Inlet mass flow rate of air V/s CPD


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We have GE frame 9E Gas Turbine with Speedtronic Mark IV control system. I want to know inlet mass flow rate of air in compressor.We have no measurement means for inlet mass flow of air.

Can i calculate from CPD .If mass flow rate will increase obviously CPD will increase and vice versa but What is the relation of inlet mass flow rate of air Vs CPD exactly can anyone tell me . Is there any expression???

Is there any other method to indirectly calculate inlet mass flow rate of air.

Phil Corso, PE

Responding to BSJhala's 16-Dec-06, 9:29am query... I certainly could not be classified a GT compustion expert, but I know enough about boilers to ask:

Have you compared CPD results against (doing it the old-fashioned way) a fuel/combustion-gas analysis?

Phil Corso, PE
If you could describe what you intend to do with the data and how accurate you require it to be, it might be possible to derive some method of calculating the mass flow-rate of inlet air.

But to this author's knowledge, there isn't any reliable and/or accurate method of determining the mass flow-rate of inlet air without instrumentation in the inlet. The reference cited in the previous post seems to have been about the easiest method from an installation stand-point, and seems to be reliable.

Perhaps with some instrumentation and empirical data, some relationship could be established between CPD and the mass flow-rate of inlet air, but it would likely require at least a temporary installation of instrumentation.

But without some idea of what the data is to be used for and what degree of accuracy is required, it's pretty difficult to comment or provide any direction/information.

Actually I want to find out A/F ratio and hence mass of flue gases to find out effectiveness of boiler components.

We have no means of finding mass flow of air,also no measurement of gas flow.Hence No measurement of A/F and mass of flue gases.
Is there any other approach to find out this??

Need Immediate help

Phil Corso, PE

Rsesponding to BSJhala's 22-Dec quest... it can be done, within reasonable limits, with a fuel/combustion gas analysis!

Phil Corso, PE
Responding to Phil Corso's 23 Dec response to BSJhala's 22 Dec question....

Can you provide some details of how this would be done without instrumentation? This author can't think of any method--other than to have some idea of the rated air flow through the compressor at rated output (which isn't published anywhere this author knows of) and then using the fuel flow-rate measurement to approximate the air/fuel ratio--but that's all it would be is a crude approximation.

Heavy-duty combustion turbines flow a LOT of "excess" air--meaning MUCH more air is flowing through a turbine than is required for stoichiometric combustion of the fuel. That excess air is used for cooling and dilution of the combustion gases in a unit equipped with conventional combustors. It's used for premixing the air/fuel mixture in units equipped with DLN (Dry Low NOx) combustors.

So, it doesn't seem possible to work backwards from a stoichiometric ratio to determine air flow if one knew the fuel flow--on a heavy-duty gas turbine, because of the excess air flow.

But if you know of, and can provide details about, some relatively simple method of combustion analysis without knowing the air flow (only the fuel flow) of a heavy-duty combustion turbine, we'd all like to learn about it. Mr. BSJhala, especially, 'cause he seems to be in something of a hurry to do something which, without instrumentation and/or detailed documentation seems to be fairly difficult--nay, impossible--to do.

If one knew the O2 content of the exhaust gases of the gas turbine, would that information, along with the fuel flow-rate, be helpful in APPROXIMATING the air flow-rate?, presuming stoiciometric combustion? This information is sometimes available from emissions monitoring equipment....


Phil Corso, PE

Responding to markvguy's 24-Dec-06, 11:16 am comments... I too, realize that absolute representin excess air, are generally meaningless. However, they are an excellent tool when used for comparative purposes!

So in answer to your direct query... Yes, it is possible. Knowing the O2, plus CO, plus N2
(the latter is not a must), constituents of flue-gas will yield an excess-air figure using one simple formula. Accuracy can be increased if the constituents of the fuel are known.

As for your question about fixed instruments... None is required! As an EIT, I cut my Fuel-Oil Cumberology teeth on a manually operated Orsat analyzer. Initially on boilers, then furnaces, and much later comparing performance of identical gas-turbines.

Regards, Phil Corso, PE ([email protected])
Responding to Mr. Corso's response of 26 Dec 2006:

Can you tell us and Mr. BSJhala what he would need to have (data-wise) and how to calculate the inlet mass flow without installing any instrumentation? (This is, of course, assuming he has some exhaust emissions information--which he may or may not have)?


Phil Corso, PE

For markvguy and Mr. BSJhala... The only instrument required is an Orsat Analyzer or equivalent, and the tech or personnel to operate it!

For good references I used:

1) Combustion Section of my EE (Pwr) Course text:

Skrotzki, B.D., & Vopat, W.A. "Power Station Engineering"; McGraw-Hill Book Co.; 1960

And, 2 Publication:

Dukelow, S.G.; "The Control of Boilers";2nd Ed;1991
Tell us the ID & Wall Thickness of duct as well as confirm the flow rate, pressure & temperature. I would highly recommend use of VERABAR Flow Element or Thermal Mass Flow Meter for the said application.
I have done some calculations based on two equations and calculated Ma and Mf which is slightly different than Design.

I have Used two expressions

(1) (Wturbine - Wcomp)*shaftEfficiency=Output(MW)

(2) Heat added in Combustion Chamber

Solving This two equations I get Ma anf Mf.If I anyhow i can find out exact Cp for Compressor , Combustion , expansion results will be near to design.
Can Any one help in finding out this...
Is my approach right???

My Email Is [email protected]
Mr. Jhala (Bapu) kyana Chho?

It seems that you want system to monitor the boiler combustion efficiently. For that I believe you need following:

1. Zirconia Oxygen Analyzer
2. CO2 Analyzer
3. Air & Fuel (gas) flow meter - Thermal mass flow is an ideal choice for this.
4. Chartless Recorder with math function which takes data from all sensors & does the math calculation and gives you an idea about boiler efficiency. This is very simple system. Complexity of system increases as your requirement.

Hope this helps.

CPD is related to air flow, but is also a function of fuel flow. You should have noticed that when the IGVs (Inlet Guide Vanes) are fully open and the unit is not yet at Base Load, that CPD will increase as fuel is increased. Even when the IGVs are fully open, CPD will vary with fuel flow. This is because the unit is spinning at a constant speed (for generator drive units connected to a reasonably stable grid...) so air flow is relatively constant and the back-pressure in the combustor increases as more fuel is burned. So, there's no way to directly relate CPD and air flow. You really need some instrumentation or some really good constants for your equations--and even then you are only approximating air flow and fuel flow.

Are the boilers fired or unfired? Meaning, do they have duct-burners or not? Is your steam production down from normal for the same exhaust gas temperature? If the boilers are fired, are you having to fire them harder (use more fuel) to generate the same steam flow/temperature?

You really need some instrumentation to do what you're asking--whether or not the boilers are fired or unfired. And if the boiler is fired it would seem you would need to know what the O2 content of the turbine exhaust gas is, as well as the O2 content of the stack gas (boiler exhaust).

This author has seen boiler plates come loose and insulation be blown out from behind the plates and get "caught" on the boiler tubes/tube fins, greatly reducing boiler heat transfer and efficiency--on both fired and unfired boilers. This also increased gas turbine exhaust duct back-pressure, which caused the gas turbine efficiency and power output to decrease.