Hello,

I have some questions regarding the operation of the Extraction Turndown Valve during low load conditions in a gas turbine system. The control logic I have reviewed is as follows:

1. The CA_CRT (Combustion Reference Index) is calculated using the CTD (Compressor Discharge Temperature) and CTIM (Compressor Inlet Temperature).
2. This value is then compared to the current CA_CRT. If the current CA_CRT is equal to or lower than the required CA_CRT by the turbine, the Turndown Valve opens.

I would like to understand the following points:

1. Why are CTD and CTIM used to determine the operation of the Turndown Valve? - What is the reason for the influence of compressor inlet and discharge temperatures on the operation of the Turndown Valve?
2. What is the purpose of this control logic? - Is this to maintain combustion temperature, or is there another reason? For example, does it aim to maintain combustion stability, enhance efficiency, or serve another purpose?
3. Is this related to surge prevention? - If so, could you please explain how this control logic contributes to surge prevention?

Thank you.

 

@hohoim2,

WHAT MACHINE ARE YOU SPECIFICALLY REFERRING TO? (I'm presuming it's a GE-design heavy duty gas turbine (that presumption was made from the signal names you provided) and that's a Frame 6B, or A Frame 9E, or a Frame 9F/FA, or a Frame 9H/HA machine (or machines).

Because it's difficult for us to respond without knowing even just a little bit more about the machine(s) in question. The manufacturer; where the machine was packaged (I'm presuming it was in Belfort, France; but it may have been in India or ???); what the Frame size is; what the turbine control system is; what loads you are experiencing this phenomenon; if it is a GE-design heavy duty gas turbine with a GE Mark* turbine control system is the Mark* running the ARES--Adaptive Realtime Engine Simulation (or MBC, Model-Based Control)--software in addition to the application code for operating, monitoring and protecting the machine; and lastly, does the machine have DLN (Dry Low NOx) combustors, and if so, which version (I, 2, 2.6 2.6e, or???)?

Contrary to popular (and false) belief every GE-design heavy duty gas turbine is not like every other GE-design heavy duty gas turbine. Yes, they all suck (draw air into the axial compressor, squeeze (compress the air), burn (combust fuel with the compressed air), and blow (exhaust--either to the atmosphere or into a HRSG (Heat Recovery Steam Generator--a boiler) or some other process. BUT, often the similarities end at that point because different packagers of GE-design heavy duty gas turbine have their own auxiliaries and have even started naming functions and devices differently than the original designers and developers of the machines did.

I'm SWAGing (Scientific Wild-Arsed Guessing) that the "Extraction Turndown (Control) Valve" is what was affectionately called the Inlet Bleed Heat control valve for decades (a really, Really, REALLY poor name for its function, but that happens sometimes). It may even be that the site prefers to call the valve the Extraction Turndown Valve instead of the Inlet Bleed Heat control valve--it's possible, and if GE Belfort was involved it's even likely.

Basically, for GE-design heavy duty machines with DLN combustion systems the original designs of the combustion systems would not allow them to run in low NOX emissions mode much below about 70-80% of rated machine load. This was because in order to run at loads below that point it required reducing the mass flow of air through the machine to a point at which the combustion (burning) became unstable AND the axial compressor was outside of its mechanical design and safety margins. The engineers went back to the drawing board and came up with a method of extracting a small amount of axial compressor discharge air (CPD--compressor discharge pressure) and recirculating it back to the axial compressor inlet. The net effect of this was to help protect the compressor when the IGVs (Inlet Guide Vanes) were closed below the axial compressor design opening in order to allow the machine to remain in low NOx emissions mode at lower loads (or achieve low NOx emissions at lower loads when loading the machine). This is called "turndown"--remaining in low NOx emissions mode at loads lower than would otherwise be possible, and as described above it's accomplished by extracting some axial compressor discharge air and recirculating it back to the axial compressor inlet (bellmouth).

