If possible, I hope to know the reason why Combined cycle efficiency (or heat rate) is the highest (or the lowest in the heat rate) in ISO temperature. (59 F, 15&#8451 and why it becomes lower at higher temperature. I would very much more appreciate it if some documents stated about this issue are available as well. Thanks.

 

In Gas turbine, main medium we are using to rotate is air like steam in steam turbine. Like how the variation in steam properties affect the performance of steam turbine, variation in air parameters also affect the performance of gas turbine and in turn combined cycle efficiency. At lower temperature air density will increases which increases the mass flow rate of the air that pass through the gas turbine. similarly at high humidity condition also air density is more due to the presence of moisture in air. In that condition also mass flow rate of air increases. As the mass flow rate increases which will definitely gives high efficiency.

 

It is a mistake to assume that because the gas turbine is capable of more power output at low ambient temperatures that it is more efficient at those temperatures. If you look at the performance curves provided by the manufacturer, you will see that the heat rate also increases as the ambient temperature decreases (increase in heat rate = decrease in thermal efficiency). What that means is that while the power output increases the fuel flow increases disproportionately.

The fact is, the gas turbines are designed for optimum performance at one specific set of ambient conditions (temperature, barometric pressure, relative humidity), and any deviation from that condition will result in some loss of efficiency. The industry has generally standardized on the optimum design point - namely ISO conditions, which makes it easier for users to compare the performance of competing machines. Sometimes performance is "tweaked" to be optimum at some other point, but no matter what point is chosen there will be a limited amount of time any given machine is operated at exactly that point.

 

dear experts,

it is clear why at higher temperatures, gas turbine output is lower but I cant understand why gas turbine control system does not control the output power?

I mean, why the output is not maintained at design power at temperatures other than design temperature by setting fuel flow rate by control system?

in fact, I want to know, is there such a control system that controls this problem?

thanks,

 

Most any gas turbine control system can be made to maintain constant output power--ignoring the parts life of the turbine.

The reason power output varies when at Base Load is that when at Base Load the internal firing temperature remains constant and at the optimal value for both power output AND parts life.

If you want to have someone reprogram your control system to maintain output regardless of internal temperature I'm sure there is someone who will take your money to do that. And immediately notify your parts supplier of the likelihood you will be spending even more money to buy parts becuse of the damage caused by exceeding the internal firing temperature for optimal parts life.

But the output would remain constant all times of the year regardless of ambient temperature.

Oh, and the likelihood of a catastrophic failure due to premature parts failure due to excessive internal firing temperatures will also increase at the same time as the power output remains higher than otherwise would have been possible.

But the power output would remain constant at all times of the year regardless of ambient temperature.

It's only a matter of money--cost to reprogram; increased cost of spare parts; increased number & cost of maintenance outages; lost production if a catastrophic failure occurs before a planned outage.

But the power output could be made to remain constant at all times of the year regardless of ambient temperature.

 

The power from the gas turbine is limited by material constraints. i.e. you must not get the temperature of the exhaust gases too hot or you will cause severe equipment damage.

To control exhaust temperature, you control fuel flow. As ambient temperature INcreases, air density DEcreases. When you have less air per unit volume - even after compression - it takes less fuel to reach the same temperature.

When you add less fuel to lower mass flow you get less power.

It's a physical/law of nature problem, not a control problem.

-Tina

 

thanks CSA and Tina

from your explanation, I got that although we want a certain power output at each temperature but the control system must consider a constraint which is a constant Temperature not constant power.

but something that is yet unclear for me, is which temperature should be constant:

"turbine inlet temperature(TIT) or turbine exhaust temperature(TET)?"

--Do we control TET to make TIT constant and in this way, TET is varied with ambient Temperature change? or vise verse?

--Does it depend on type of cycle? for example:
for gas turbine power plants: TIT constant and
for combined cycle power plants or co-generations: TET constant (for more important matter of recovering temperature or inlet temperature to HRSG)

--Or is the load level(full load or part load) important in selecting the control logic?

thanks anyone who kindly help me...

