spend extended amounts of time running in this reduced output/speed state. (In which case, you would be better served by smaller GT units.)
I stand to be corrected, however I do not believe that "correction" curves exist for the Frame-6 machines, for IGV position. (If they do exist, I would like to have a copy of!)
j79guy
RE: Calculating part-load output to base-load (ISO)
Is there any theoretical reason behind GT showing such a behavior at part-loads? I want to get a grasp of it conceptually.
I agree with you on this point that the gains pertaining to such modification (preheating inlet air at part-loads)won't be significant (i did a bit of modelling last weekday), but what makes GT run better at higher temperatures at part-loads?
And about the correction curves for IGV position, I couldnt find any and am not sure if they exist. If there are any, i'll do share with you.
Meanwhile, a big thanks for posting your views.
RE: Calculating part-load output to base-load (ISO)
Typically these machines are most efficient at full load because for operating speed (typically synchronous) the mass flow of air through the machine is the same and the efficiency is determined by the fuel addition and the temperatures that the turbine section inlets see. Part load, same air flow, (ignoring IGV's for this point) less fuel, lower temperatures across the turbine stages, less efficiency, lower heat rates at partial load.
The jet engines that are the core of the Aeroderivitave turbines such as the LM's and others mentioned earlier on the other hand, were designed originally to be the "motors" out there under the wing propelling an airplane; delivering gosh awful amounts of power at take off and while climbing through the thick air of low altitudes but spending most of their time cruising at lower fuel delivery rates at some very high (and thin plus very cold air) altitudes.
These turbines when used as Gas Generators for a Power Turbine typically operating at synchronous speeds (GE has just announced an uprate for the LM-6000's which is basically the result of operating the power turbine at higher speeds and reducing to synchronous speed via a gear box) can operate at whatever speed needed in order to deliver the right amount and right temperature gas to satisfy the demand of the power turbine.
Other than the overall fuel input vs. the shaft HP out, I am not sure the "in-between" can be correlated. If I am wrong, the smart guys on this board will set me straight.
If I am right, then that has to be factored into the discussion above.
rmw
RE: Calculating part-load output to base-load (ISO)
In an ideal world, axial flow compressors would have variable stators on every stage, or each disk running at a different speed. Neither are practical! Compressors as a whole are the same as a single airfoil, most efficient at one speed, mass flow. The small heat rate bump you are observing, is a small anomaly, effecting the compressor in a good way. Hope this helps.
j79guy
RE: Calculating part-load output to base-load (ISO)
Even still, in looking at heat rate curves, they are not perfectly linear to ambient temperature, and follow a slight "sine wave", with bias towards higher overall efficiency at lower ambient temps. The warmer air, I still think is putting the compressor into a slightly more efficient operating condition, for that particular point.
j79guy
RE: Calculating part-load output to base-load (ISO)
And thank you. As an old frame guy who is now working more in the aero world than the frame world anymore, I have often looked at those complicated looking linkages and wondered what the heck they were doing and when did they do it. My simple mind can wrap itself around an IGV or even a two shaft machine like some of the old Fr 3 & 5's that I saw years ago, but all those stages of linkages were too much for me to comprehend. I only assumed that it was an offshoot of the original engine design where such a wide range of temperatures and air densities had to be dealt with for airplane flight.
Your explanation helps me a lot.
rmw