So, by extracting air from the axial compressor discharge to reduce the mass flow of air through the machine and recirculating the air being extracting back to the axial compressor inlet (where it raises the inlet air temperature a little bit--which reduces the density of the air which also helps to protect the compressor) the IGVs can be closed below the original design minimum operating opening which helps the machine get into or remain in low NOx emissions mode over a wider range of loads (the "turndown" function of the system).

So, it would seem that someone has changed the name of the Inlet Bleed Heat control valve to be the Extraction Turndown control valve. But, the function is still the same--if it's a GE-design heavy duty gas turbine running DLN combustion system.

You should notice that at the same time the Extraction Turndown control valve is opening the IGVs are probably closing are just ready to begin closing (when unloading the machine), or it's closed below the normal minimum operating angle (57 DGA?) and is nearing 57 DGA (as the machine is being loaded). They all work together to help protect the axial compressor so that the machine turndown (sometimes called the turndown ratio) can be increased which means the load range of the machine is enhanced (widened) while operating in low NOx emissions mode.

Anyway, without knowing a lot more about the machine--and guessing it's a GE-design heavy duty gas turbine with a DLN combustion system--that's all I can add.

 

@hohoim2,

WHAT MACHINE ARE YOU SPECIFICALLY REFERRING TO? (I'm presuming it's a GE-design heavy duty gas turbine (that presumption was made from the signal names you provided) and that's a Frame 6B, or A Frame 9E, or a Frame 9F/FA, or a Frame 9H/HA machine (or machines).

Because it's difficult for us to respond without knowing even just a little bit more about the machine(s) in question. The manufacturer; where the machine was packaged (I'm presuming it was in Belfort, France; but it may have been in India or ???); what the Frame size is; what the turbine control system is; what loads you are experiencing this phenomenon; if it is a GE-design heavy duty gas turbine with a GE Mark* turbine control system is the Mark* running the ARES--Adaptive Realtime Engine Simulation (or MBC, Model-Based Control)--software in addition to the application code for operating, monitoring and protecting the machine; and lastly, does the machine have DLN (Dry Low NOx) combustors, and if so, which version (I, 2, 2.6 2.6e, or???)?

Contrary to popular (and false) belief every GE-design heavy duty gas turbine is not like every other GE-design heavy duty gas turbine. Yes, they all suck (draw air into the axial compressor, squeeze (compress the air), burn (combust fuel with the compressed air), and blow (exhaust--either to the atmosphere or into a HRSG (Heat Recovery Steam Generator--a boiler) or some other process. BUT, often the similarities end at that point because different packagers of GE-design heavy duty gas turbine have their own auxiliaries and have even started naming functions and devices differently than the original designers and developers of the machines did.

I'm SWAGing (Scientific Wild-Arsed Guessing) that the "Extraction Turndown (Control) Valve" is what was affectionately called the Inlet Bleed Heat control valve for decades (a really, Really, REALLY poor name for its function, but that happens sometimes). It may even be that the site prefers to call the valve the Extraction Turndown Valve instead of the Inlet Bleed Heat control valve--it's possible, and if GE Belfort was involved it's even likely.

Basically, for GE-design heavy duty machines with DLN combustion systems the original designs of the combustion systems would not allow them to run in low NOX emissions mode much below about 70-80% of rated machine load. This was because in order to run at loads below that point it required reducing the mass flow of air through the machine to a point at which the combustion (burning) became unstable AND the axial compressor was outside of its mechanical design and safety margins. The engineers went back to the drawing board and came up with a method of extracting a small amount of axial compressor discharge air (CPD--compressor discharge pressure) and recirculating it back to the axial compressor inlet. The net effect of this was to help protect the compressor when the IGVs (Inlet Guide Vanes) were closed below the axial compressor design opening in order to allow the machine to remain in low NOx emissions mode at lower loads (or achieve low NOx emissions at lower loads when loading the machine). This is called "turndown"--remaining in low NOx emissions mode at loads lower than would otherwise be possible, and as described above it's accomplished by extracting some axial compressor discharge air and recirculating it back to the axial compressor inlet (bellmouth).

