from the Forum department...
firing temperature calculation
 Posted by sheila on 17 March, 2009 - 6:20 am
hi every one
I am a maintenance engineer and I am trying to estimate the proper interval for gas turbine overhaul, so I need to calculate firing temperature but I don't know how calculate it. It's my pleasure if any body could help me.
sheila

 Posted by CSA on 18 March, 2009 - 1:42 pm
I presume you are asking about the turbine firing temperature rating, not how to calculate the exhaust temperature reference.

GE defines firing temperature as either the temperature of the combustion gases leaving the first-stage turbine nozzles or the temperature of the combustion gases entering the first-stage turbine nozzle. The distinction is whether or not the first stage turbine nozzles are air-cooled or not. The air used for cooling the first-stage turbine nozzles discharges into the combustion gas flow-path, decreasing the gas temperature very slightly so the idea is that the gas temperature entering air-cooled nozzles can be increased very slightly, increasing output very slightly.

Firing temperature is not measured (today!), and is only a theoretical number. When the unit is operating on CPD-biased exhaust temperature control the unit is theoretically operating at a constant firing temperature regardless of ambient conditions. CPD-biased exhaust temperature control is a way of "estimating" a constant firing temperature using compressor discharge pressure and exhaust temperature. Empirical data defines a direct relationship between firing temperature, compressor discharge pressure, and exhaust temperature.

So, since firing temperature can't be (today!) and isn't (today!) measured, exhaust temperature control is how the turbine control system limits firing temperature to protect the machine while maximizing output.

Machines equipped with DLN combustors have a "firing temperature reference" function that does a reasonably good job of trying to calculate a firing temperature, but it's just an approximation of firing temperature and is only used for determining combustion mode switching. On a new and clean machine, it is a pretty good indication of firing temperature, but it's not "the" firing temperature.

I don't know how to calculate a machine's firing temperature. I think it's more of a machine rating defined by components (compressor, combustors, nozzles, shroud blocks, seals, etc.) more than a calculated number. I have seen firing temperatures listed on gas turbine performance "curves", typically used to estimate gas turbine output based on ambient temperature.

I think this is more or a question for your GE parts and/or service representative. Give him or her your turbine serial number, and they should be able to tell you very quickly what the machine's firing temperature rating is.

Hope this helps!

 Posted by sheila on 28 March, 2009 - 1:37 am
Dear CSA,
Thanks u so much for helping me.

I think my mistake originate from a chart with "Delta firing temp." in x axis versus "maintenance factor" in y axis. This factor takes "the load effect on part life" into account. If it doesn't possible to calculate firing temp. so what should i do with this factor? If it is necessary i can post u the estimating overhaul interval instruction.

It's my pleasure if u could help me.
regard highly

 Posted by ENGINEER on 20 March, 2009 - 9:01 am
hi sheila
I give you equ. that=
Tf=Tx* pcd/p wher:
Tf= firing Temp.
Tx= exhuste Temp.
pcd= air comperessor pres.
p= baromatric pres.

best wishes

copyplc@yahoo.com

 Posted by CSA on 20 March, 2009 - 6:20 pm
So, let's give this a try:

Let's take the isothermal exhaust temperature (Tx) from a Frame 7EA, 593 deg C, and a typical compressor discharge pressure (pcd), 10.87 barg, and sea-level barometric pressure (p), 1 bara.

Tf = 593 * 10.87 / 1 = 6445.91 deg C.

That doesn't seem remotely correct.

 Posted by Combustion Engineer on 18 April, 2009 - 11:13 am
Dear CSA,

The correct formula to calculate the firing temperature comes from the basic thermodynamic equations. Here the temperature needs to be put in "absolute" values and not the way CSA has calculated.

Say the values are now in English units. The correct equation is:

Tfire = (TTXM+460) x (Press Ratio)exp((k-1)/k)-460

For air-fuel mixture, you will have k = 1.26 approx.
For the parameters that you have, you will get T-fire = 2090F.
Hope this helps.
Combustion Engineer

 Posted by Mustafa on 24 March, 2009 - 3:06 am
Dear Engineer

It is seems there is writing mistake as also mentioned by CSA. You are requested to corrent the same.

With Regards

 Posted by Me I on 26 March, 2009 - 8:33 am
Thanks alot engineering maybe your equ.not correct exactly so must you to writting another equ.more accuracy.....OK ???

best regard

 Posted by David Davis on 27 March, 2009 - 11:19 am
Hello, our company, Advanced Automation Technolgies, Inc., actually calculates firing temperature of gas turbines by measuring the inlet compressor air mass flow with ultrasonics and adding in the fuel gas information. It is a very accurate estimation and our flow measurement system installed in the inlet system is very accurate within 0.5%. We validate our information by our ability to predict the exhaust gas temperature and 02 content. Even the OEMs are taking notice.

