we tested a new fogging system in ge ms6001. When we compare the rated capacity (base load mode) without fogging system it achieved 31.2 mw, inlet temp 31 oC, exh temp 551 oC, cpd 9.78 bar. with fogging system it achieved 32.8 mw, inlet temp 24 oC, exh temp 545 oC, cpd 10.05. As u can see from the data, when we use the fogging system, the cpd will increase and exh temp will decrease so it limits the capacity. Is there any way that we can increase the rated capacity to more than 32.8 mw?

thank you

 

Take a look at your Control Specification, if you have one. From there you will see that TTRXB (Base Load Temp. Control Set-point)is directly related to CPD, this is to linearize the Firing Temp. Sure you got more Mw when you used the inlet fogging by increasing the CPD, but this increased the cooling effect by the increased airflow so your Base set-point was slightly reduced. The answer to your question is really No, you cannot get any more Mw without over firing the machine.

 

Glad you tackled this one, Bob.

It doesn't seem right, and yet the originator said that when CPD went up and exhaust temperature went down that power output increased. Everybody thinks that when Base Load is selected that exhaust temperature should increase when power increases. But it doesn't. The slope of the exhaust temperature control curve is negative. As CPD, on the x-axis of the plot, increases the corresponding exhaust temperature, on the y-axis of the plot, will decrease *for the same firing temperature* which is what the curve represents: a *constant* firing temperature.

If one plotted MW on the x-axis versus exhaust temperature on the y-axis while running at Base Load, the curve would look just like the Base Load exhaust temperature control curve: it would have a negative slope.

The increased air (mass) flow which occurs with an increase in CPD causes the exhaust temperature to drop while running at a constant firing temperature, which is what Base Load is. Doesn't seem right, but that's the way it works! Fuel flow even increases as CPD increases while on Base Load, but exhaust temperature decreases. Doesn't seem right, but that's the way it works.

I believe what the originator is really saying is: Why didn't the power output increase more with the addition of the fogger? One would think that the output would have increased more than approximately 5% in a dry ambient when running the fogger. Is the fogger running properly? Atomization is the key, and too many big drops of water will not be as effective as a well-atomized spray or fine mist.

Is there a lot of water running out of the inlet drain while the fogger is running? This would most likely indicate poor atomization or excessive water.

How much did the compressor inlet air temperature drop when the fogger had been running for 30 minutes or so over the ambient temperature?

What does the fogger manufacturer say? What kind of power increase did they estimate would occur with their system?

When the unit is running at Base Load, it's already putting out maximum power based on the exhaust temperature control curve. By fogging, you've increased the mass flow through the unit by cooling the inlet air, which has allowed a higher fuel flow which has resulted in more power output, even though the exhaust temperature decreased. Which is what should happen; a decreasing exhaust temperature while running at Base Load does not mean the power output is limited. It actually means the power output has increased, as you can see from the data.

If you're looking for more power output as a result of the addition of a fogger (more than the 5% you've already seen), you should ask the fogger manufacturer if there's anything you can do to optimize the system, or if that's the limit of the fogger's capacity. Many times during the addition of something new like this fogger, there is testing and tuning and training and more testing and tuning and training before everything is running in top condition. It's not clear if this data is from the first attempts, or after some period of time when all the kinks have been worked out and the system is running exactly as it should.

When was the last time the compressor was washed off-line? What is the inlet filter differential pressure (are the inlet filters clean?)? When was the last maintenance outage on the unit?

Again, what kind of performance increase were you expecting? Did the fogger manufacturer make any kind of performance guarantee?

But, from the data you provided the control system is working properly. As CPD increased while at Base Load the power output increased. And, the exhaust temperature decreased just as it should and would be expected to. If you're not getting the "bang for the buck" that you expected with the new fogger, it's probably not the turbine control system which is the problem; it seems to be working just as it should.

 

Thank you, Mr. Bob, for your explanation, but I'm still confused. Why will decrease on comp inlet temp increase the CPD? And if anyone has any experience on fogging system, how can the fogging system reduce the SFC?

Thank you.

 

What is fogging?

au revoir
Rahul

 

I don't think it is the decrease in inlet temp which is increasing the CPD. As the inlet temp decreases it cools the gas path and allows you to increase fuel flow / load. As the fuel flow increases and more work is done the combuster pressure increases and this in turn increases CPD.

 

If you really want to increase power output, just go with water injection into combustion chamber.

Putting a fogger increases the volumetric efficiency, which means that the saved power at the compressor will get from output.

Moreover, look for the surge margin of compressor (I am sure it will be considered by the fogger designer).

My opinion only.

Thanks,
QP

 

The state you seem to be in most of the time.

 

Thank you for all information, but somehow the supplier has made some modification and we can achieved 7% increase in rated power. Load 33.6 mw, exh temp 544oC, cpd 10.14 bar, comp inlet 23oC.
They just add some nozzle in evaporative section before the silencer.

Once again, thank you.

 

Which came first: the chicken or the egg?

The axial compressor is spinning at a constant speed and the IGVs are full open when the unit is at Base Load. Cooling the inlet air makes the air more dense. More air results in the ability to burn more fuel. More fuel results in a higher pressure in the combustor, which the axial compressor must overcome because the speed is constant and the IGVs aren't moving.

So, more air allows more fuel. More fuel results in a higher combustor pressure which results in a higher CPD.

So: Which came first?

 

The turbine runs extreeeeemly lean hence the only thing limiting fuel flow is the base load exhaust temp limit. More air, cooler air or wetter air will all cool exhaust temps and in turn increase power output / fuel flow at base load.

It was the egg.

 

Didnt quite get this one..

au revoir
Rahul