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Gas Turbine Control and Efficiency
Gas turbine temperature control system and efficiency

Dear all,
I have read some of the interesting thread here about gas turbine temperature control. I am interested in gas turbine control and its efficiency at part load.

The gas turbine operation is normally controlled by manipulating fuel flow and IGV to meet load and control turbine inlet temperature (as measured by exhaust temperature).

From what i've read, the GT can be controlled by:
1) manipulating fuel only to meet load (approximately constant air flow)
2) manipulating both fuel and IGV to meet load and exhaust temperature.

For GT with simple cycle operation (no HRSG), at part load operation, the efficiency of fuel control only can be higher than control with fuel and IGV control.

The GT that I look for shows high turbine exhaust temperature of about 1100 deg F even in part load (ambient temp of 80-90 deg F), with the brochure indicating exhaust temperature of just 700-800 deg F (iso).

Can we just use fuel control only (to reduce exhaust temp and increase efficiency)? or the GT is automatically configured to use both IGV and fuel control for simple operation at part load operation?



>The GT that I look for shows high turbine exhaust
>temperature of about 1100 deg F even in part load (ambient
>temp of 80-90 deg F), with the brochure indicating exhaust
>temperature of just 700-800 deg F (iso).

What kind of combustion system does the GT you look for have? Conventional combustors or DLN combustors?

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Also, does the turbine you look for exhaust to atmosphere (simple cycle), or into an HRSG (Heat Recovery Steam Generator; a boiler to produce steam for a steam turbine and/or nearby process; combined cycle)?

The IGVs can be used for MANY things, and their use depends on the type of combustion system (conventional or DLN) and the application of the GT (simple- or combined cycle).

When the GT exhausts into an HRSG to produce steam, using the IGVs to increase the exhaust temperature at Part Load (less than Base Load) actually improves the overall plant efficiency even though it slightly decreases the GT efficiency at Part Load. So, even though the heat rate for the GT increases at Part Load when the IGVs are modulated to maximize exhaust temperature going into the HRSG to produce steam, the overall plant efficiency (heat rate) decreases because the steam production is increased. So, it's not a net loss.

If the GT exhausts directly to atmosphere there is no need to maximize exhaust temperature so the IGVs are not modulated to increase exhaust temperature--for conventional combustor-equipped units. This maximizes efficiency at Part Load, but the GT is still not as efficient at Part Load as it is at Base Load (the definition of which is that the IGVs are at maximum operating angle).

For units with DLN combustors, the IGVs are modulated to maintain stable flame which means that the air flow has to be reduced at Part Load operation in order not to "lean out" the fuel/air mixture excessively and have stable combustion while still reducing emissions to the extent possible. So, whether or not the GT exhausts to atmosphere (simple cycle operation) or into an HRSG (combined cycle operation) the IGVs have to be modulated in order to control air flow to maintain stable combustion.

The effect of modulating IGVs to maintain stable combustion at Part Load for DLN combustor-equipped machines is to increase exhaust temperature (and turbine inlet temperature). It's something of an undesirable effect, particularly if the unit will be operating in simple cycle mode. But, if the unit is operating in combined cycle mode it's actually desirable because, again, overall plant efficiency increases at Part Load AND steam production is maximized at Part Load.

IGVs are only modulated (closed) at Part Load during combined cycle operation of conventional combustor-equipped units to the extent that the turbine inlet temperature does not exceed the maximum design value for that operating condition. That's what sets the limit for the IGV "opening"--the calculated exhaust temperature reference, TTRX, which sets the turbine inlet temperature. It would be ideal if the IGVs could be closed even further in some operating conditions which would result in elevated exhaust temperatures and turbine inlet temperatures--but that would be bad for the turbine section (nozzles and buckets) and the GT exhaust diffuser. IGVs are NOT modulated to control turbine inlet temperature or exhaust temperature, rather as fuel is increased when loading the IGVs are opened while maintaining a stable turbine inlet temperature (the exhaust temperature, at higher loads, will drop for the same turbine inlet temperature--recall the exhaust temperature reference curve has a negative slope). It's really mistake to say the IGVs are used to control turbine inlet temperature or exhaust temperature. They are modulated in response to the turbine inlet temperature which is a function of exhaust temperature and CPD (which is also affected by IGV angle).

It's really a tricky thing to explain without graphs and drawings, but for machines operating at Part Load with DLN combustors, or machines operating at Part Load with conventional combustors operating in combined cycle mode, the IGVs are modulated to control air flow.

For machines with conventional combustors reducing air flow results in increased exhaust temperatures--which is what is desirable at part load when producing steam in an HRSG with the GT exhaust.

For machines with DLN combustors, controlling air flow is necessary to maintain stable combustion while reducing emissions and the effect of controlling air flow with the IGVs is that the exhaust temperatures are increased at Part Load. If the unit is exhausting into an HRSG, that's all well and good. If the unit is exhausting to atmosphere (simple cycle mode), IGV modulation to control combustion air flow is still a MUST, which results in higher exhaust temperatures and decreased Part Load efficiency, but it's necessary for proper and stable combustion. IGV modulation can't be avoided for units with DLN combustion systems; it's a necessary "evil."