 

Moshen,

If you wish to call the temperature of the hot combustion gases entering/leaving the first stage turbine nozzle "TIT", that is your prerogative. At least one gas turbine manufacturer chooses to call it "firing temperature" (which is NOT the same as "flame temperature"--which is where combustion occurs very close to the fuel nozzle tip in a conventional combustor). (NOTE: I'm referring to single-shaft, heavy duty gas turbines, not multi-shaft heavy duty gas turbines or multi-shaft aeroderivative gas turbine.)

Firing temperature (or "TIT") is not measured. There are no temperature sensing devices installed at the first stage turbine nozzle inlet. That's because the temperature (of the hot combustion gases entering/leaving the first stage turbine nozzle of a single-shaft heavy duty gas turbine) can be as high as 2200 deg F for some machines (slightly lower for others). Any temperature sensor (prior to the invention of infrared temperature sensors) that might be placed in this area would likely fail often (requiring replacement), and, further there is a high degree of stratification of gases, meaning the gas flow is not always uniform through its cross-section. Lastly, should a temperature sensor (most likely a pyrometer--look them up using your preferred Internet search engine) fail there is a high likelihood that some portion of it may break free and strike the rotating first stage turbine nozzles, causing catastrophic damage.

So, firing temperature ("TIT") is not typically measured.

However, over decades of development and data-gathering it has been learned that firing temperature ("TIT") can be approximated very closely by correlating axial compressor discharge pressure (which varies with ambient temperature when the turbine is operating at Base Load) and exhaust temperature. This relationship is used to control firing temperature ("TIT") by monitoring axial compressor discharge temperature and exhaust temperature, and a nearly linear--and negatively sloped--relationship can be developed and used to program the turbine control system so that a constant firing temperature ("TIT") can be maintained when the unit is being operated at Base Load as ambient conditions (temperature, primarily) change.

There is a second reason it's desirable to maintain a constant firing temperature ("TIT") is to reduce the thermal stresses caused by cycling the firing temperature to maintain a constant power output at Base Load. By keeping the internal hot gas path parts at a constant temperature during 'full load' operation the hot gas path parts (the expensive ones!) will last the longest while still producing as much power as possible (while maintaining a constant firing temperature ("TIT")).

So, maintaining a constant firing temperature ("TIT") is desirable to optimize hot gas parts life and to prevent what's called "over-firing"--admitting excessively hot gases into the turbine section, which will definitely reduce hot gas path parts life, but may also result in a catastrophic failure of hot gas path parts. Which is extremely costly to repair, not to mention the lost production during the repair (which necessarily takes some time to complete).

But, if you want to spend some money to re-program your turbine control system to maintain a constant power output (most likely the highest power which could be produced on the coldest day) you are free to do so if the turbine is not covered under any kind of warranty and management is willing to accept the risk (nay, likelihood) of catastrophic failure by doing so.

Also, don't forget to tell the company that insures the equipment at your site what you are doing; they will likely be interested to know this, as well.

And, inform outside operations personnel that they shouldn't venture very close to the operating turbine after the re-programming has been completed. Catastrophic failures of rotating equipment have been deadly (to human personnel).

But, you can certainly re-program your turbine to maintain power at all times.

If you are willing to accept all the consequences of doing so.

As Tina has also said, there is an upper limit of exhaust temperature above which the materials in the turbine exhaust (diffuser; duct work; stack; HRSG components, if one is being used) will be damaged.

Just another set of factors to be considered when re-programming your turbine control to maintain power regardless of ambient conditions.

Write back to let us know how this works for you.

 

Ok, Thanks CSA

 

Mohsen,

If you go up to otised's response earlier in this thread, you have the best answer to your dilemma. Tina and I just tried to explain how a typical gas turbine is configured to operate at "full" power output during different ambient conditions while still maintaining optimal hot gas path parts life and reliability.

If you can keep the axial compressor inlet temperature near rated during turbine operation the power output will be more near rated and optimal.

Of course this takes energy, but if you do the maths it might be advantageous. Methods include foggers and chillers. Both methods will increase the maintenance and operation requirements; they are not "set and forget" options.

