I am working in GE7FA gas turbine. GT at part load operation exhaust temperature is maintaining high-650 deg.C though the IGV contains huge cushioning. What could be the reason to maintain the high exhaust temperature at part load condition other than the consideration of improvement of part load efficiency?

Regards,

[email protected]

 

Does the GE 7FA have conventional or DLN combustors?

 

Sir,

>Does the GE 7FA have conventional or DLN combustors? <

GE 7FA machine with DLN 2.6

 

I would suggest that you need to study the available documentation on how the DLN combustion system on your turbine works.

The Inlet Bleed Heat system in conjunction with the Inlet Guide Vanes are used to control the air flow through the turbine, like the throttle plate of a carburetor or a fuel injection manifold. It's necessary to limit air flow to prevent leaning the air/fuel mixture too much which will cause instability and combustion problems.

The problem with using the Inlet Guide Vanes to control (limit) air flow is that it causes the exhaust temperature to go up (less air for the same fuel means higher exhaust temperatures).

So, on machines with DLN combustion systems, the exhaust temperatures will usually always be very high during most of the load range, not like a machine with conventional combustors.

But, do find the available documentation provided with the unit; it will have a lot of good, basic information. And not just on the combustion system!

 

Sir,

Normally in the gas turbine part load operation, maintain the load by regulate the fuel control valves, but exhaust temperature is controlled by the IGV. If GT is under base load operation i.e. IGV is in full open condition, maintain the GT exhaust temperature only on the Fuel gas control valves. Based on the corrected exhaust temperature reference set point (TTRX) the Fuel gas control valve will regulate to maintain the GT exhaust temperature(TTXM) irrespective of the load.- there is no option to control the exhaust temperature in this condition except fuel gas control. At part load operation IGV will modulate with co-ordination of IBH to keep Pre-mix operation into service below the 80% of the Compressor load also for DLN.

My question is, even though we are having lot of cushioning in IGV to reduce the exhaust temperature of Gas turbine at part load, Why we are maintaining high exhaust temperature at part load operation as compare to the base load operation.

waiting here for your immediate reply
regards

 

Units with conventional combustors not operating in IGV exhaust temp control at Part Load (below Base Load) MAY have the IGVs full open at some part load conditions. This is usually for simple cycle machines not wanting to maximize exhaust temperature to maximize steam production at gas turbine part load. On such units, at part load operation with the IGVs full open, there is no "exhaust temperature control"; just Droop Speed Control. Exhaust temperature control doesn't kick in until Base Load.

DLN combustion control is very different from convention combustion control. Because in DLN combustion the air/fuel mixture is very lean at all times, and because the axial compressor is rotating at a constant speed, the only "adjustment" for controlling air flow as fuel flow is changed is the IGVs (and in some circumstances), the IBH system. IBH is usually the means by which premix operation is extended versus units without IBH (which is usually called "turndown"), but I believe that all F-class machines use IBH so they all operate more or less similarly in this respect (turndown).

DLN combustors, by virtue of their design (with no "moving parts" to control air flow), must have some other means of controlling air flow (the IGVs) to prevent excessively leaning-out the air-fuel mixture at part load if the IGVs were wide open at part load. An effect of closing the IGVs to limit the air flow is that for the same fuel flow the exhaust temperature will be higher (no cooling or dilution effects from higher air flow).

The design of the DLN combustor is such that at Base Load with the IGVs full open, the flame stability is at it's best, and the air/fuel mixture is optimized and is very lean to keep NOx emissions very low. If the IGVs were left open as fuel was reduced, the air flow would stay relatively high and the air/fuel mixture would become even more lean, which would reduce the flame stability, increase dynamic pressures in the combustion cans, and negatively impact emissions. If the air/fuel mixture leans out too much, the unit will be tripped.

So, the IGVs, as the only method of controlling air flow while the compressor is operating at a constant speed, are used to reduce air flow as fuel is reduced. And the effect of that is to increase exhaust temperature above what would experienced if the IGVs are left full open.

Conventional combustors have a much more stable flame, and a higher range of air/fuel possible air/fuel ratios.

 

The primary purpose of the Inlet Bleed Heat is to keep the compressor out of surge (pulsation).

Inlet guide vanes are also used for this purpose during startup. Once the unit is up to operating speed there are 2 types of control for the IGV's:

1. Simple cycle applications: The IGV's open early in the loading cycle and stay open. This minimizes exhaust temperature for part loads. DLN may have some impact on this, but once the unit reaches emission compliance mode, the guide vanes should go full open.