I hope this helps even a little bit. If there was ONLY conventional combustors or ONLY DLN combustors, there would be only one answer. But, since you chose not to tell us what kind of combustion system the unit you look for has, the answer is more complicated and nuanced.

Please don't think of the IGVs are being anything more than controlling air flow--because that's all they really do. And, controlling air flow results in exhaust temperature variations. And there are limits to exhaust temperature variations, and turbine inlet temperature variations. Those limits determine the IGV angle--and they are closed-loop, meaning there is a temperature reference and the IGVs are modulated (opened/closed) as necessary to make the actual temperature equal to the temperature reference. And in NO case can the exhaust temperature ever exceed the calculated exhaust temperature reference/limit, TTRX, which defines the turbine inlet temperature. That's all; the IGVs are modulated to control air flow (which isn't always measured, and is rarely used as a setpoint/reference for IGV angle). The effect of controlling air flow is to vary exhaust temperature, which affects turbine inlet temperature.

CSA, Thank you for your reply.

The gas turbine that i am looking for is operated in simple cycle only (NO HRSG, just gas turbine). The gas turbines are used for offshore application to generate electricity. The GTs have been used for about 18 years.

So lets limit to gas turbine operating in simple cycle mode only (NO HRSG). If the GT uses DLN, it means that IGVs are being used to control air flow, hence the IGV is not available to be adjusted.

So, to increase efficiency:
for GT having conventional combustor and operated in simple cycle, are the IGV automatically adjusted to maintain TTRX (closed loop)? or are they automatically opened to give lower TTRX in simple mode (closed loop)? or should it be done in open loop (manually), considering GT has override control (to avoid important problem)?

Thank you in advance.

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For a machine of that vintage (age) with conventional combustors and operating in simple cycle mode the IGVs will remain closed as the unit is loaded from generator breaker closure until the exhaust temperature (TTXM) reaches either 700 deg F or 900 deg F (I can't recall which, but I think it's the latter). At that point as the fuel is increased to increase load the exhaust temperature will tend to increase--but the IGVs will automatically open to maintain the setpoint (700 or 900 deg F), until the IGVs are fully open (at maximum operating angle). At that point as fuel is increased to increase load, the IGVs will be at the maximum operating angle so increasing fuel will cause the exhaust temperature (TTXM) to increase. Once the exhaust temperature (TTXM) reaches the exhaust temperature reference/limit (TTRX), the unit will be at Base Load.

Is there a method for measuring heat rate (efficiency) on the GT? Efficiency at Part Load is never going to be high (a low heat rate); even if the IGVs are not being modulated to maintain a high GT exhaust temperature. It's just not the way the machine is designed--which is to run at Base Load (that's the way GE-design heavy duty gas turbines were designed to run: at Base Load) and so the efficiency is best (the heat rate is the lowest) at Base Load when the IGVs are fully open and the GT exhaust temperature is equal to the exhaust temperature reference/limit.

The best way to keep GT efficiency high at any load is to make sure the inlet air filters are clean and the axial compressor is clean. A dirty axial compressor is an inefficient axial compressor, and since the axial compressor consumes the majority of the torque produced by the turbine section keeping the axial compressor clean increases the overall GT efficiency.

But, unless bonuses are being paid for optimizing efficiency AND there is an accurate method for measuring efficiency (heat rate) AND the unit can be shut down for off-line axial compressor water washes as needed, the efficiency of the GT is going to be whatever it is because of the conditions the unit is being operated at and the design of the machine. It's probably likely that on an off-shore rig the unit doesn't run at Base Load very often (if at all), so the efficiency will never be as high (lowest possible heat rate) as it could be--and that's just a condition of operation in that application.

There's nothing an operator or control system can do--other than to try to keep the axial compressor and inlet air filters as clean as possible if trying to maximize GT exhaust temperature. The IGVs can't be scheduled (programmed) to do anything differently than they are currently doing (unless someone has selected Combined Cycle Mode for some unknown reason and the IGVs are maximizing exhaust temperature--which would be odd if the unit is exhausting to atmosphere and not into an HRSG).

The reason the IGVs try to maintain the 700 or 900 deg F when at Part Load is that the designers are trying to minimize combustor pressure oscillations at part load with the IGVs--and it has proven to be very effective. Combustor pressure oscillations can do to two things: first, they can cause accelerated wear on parts that rub against each other (hula seals; fuel nozzles on the combustion liners; transition piece seals to the first stage nozzle segments), and, second, they can lead to flame instability (in the worst case), which can cause loss of flame trips. So, it's best NOT to try to change the way the IGVs hold the intermediate exhaust temperature at Part Load. (In case you were wondering if that might be a possibility.)

Hope this helps! Please do tell if there is a highly accurate method for measuring heat rate (performance; efficiency) on the unit(s) at Part Load. Because if there isn't, then it would likely be very difficult to tell the results of any attempt to increase Part Load efficiency.