Not only do we get Turbine Inlet Temperature, we get Turbine efficiency and exhaust gas mass flow. A lot of great diagnostic information, including icing buildup.

David Davis
713-304-1840
david.davis@aatgroup.com

 Posted by David Davis on 5 August, 2009 - 12:27 pm
I'm updating this post because of the many inquiries we are getting for Turbine Inlet Temperature calculation. Earlier this year, we changed our name from Advanced Automation Technologies, Inc to Real Time Power, Inc, to reflect our true market.

Our new contact information is below.

Real Time Power, Inc.
22136 Westheimer Pkwy #617
Katy, Tx 77450
+1 832 437 4040
www.rtpsys.com
sales@rtpsys.com

Please see our website for information on our turbine air flow measurement system, TIT (Turbine Inlet Temperature) features, emissions prediction, and real time power information.

David Davis
Real Time Power, Inc.
david.davis@rtpsys.com

 Posted by sheila on 30 March, 2009 - 3:39 am
Dear CSA
but as u know operating load amount affect overhaul interval highly because with changing the load the firing temp. and consequantly the combustion and hot gas path parts life time also will change.
do u have any suggestion for me how consider this effect?
thanks so much

 Posted by CSA on 30 March, 2009 - 12:03 pm
Sheila,

When you post questions you should post some information about the equipment you are asking about. Most of the questions on this site are related to GE-design heavy duty gas turbines; we have very few posts about Siemens units. If we had known in the beginning this was about a non GE-design unit, you might have received a more appropriate reply.

Personally, I'm not familiar with Siemens units and how they calculate "firing temperature". I am familiar with how combustion turbine maintenance outages are determined, and each manufacturer seems to do so a little differently, but basically the same.

The length of time at Base Load or Peak Load or any other higher load is to be factored into the maintenance outage planning. Less time at Base Load would generally prolong the interval between maintenance outages, since typical maintenance outages are usually stated for units operating continually at Base Load (which is something of an ideal operating condition for the turbine and is the most efficient operating condition).

Units operating occasionally at loads higher than Base Load (Peak Load, or Peak Reserve Load) usually multiply the time operated at these loads by some factor greater than 1.0, since the firing temperature at these loads is higher than Base Load and therefore the hot gas path parts are subjected to higher thermal stresses.

I'm afraid I can't help with your specific question as it seems to be related to Siemens units and their maintenance planning recommendations. I would suggest contacting Siemens for assistance with your questions and planning; they would likely be very helpful if they will be selling parts and/or services for the outage(s).

But, again, as you have opened several posts about "generic" equipment (gas turbine, compressor) please provide some basic information such as manufacturer, rating, drive (generator or mechanical (compressor)), etc. We can probably be more helpful with some more specific information.

 Posted by sheila on 30 March, 2009 - 11:38 pm
Dear CSR
I am working with a GE-Design heavy Duty gas turbine: MS-5002C and a Siemens gas turbine type 10MV 2A. It's my pleasure if you provide me for some information about above GE gas turbine. Some additional information is:
Simple cycle
Centrifugal compressor drive
No water/steam injection
Fuel: natural gas
Continues operation
Thanks so much

 Posted by CSA on 31 March, 2009 - 11:16 am
GE have a publication for help with planning gas turbine maintenance: GER-3620. I'm sure you can contact any GE Energy Sales or Service office and give them that publication number and ask for a copy and they will send it to you.

It has everything you're looking for.

Or, perhaps someone here has an electronic copy they will send you if you post your email address (to avoid spam, I would suggest posting it something like this your dot name at company dot com).

 Posted by sd on 31 March, 2009 - 11:28 pm
I have uploaded the file here. Hope you guys can get it:
http://rapidshare.com/files/215997195/ger3620k.pdf.html

 Posted by sheila on 1 April, 2009 - 9:37 am
I have that publication but I have two problem with that.

First: There is a graph in that publication which shows "maintenance factor result from increasing load from base load" against "delta firing temperature again resulting from increasing load from it's base amount"

Now I have two question:
1. How should I know that in which load my turbine is working?
2. What is delta firing temperature correspond to the increased load?