2. Combined cycle applications: The IGV's are modulated to maximize exhaust temperature to improve combined cycle performance (maximizes steam temperature and keeps it relatively constant over a large load range).

 

Dear Bhagvan,

Is your GT open cycle or connected with Waste Heat Recovery Boiler?

The exhaust Gas temperature will be maintained at higher side for maintaining the required steam temperature at part load of GT.

Regards.
Pandey

 

Dear Sir,

Thanks to all,

Basically GT was introduced to operate for peak load i.e. designed a gas turbine for frequency response reserve. When ever the frequency drops, standby GT's will come into picture to maintain the frequency.

Gas turbine work ratio(Net work output ratio is based on the Turbine inlet temperature which is equal to the maximum cycle temperature. when ever the turbine inlet temperature is high, Net work ratio also high. This is possible only to maintain the maximum turbine inlet temperature by maintaining higher turbine exhaust temperature with reference to Brayton cycle process 3 to 4)

So, If we are maintaining lesser exhaust temperature in the part load leads to reduce the efficiency.

As per your explanation GT exhaust temperature is high for combined cycle plant to maintain the sufficient heat input to the HRSG. But If we reduce the exhaust temperature by increase the mass flow of GT(More opening of IGV) which will source to the HRSG will increase with out effect the thermal shock(At higher quantity of mass flow of flue gas with lesser temperature, more quantity of steam will generate which is useful to create the more cooling media in the boiler vessels.

and also due to higher mass flow of air, compressor power consumption will increase and net work out put will drop. So, to maintain the constant load at any condition, fuel gas input will increase.

Irrespective of load, always design consideration like GT exhaust pressure, temperature and surge protections will consider for operation of IGV.

Basically Inlet Bleed heat is designed to avoid the anti icing and keep Pre-mix into service at lower compressor loadings. Normally two types of combustions- Diffusion and Pre-Mix.

If we keep in the diffusion combustion, combustion temeperature will be high due to rich combustion for initiating strong flame. Once 80% of load reaches it will convert to pre-mix combustion which is lean combustion.
Rich combustion leads to increase the NOx. So to keep the Pre-mix into service at lower loads also by increase the compressor loading by increase the compressor inlet temperature.If the load is increasing corresponding IBH flow will reduce.

waiting here for your reply

Regards

 

You don't ask very many questions. And, your statements are kind of confusing. It's not clear if you're asking for confirmation of your understanding or if you're making statements about gas turbine operation.

GE-design Inlet Bleed Heating (IBH) was originally designed for anti-icing (but, I'd be willing to bet that you have no need for anti-icing at your plant).

Just because a unit has IBH <b>does not</b> mean it is used as an anti-icing function, which requires more equipment and control system functionality than an IBH system used for DLN combustor-equipped machines does. It's been said before on control.com: GE generally doesn't recommend anti-icing controls for their heavy duty gas turbines unless there is an un-natural source of humidity that can be drawn into the axial compressor (turbine) inlet at low ambient conditions.

Just as otised said: On a unit equipped with DLN combustors IBH is used in conjunction with operating IGVs at less than "normal" axial compressor design conditions to prevent axial compressor damage.

When the designers of the DLN combustion system had a need for recirculating a portion of the axial compressor discharge air they initially used an IBH system design. Unfortunately, the name of the system was never changed to reflect the new usage, even though the physical system has evolved over the years to something much more than the anti-icing system it was initially.

It's important to remember: DLN combustion was applied to existing turbine, and axial compressor, designs and the IGVs were not usually designed to operate at angles less than approximately 57 degrees.

As noted previously, it's necessary to reduce the air flow through the turbine to maintain a proper air/fuel ratio that is already very lean at low fuel flows (loads). (Remember, the axial compressor is spinning at a constant speed which can't be varied.)

So, the only way to reduce air flow is to reduce the IGV angles below axial compressor design conditions, and to do so without causing axial compressor damage the IBH system is used to recirculate a portion of the compressor discharge back into the axial compressor inlet which serves to heat the inlet air which serves to reduce the density of inlet air which serves to provide some additional safety margin at low IGV angles. Lower density of air decreases the work done by the compressor.

The points at which "full" premix operation is achieved differ depending on the turbine and it's equipment and the version of DLN combustion (DLN-I, DLN-2.0, DLN-2.6, and DLN-2.6e, etc.). But for most DLN-I equipped units operating without IBH (or without IBH being active), the transition from diffusion flame to premix operation typically occurs at about 80% of rated output.

