In an aeroderivative GE turbine sometimes there is a spread between the temperature measured by the thermocouples,sometimes as high as 40,does anyone know,why that happens?
Why is there a large spread in the temperature measured by the exhaust thermocouples?
- Thread starter nikidi.control
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yes,and the machine is running normally
No,they are thermocouples,but it happens in all 3 of the gas turbines,so we have 3 units,and it is happening in all of them
thanks Dave,lets see what they say
There is an allowable spread on GT exhaust temperature ...
There can be an alarm and a trip if Exhaust spread is above a designated /fixed level ...
There can be an alarm and a trip if Exhaust spread is above a designated /fixed level ...
High exhaust spread can be dangerous and damge the GT ...
It can be due to some friction on the Hot gas path elements like blades ..turbine wheels...
It can be due to some friction on the Hot gas path elements like blades ..turbine wheels...
It can be also a combustion problem
I suggest you to have a read on this interesting attached here article/document
Exhaust temperature spread is calculated by a combustion software which determines the spread limit and the first, second and third spreads. All of them, are used in determining the alarm and the trip for the gas turbine.
First spread is the difference between the first maximum thermocouple reading and the least value thermocouple reading. Second, second highest thermocouple minus the second lowest
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First spread is the difference between the first maximum thermocouple reading and the least value thermocouple reading. Second, second highest thermocouple minus the second lowest
…
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Why the gas turbine exhaust gas temperature gets higher value in startup then its back to normal?
There are two significant EGT rises in a jet engine start up before it stabilises with other engine parameters.
The engine start up begins when the pilot selects the engine mode selector to the start position. When this happens, the bleed air either from the onboard APU or from a ground starter unit runs an air starter motor which is mechanically geared to the high pressure compressor, N2. As the N2 rotates, you wait until a certain motoring speed is reached. This will depend on the type of the engine. Once N2 reaches this speed, fuel is introduced by the pilot using the fuel switches or the engine master switch, which sends fuel into the combustion chamber through the fuel nozzles. When the fuel is on, the first parameter that rises is the EGT. This increase in EGT is sharp. Why this happens is because, there is excess fuel in the chamber compared to air which leads to a hot burn. But as soon as this excess fuel is burnt off, the EGT rise rate reduces.
Then comes the final EGT peak. This occurs due to the acceleration of the compressor stages of the engine. Because of the increased acceleration, more fuel is introduced into combustion chamber which rises the EGT a second time. But because the compressor keeps on accelerating, the air to fuel ratio reaches a mean value for engine to operate at idle. At this point, the EGT reaches a peak and slowly stabilise at idle value.
The peak EGT the engine reaches is an important value because constant high start up values can deteriorate the health of the engine. When I used to operate Dash 8s we checked this peak value at every start up. If the APU is weak or the ground power unit is not up to the mark, the EGT or the ITT temperature can reach very high values, sometimes even turning on the ITT over temperature light. Some of our Dash 8s had such weak APUs that we start the aircraft with both the APU and ground power unit connected to the aircraft electrical system, to reduce the peak temperature. Dash 8 of course did not use air to start the engines, but the APU, GPU or the batteries power the aircraft onside DC generator which acts as a starter generator for the start up.
Why the gas turbine exhaust gas temperature gets higher value in startup then its back to normal?
There are two significant EGT rises in a jet engine start up before it stabilises with other engine parameters.
The engine start up begins when the pilot selects the engine mode selector to the start position. When this happens, the bleed air either from the onboard APU or from a ground starter unit runs an air starter motor which is mechanically geared to the high pressure compressor, N2. As the N2 rotates, you wait until a certain motoring speed is reached. This will depend on the type of the engine. Once N2 reaches this speed, fuel is introduced by the pilot using the fuel switches or the engine master switch, which sends fuel into the combustion chamber through the fuel nozzles. When the fuel is on, the first parameter that rises is the EGT. This increase in EGT is sharp. Why this happens is because, there is excess fuel in the chamber compared to air which leads to a hot burn. But as soon as this excess fuel is burnt off, the EGT rise rate reduces.
Then comes the final EGT peak. This occurs due to the acceleration of the compressor stages of the engine. Because of the increased acceleration, more fuel is introduced into combustion chamber which rises the EGT a second time. But because the compressor keeps on accelerating, the air to fuel ratio reaches a mean value for engine to operate at idle. At this point, the EGT reaches a peak and slowly stabilise at idle value.
The peak EGT the engine reaches is an important value because constant high start up values can deteriorate the health of the engine. When I used to operate Dash 8s we checked this peak value at every start up. If the APU is weak or the ground power unit is not up to the mark, the EGT or the ITT temperature can reach very high values, sometimes even turning on the ITT over temperature light. Some of our Dash 8s had such weak APUs that we start the aircraft with both the APU and ground power unit connected to the aircraft electrical system, to reduce the peak temperature. Dash 8 of course did not use air to start the engines, but the APU, GPU or the batteries power the aircraft onside DC generator which acts as a starter generator for the start up.
The reason behind the spread may vary depending on the specific aero-derivative GE turbine. Some possible reasons for the temperature spread include faulty thermocouples or inaccurate sensors. Inaccurate readings can be caused by factors such as fouling from sweat and oil particles, wind speed fluctuations during measurement, or variations in ambient air temperature. If you experience a considerable temperature spread between thermocouple readings, it might be worth consulting with a technician to ensure that your GE turbine performs optimally.
Hello,I will speak to the technicians and let you guys know
Hello,still nothing
First of all, the write-up posted to post #11 of this thread is primarily for can annular type combustion systems--as are typically used on GE-design heavy duty gas turbines. Can annular combustion systems have multiple combustors (often called 'combustion cans') around the circumference of the axial compressor discharge. Most aeroderivative combustion turbines use a single annular combustor--one that is continuous around the circumference of the hot gas section (continuous but made up of more than one section).
Secondly, the write-up provided in post #11 to this thread is copyrighted material. And I don't see any claim of having reprinted (or posted) the information with the permission of the firm holding the copyright. (The copyright is CLEARLY posted at the bottom of the second page of the write-up....)
Thirdly, the question to the original poster is: How is the spread problem resolved on the aeroderivative units on the site when the spread becomes too large? Are the T/Cs replaced and the problem goes away? If so, that would indicate the problem is failed or failing T/Cs, possible poor quality T/Cs and/or interconnecting cabling/wiring. So, the method for solving the spread problem over time can tell us a lot about the nature of the problem.
Are individual fuel nozzles (sometimes called 'fuel injectors' on aeroderivative combustion turbines) replaced to reduce or eliminate the temperature spreads? Again, that would indicate it's not an instrumentation problem, but a fuel quality or nozzle/injector problem.
If the annular combustor has cracks or is missing sections and has to be replaced that's another indication of how the problem starts and what it might be. Could even be leaky seals between the annular combustor sections.
So, if you understand how the temperature problems are solved on the site that would help understand what the cause(s) might be. