Calculating part-load output to base-load (ISO) 1

[5414463]

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?

 

[j79guy]

Correction to ISO conditions to my knowledge, is universal. The corrections are for altitude/barometric pressure, ambient air temperature and relative humidity, which effect the performance of gas turbine engines equally at all performance points. Obviously, as with all GT engines, performance variation between like models becomes more apparant at lower loads, especially those units with variable stator vanes. (Calibration tolerance.) The correction formulae to ISO conditions used on our test cell, are the same for partial and full loading.
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

 

[stgrme]

Contrary to j79guy's statement above, the ISO-rated elevation for land-based gas turbines is sea level.

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.

 

[j79guy]

stgrme; As per General Electric test cell correction factors for LM class engines, (GE Manual GEK9262) and for Rolls Royce aeroderivitive units (Avon, Spey and RB211.)(Rolls Royce Technical Manual M-Av1533-G) are all using 1000' altitude and 59 Degrees F. as the standard of which to correct performance data to. I have however also occasionally seen sea level/59 Degress F. as a reference for base performance numbers for land based units for GT units other than those models I mention, thus I must raise a point in suggesting that 5414463 refer to his manuals to see what his "ISO" is. For the GE LM class machines, with six or seven rows of variable incidence stator vanes, depending upon engine model, the manuals do not consider the position of the variable stator vanes in correcting back to "ISO" conditions. In checking my test cell procedures for the Rolls Royce Avon machines, I also do not see where there are specific correction curves for IGV or bleed valve position. Ambient conditions at time of test, effect a GT engine as a whole, equally throughout it's entire operating range. 1% output change per 10 Degrees F. delta, 3.7% output change per 1000' altitude delta. (Rule of thumb, but pretty darn close.)
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

 

[5414463]

Thanks for all the info guys!
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!!

 

[5414463]

sorry guys, in a hurry i put a wrong unit in my previous post. So to avoid any misunderstanding:

Pambient=1013.26 mbara

 

[j79guy]

To answer your questions from my opinions, and past GT operating experience:

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

 

[5414463]

j97guy! Thanks for the input.

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.

 

[rmw]

I want to throw a thought in here. It may not be right but it occurred to me as I read this thread. Yours is a Frame 6 and about the only variable in the air flow stream is the IGV's, (ignoring any bleeds). This is typical of frame machines throughout the range as far as I know (for the older models I am familiar with at least.)

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

 

[j79guy]

5414463; The small heat rate "bump" you are experiencing comes from the compressor. Compressors really are constant speed devices, with modifications to help efficiency at off-design operating points. Any compressor has an efficiency "sweet spot", where they operate at peak. This point, if you have access to a compressor map from the OEM, can be identified at a specific speed, compression ratio, and mass flow, biased somewhat by ambient conditions. (Temperature, barometric pressure, inlet losses.) To help with off design points, the manufacturer added variable incidence stator vanes to help the LP section of the compressor cope with the limited flow capacity of the HP stages. Once the compressor is up to full speed, the variable stators are fixed on the open-travel stops, and the compressor then operates as a constant speed device. When operating the unit at low outputs/speeds, even though the variable stators are performing their function, the point in the compressor with the first row of fixed stators is not in an aerodynamic efficient condition. (The main reason fatigue failures occur at this point in the compressor.) At certain ambient conditions, i.e. hotter than ISO conditions, the inlet air is less dense, thus the LP stages of the compressor will deliver less mass net flow to the first row of fixed stators, putting them into a slightly more aerodynamic efficient profile. In effect, the warmer, less dense inlet air is helping to cover up a bad operating area in the compressor map.
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

 

[j79guy]

rmw; Good point, as an aeroderivitive guy, I fall into the trap that all GT's are multiple spool. Frame machines typically are single spool, constant speed. Which begs the question; 5414463, what application is your Frame-6 unit being used? Electric generation, or mechanical drive?
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

 

[rmw]

j79guy,

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