Good luck with your understanding. If you have specific questions, please phrase them in the form of a question.

 

Good evening everybody,

My machine(GE7FA) was designed to operate the anti icing. Why because my Reference site conditions(RSC) are- 1.01325bar ambient pressure, 46 Deg.C temp and 100% RH.

At the time of winter it is coming below the 2 Deg.C and 30%RH. Normally below the 7.2 Deg. C of compressor inlet temperature my evoporative cooling system will trip.(As per the normal criteria less than 42Deg. F not allowed)

In the winter condition, irrespective of load condition IBH will come into picture to maintain the compressor inlet temperature.

Thank u for your explanation but still I am having doubt on your explanation i.e.

1. IBH only for Anti-icing?

2. what is the suitable condition(80% rated output normally required to keep Pre-mix into service for dual type combustion machines like diffusion and pre-mix type) for pre mix operation and what is the relation between the DLN to Pre-mix combustion(pre mix operation NOx levels are Low)?

3. Is it Quantity of air flow requirement(Basically air to fuel ratio is high for gas turbine to create cooling media in the combustion chamber) for combustion at part load will change by keeping 5% recirculation?

4. By increase the inlet temperature due to IBH, compressor outlet temperature also change i.e. cooling media(air) to the combustion chamber with high temperature. So, how the quantity of air flow will be fixed at different compressor inlet temperature conditions?

regards
Bhagavan

 

Good morning, Bhagavan,

Questions. This is better; thank you.

Please understand, there are many aspects to the questions you are asking. A former colleague of mine was always heard to say, "Engineering is a series of compromises." That's a very simple, powerful and true statement. The compromises include physical principles as well as economic considerations.

A lot of things have to work together for DLN combustion to produce low emissions reliably. Please also remember that DLN combustion is an "auxiliary" to gas turbines. By that I mean, that DLN combustion systems were fitted to turbines that were originally designed for conventional, diffusion flame combustors. So, some compromises had to be, and were, made.

Before we go any further, I want to be very clear: I am not a combustion engineer or a turbine designer. All of my understanding has come from years of on-the-job training and listening to conversations and talking with combustion engineers and turbine designers and reading anything I could get my hands (and eyes) on. Almost none of this stuff is written any where in one place (IP, Intellectual Property considerations; trade secrets; and all that), but bits and pieces of it can be found in a lot of diverse and different documents. This makes gaining knowledge and understanding very difficult.

So, all of the below is based on my understanding, from what limited information was presented in various training courses, what was gleaned from being involved in some development activities (as a technician!), from just picking up bits and pieces here and there over the years, and reading anything I could find on the subject. I have not written anything I have not been told by reputable sources and have to come know and understand, or read in publicly available documentation or that would be considered to be a trade secret or proprietary information.

1. IBH was initially designed for non-DLN machines and used for anti-icing protection. I'm very confused about your site and your questions and statements.

On the F-class machines I'm familiar with IBH was only provided for start-up and shutdown compressor protection because the IGVs were operated below approximately 57 degrees on units with DLN combustors. It's a "benefit" that IBH can be used for both compressor protection during start-up and shutdown and for anti-icing protection; it reduces the costs of having two similar systems for two different conditions, as well as the complexity of the control system.

I would suggest that you refer to the condition whereby the IBH system is being used for anti-icing protection as 'anti-icing', not 'IBH'. In general, IBH is not user-enabled or disabled on F-class units (it can be on some B/E-class units with DLN-I combustors). So, it's probably going to be enabled during start-up and shutdown and doesn't require user/operator assistance or input. In that case, I would suggest you refer to the operation of the IBH system during start-up and shutdown and part load operation as 'compressor protection'.

2. The suitable condition for Pre-mix operation is a function of flame stability, combustor dynamics, and emissions level. These conditions are simulated in combustion laboratories and each one has an acceptable range. The results of these tests and development are proprietary.

The challenge (for any manufacturer) is to design a system that can be mass-produced and applied where the acceptable ranges "overlap" each other so that the flame is stable, the combustor dynamics are acceptably low, and the emissions meet desired levels. This is a function of the hot gas path components (fuel nozzles, nozzle orifices, combustors, etc.), the <b>expected</b> fuel characteristics, the site ambient operating conditions, and the fuel control valves chosen for the application.