So, by extracting air from the axial compressor discharge to reduce the mass flow of air through the machine and recirculating the air being extracting back to the axial compressor inlet (where it raises the inlet air temperature a little bit--which reduces the density of the air which also helps to protect the compressor) the IGVs can be closed below the original design minimum operating opening which helps the machine get into or remain in low NOx emissions mode over a wider range of loads (the "turndown" function of the system).

So, it would seem that someone has changed the name of the Inlet Bleed Heat control valve to be the Extraction Turndown control valve. But, the function is still the same--if it's a GE-design heavy duty gas turbine running DLN combustion system.

You should notice that at the same time the Extraction Turndown control valve is opening the IGVs are probably closing are just ready to begin closing (when unloading the machine), or it's closed below the normal minimum operating angle (57 DGA?) and is nearing 57 DGA (as the machine is being loaded). They all work together to help protect the axial compressor so that the machine turndown (sometimes called the turndown ratio) can be increased which means the load range of the machine is enhanced (widened) while operating in low NOx emissions mode.

Anyway, without knowing a lot more about the machine--and guessing it's a GE-design heavy duty gas turbine with a DLN combustion system--that's all I can add.

We have a GE 7FA type gas turbine with a Mark VIe control system. The turbine is rated for 188MW, but it is typically operated at a low load of around 100MW, during which the turndown valve opens. I am not familiar with ARES or MBC, but I know that our system uses DLN2.6. The turndown valve is not the IBH; it extracts air from the 9th and 13th stages of the compressor and sends it to the 2nd and 3rd stages of the turbine for cooling purposes.

Additionally, there are a total of 4 turndown valves, with two each extracting air from the 9th and 13th stages of the compressor. The air extracted from the 9th stage is sent to the 3rd stage of the turbine, while the air extracted from the 13th stage is sent to the 2nd stage of the turbine.

 

Okay, so it sounds like the OEM has some new control schemes and equipment that I'm not familiar with.

Have you looked in the Operations & Maintenance Manuals provided with the machine to see what it says about the extraction turndown valves?

 

Okay, so it sounds like the OEM has some new control schemes and equipment that I'm not familiar with.

Have you looked in the Operations & Maintenance Manuals provided with the machine to see what it says about the extraction turndown valves?

Those guys at GE must have made the manual terribly because it doesn't provide detailed explanations about not just the turndown valve, but other things as well. As far as I can tell, the reason the turndown valve opens at low load is to prevent surging. However, I don't understand why they would use the Compressor Discharge Temperature (CTD) for this judgment. It seems more logical to use pressure. According to the logic, the valve opens and closes based on a comparison between CTD and CTIM (correction value), and this doesn't make sense to me.

 

@hohoim2/@hohoim2,

GE manuals are not the greatest in the world; in fact they are not really very good (though the one thing GE Belfort does reasonably well is assemble good manuals).

I presume the extraction turndown valves/operation would be "covered" (probably in very brief detail if at all) it would be in the tab/section that describes the Cooling & Sealing Air system (and that's just a SWAG--Scientific Wild-Arsed Guess).

I also guess the unit you are working on has ARES/MBC--which is a simulation of how an ideal engine would run for the present ambient and operating conditions. And this simulation is considered to be proprietary information of the General Electric Company, which means it's not very well documented because they believe it gives them an edge in the combustion turbine marketplace and they don't want people to really understand or learn how it works and copy it. Actually, GE believes most all of their control schemes and philosophies are proprietary and don't want to document them very well.

You might find more information in the 7F Users Group you can join if you work at a plant that uses GE 7F machines (use your preferred search engine to find the organization's URL).

Please let us know what you discover. Thanks!