At the end I apologize u for my inability in represent my question clearly.

sheila

 Posted by Ore Rotundo on 1 April, 2009 - 5:56 am
Sheila,

I am not sure, what is your main driver behind your original question. Why would like to calculate the firing temperature? For maintenance purposes or for performance analyses? For a GE design GT, the firing temperature is calculated in the control system in the complex exhaust algorithm block.The primary exh. temp. control reference is calculated as below:
TTRXP = TTKn_I - TTKn_S * (CPD - TTKn_C) + TTRXDSP + CT_BIAS + WQJG.
The secondary exh. temp control reference is calculated as below:
TTRXS = TTKn_I - TTKn_M * (FSR - TTKn_K) + TTRXDSP + CT_BIAS + TNH_BIAS + WQJG.
For two shaft (mechanical drive) GT's the bias as as following: TNH_BIAS = TTKRXN * (100%TNH - TNH)- TTKRX2 when L3TFLT is TRUE,
else TNH_BIAS = TTKRXN * (100%TNH - TNH). All this information is also available from your OEM manuals (control spec.) or can be checked from the control panel. As you can see, it seems all a bit complicated. Again, if your intention is to evaluate the maintenance interval, there are other easy ways to do it. Use or check the historical trend of spread. Operating your GT on high spreads will shorten the life of the buckets (cold spots).
Normally the OEM identified, based on their experience, maintenance intervals. OEM advices GT specified maintenance intervals to their clients.
The standard interval is based on the fired hours:
|8k=CI|16k=CI|24k=HGP|32k+CI|40k=CI|48k=MO|. In order to determine the exactly interval hours, you need to compensate the fired hours based on the factors such as, starts, trips, fuel type etc. The plant managers are usually aiming for higher production. Therefore they would like to extend the interval hours and shorten the outage hours. One way to this is, upgrading your GT's components such as the liners, X-fire tubes etc. GE provides this upgrade under the so called EXTONDOR kit. You may contact them for further inquiries. This upgrade will allow you to extend the interval hours. Instead of performing the CI every 8k, it will be 12k and instead of HGP you may consider to do the so called LTPI. This upgrade is in general economical justified. Before reaching the HGP or LTPI your machine will produce 2X4k hrs extra. The centrifugal compressor interval hours may also accommodate this extension.

Docendo Discumus

 Posted by CSA on 1 April, 2009 - 4:48 pm
I don't believe GE calculates firing temperature in the Speedtronic, except as a combustion mode switching function on DLN machines.

Exhaust temperature reference is calculated; yes. But not firing temperature.

And Sheila's never said what kind of control system her GE-design heavy duty gas turbine has.

The exhaust temperature control reference "curve" represents a constant firing temperature, but that firing temperature is only expressed in the rating of the machine, not in the control system (except for DLN machines, and that's only a calculated firing temperature reference; actual firing temperature is still never measured).

 Posted by sheila on 5 April, 2009 - 6:52 am
Dear CSA,

In the file you posted for me there is a formula:
Factored Hours = (K + M x I) x (G + 1.5D + AfH + 6P)
In witch:
G = Annual Base Load Operating hours on Gas Fuel
D = Annual Base Load Operating hours on Distillate Fuel
P = Annual Peak Load Operating Hours

So how should I know how many my turbine's load is? when it works on base load and when peak load? That is how should I calculate P, D & G?

And if at some times my turbine work in part load or some load between base and peak, could we use this formula?

Thank u again
Sheila

 Posted by CSA on 5 April, 2009 - 11:26 pm
Sheila,

Another individual, sd, uploaded the file; the thanks for that belongs to him. I haven't downloaded the file to look at it yet.

As was said before, there *should be* timers to accumulate the time at any load above Base Load, and if the unit is capable of loads above Base Load (Peak Load, and possibly Peak Reserve Load), there should be a means for selecting that load.

You still haven't told us what kind of turbine control system is being used to control the unit at your site. I don't know how to tell you to look for the 'timers and counters' to determine the number of fired hours and the number of fired starts and the number of emergency trips.

Ask one of the operators, or one of the operation supervisors, if the unit is capable of loads in excess of (above) Base Load. Ask them if the units ever operate above Base Load, and if so, ask them to tell you how they know how much time the unit runs at loads in excess of Base Load. (They should be familiar with the timers and counters of the unit and should be providing the information you require.)

If I can get a chance this week to download the file and have a read, I might be able to refer you to the specific section, if it exists, to tell you if GE makes any distinction between Base Load and loads less than Base Load. They may not; they consider any fired time below Base Load as the same as Base Load. I'm not a maintenance engineer and have not been involved in planning maintenance outages; I've only been the "beneficiary" of maintenance planning, or the lack thereof, and the poor execution of planned maintenance (like skipping critical control system checks because the mechanical outage ran well past schedule and there was no "float" in the schedule for such problems).

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