You might have a look at Mee Fogging systems, and chiller manufacturers. Some chillers require large equipment investments (storage tanks; refrigeration equipment; pumps; etc.)--but again, depending on your site ambient conditions and requirements you might be able to justify the expense.

It all depends on your skills at using MS-Excel and MS-PowerPoint, eh?

 

I really appreciate you, because you don't leave anyone until make him clear for each question.

exactly as you said, I'm thinking about a cooling system type for a feasible study from an economical and technical viewpoint . I have to model first a combined cycle and then add a cooling system and finally compare the results. if the results were Ok then we do it in practice.

I have studied a lot about inlet air cooling systems (fogging, media, chiller,...). I prefer absorption chiller for its reliability and flexibility.

but I have a question about cycle modeling and I would thank you if help me:
--In an analytical model, how inlet mass flow is calculated for a compressor ?
some people offer a very simple method, that is for a constant volume machine:

"m = m_iso * T_iso/Tin * Pin/P_iso"

but others offer a more precise one: a trial and error procedure that finally should satisfy a constraint. i.e. gas turbine exhaust pressure equals Pamb and also with a control method (my first question above in this thread was for this matter that you responded, the firing temperature should be constant with an exhaust temperature control).

Now may you explain me that, is this procedure with this constraint and control method correct for calculation of inlet mass flow to the compressor of a combined cycle power plant?

thanks a lot,

 

Moshen,

Are you monitoring this control.com thread?

(Not that I think it's really heading in a great direction; but it is relevant to your latest question.)

It all depends on how much "accuracy" you want to have in your calculations.

If it were me, I would be contacting the manufacturer's representatives of companies who manufacture the kind of inlet cooling equipment you are considering and get them to help with the efficiency and return-on-investment calculations.

Of course, you will still need to check their work, but they do this stuff for a living in order to sell equipment. If they want to sell equipment, they have to be able to justify the capital expenditure, and the better companies (NOT necessarily the ones with the better equipment.... hint, hint, hint) do the best job of justification by providing lots of calculations and data and information. For free.

From which you can learn an awful lot about how efficiency and mass flow calculations are done by the professionals who do it for a living.

Let us know how you proceed!

 

Mohsen,

Sorry; forgot to include this link to the other thread:

http://control.com/thread/1372017375

 

thanks again CSA,

but just there is another question about control system in part load. I studied somewhere these sentences:

"in the first part of load reduction, the turbine outlet temperature is kept constant while the power reduction is obtained by closing the VIGV angle (and consequently lowering the air mass flow rate); when the minimum VIGV angle is reached, the fuel mass flow rate is decreased by the control system until the desired power output is obtained."

according to above Load adjustment criteria, it was said that when VIGV is closed, exhaust temperature is kept constant, so how firing temperature and fuel flow rate are changed?

 

Mohsen,

We have no idea what kind of turbine you are going to be simulating or installing this equipment on. Some manufacturers do things differently than others. Most of my explanations are of how GE controls their conventional combustor-equipped heavy duty gas turbines. I have an impression that you are working on a multi-shaft gas turbine, and don't know if it's a heavy duty or aeroderivative turbine.

So, I think I'll just have to step back from this thread because I don't have enough information to be of any further help. There are some fundamentals which are pretty peculiar to many gas turbines and which many manufacturers follow to one degree or another. But, having re-read your posts in this thread, you kind of jumped in to another person's thread and haven't really provided much in the way of information about your specific site and application and equipment.

And, when any kind of low emissions combustor is involved, sometimes a lot or just a critical few things change which can greatly affect operation or operational considerations.

Best of luck, and do let us know how you proceed (as well as what kind of turbine you are working with or preparing to work with).

 

sir,

just I wanted some general explanations. (if possible)

my gas turbine model is V94.2 siemens, a heavy duty single shaft one.

thank you.

 

Mohsen,

I am not familiar with Siemens units and the particulars of their control schemes. It sounds as though the unit might be exhausting into an HRSG and the IGVs are being modulated during unloading to maximize exhaust temperature.

Sorry I can't be more help.

 

Ok, thanks CSA for your help.

 

The density of the gas is reduced with higher humidity. If you treat as an ideal gas, water's molecular weight of water is 18 and air is 29.