The instrumentation used in a combustion lab to determine the proper components and air/fuel flows would make the cost of a turbine prohibitive (too expensive). There are several companies now offering various packages of instrumentation and software to monitor gas turbine combustion on a real-time basis and make adjustments while running. But, these are not required for basic turbine operation and power production.

And, every time one tries to tweak a little more out of a turbine by running it closer and closer to its limits, one increases the complexity of operation, the cost of maintenance, and, in my personal opinion, reduces the reliability and even the availability of the turbine. The instrumentation and software is just too expensive and requires a high amount of maintenance and calibration and adjustment. I question whether the power output gains over the estimated life of the machine outweigh the increased maintenance costs and outages caused the instrumentation.

In addition there is a steep learning curve for understanding all of the idiosyncracies of operation at this extreme or that extreme or without this input or that output. It can be very, Very, VERY complex and usually there's only one or two people on a site who are really knowledgeable about the system and they don't usually share their knowledge and they quite frequently change jobs because of their experience. So, the sites are left hanging when the person leaves.

The Speedtronic turbine control system is adequate and sufficient for the purposes of basic turbine operation while maintaining desired emissions (guarantees) over the expected ambient operating conditions.

I believe that GE is continuing to offer more and more enhancements through the Speedtronic to optimize F-class turbine operation so as to maximize efficiency over as much of the load range as possible. This requires some very sophisticated control algorithms and are usually options, not standard features available in a basic unit package. This would narrow some of the ranges in an effort to improve efficiency.

3. You do make some unusual associations. You seem to be referring to "cooling and dilution" air in a combustor. In a conventional combustor with a pure diffusion flame, cooling and dilution air is required to reduce the temperature of the flame to an acceptable level for passing through the turbine nozzles and buckets. This is done by placing slots and holes in the combustion liner through which a relatively large portion of the axial compressor discharge air flows to reduce the flame temperature. Some of the compressor discharge enters the combustor near the fuel nozzle and is used for combustion, and in a conventional combustor the air/fuel mixture is much richer than in a DLN combustor.

In a DLN combustor, almost all of the air enters the combustion liner through the end of the liner near the fuel nozzles. This is so that the air/fuel mixture is a lean "as possible". A lean air/fuel mixture burns much cooler (hence, the reduced NOx formation!), and therefore the combustion gases don't require nearly as much cooling and dilution air flow as a conventional combustor requires. There are usually very few cooling and dilution slots and holes in the body of a DLN combustion liner.

But, directing most of the axial compressor discharge air into the area of the combustor where the fuel nozzles are presents a problem for low load operation (low fuel flows). Premix combustion requires a very lean air/fuel ratio, and the air flow through the turbine is a function of the IGV angle (primarily). The designs of most F-class compressors are such that there is a minimum IGV angle below which the compressor operation becomes unstable and the compressor can suffer catastrophic failure.

However, by recirculating a portion of the axial compressor discharge flow to the compressor inlet the temperature of the air entering the compressor inlet is increased which reduces the density of the air entering the compressor (and also reduces the efficiency of the turbine) the compressor protection is improved.

So, the primary purpose of recirculating compressor discharge air through the IBH control valve and into the axial compressor inlet is to protect the compressor, since cooling of combustion gases/flame temperature isn't as important because the combustion is occurring at a lower temperature than in a unit with conventional, diffusion flame combustors. The air flowing into the combustor is a function of the IGV angle. The IGV angle, if too low without some other method of protecting the compressor, can cause compressor failures. So, the IBH system is used to protect the compressor at low IGV angles.

4. I don't really understand this question, and I think the answer is in 3, above. The combustors have fixed openings into which axial compressor air flows. Again, there are acceptable ranges of operation which each turbine is designed for.

 

If the gas turbine is working in combined cycle mode then it is desirable to have high exhaust temp for better recovery in HRSG for improving combined cycle efficiency.

That is why you have option to run in combined cycle mode and in open cycle mode

 

If IGV will open fully at part load condition the inlet air flow will increase and your compressor pressure ratio will change..

> My question is, even though we are having lot of cushioning in IGV to reduce the exhaust temperature of Gas
> turbine at part load, Why we are maintaining high exhaust temperature at part load operation as compare to the
> base load operation.

 

That is only when you are driving your gas turbine in conjunction with an HRSG. So that at low loads (when you have low exhaust gas mass flow) you can compensate for the low steam production via high exhaust gas temperatures.