difference between AFQ & AFQD signals for GE turbine

A

Thread Starter

Ali

I would like to know the difference between two signals existing in GE MARK VI for 7F DLN2.6 Machines:

1)AFQ: compressor inlet air mass flow.
2)AFQD: compressor inlet dry air mass flow.

What is the difference physically?

In Ideal condition their reading should match or differ? they are exactly matching in one machine, while there is a slight difference between them (400 AFQ &394AFQD).

I am in doubt which condition is better in term of performance?

Thanks in advance.
 
Ali,

Without being able to see the application code running in the Mark VI at your site, it's rather difficult to say with any certainty--but we can make some presumptions and point you in the proper direction to understand the difference.

AFQD seems to be a measurement of axial compressor inlet air flow with the moisture content subtracted from the calculation. To do this it's probably likely there is some kind of humidity sensor, likely a specific humidity sensor, or a dew point sensor the output of which can be converted to specific humidity--and then the humidity (moisture) can be subtracted from the total air flow (AFQ) to derive the dry air flow (AFQD).

Someone needs to review the application code running in the Mark VI, using Toolbox, to determine exactly what's being done. Use the 'Finder' feature of Toolbox to look up where the signal AFQD is written to, and following the signal "backwards" through the application code it will likely become very clear how the "dry" air flow calculation is performed after the "total" ("wet") air flow calculation is performed.

As for whether or not they should be the same--well that depends on the humidity of the air entering the axial compressor at your site. Is it a humid ambient? Is there any evaporative cooling on the inlet air? I would say that, in general, the two should not be the same (unless the air entering the axial compressor is VERY dry), and that the possible cause for no difference would be an erroneous humidity/dew point sensor input.

It's also very possible that a dry air flow calculation is a purchased option that was not purchased at the time the turbine was ordered. GE quite frequently leaves unused signals in application code. If there is no moisture/humidity/dew point sensor on the inlet providing a signal to the Mark VI that can be used to calculate moisture in the inlet air, it's very likely this option was not purchased.

Toolbox can also be used to see where the two signals are used in the application code--if at all. Sometimes they are just provided for monitoring, or for use in some performance/heat rate calculation performed by another control system (DCS) or computer in the plant. Again, without being able to see the application code running in the Mark VI at your site, it's very difficult to say with any certainty what the signals are being used for, and how--exactly--they are calculated or determined.

Hope this helps!
 
Dear CSA,

I checked the toolbox and found that we have a faulty dew point sensor in the second unit and since that time AFQ & AFQD started to match and they were different before the fault happened.

I found the block in mark VI but I was unable to find how this calculation is going.

Thanks for your explanation that guided me to find the answer for my doubt. you are really super expert.
 
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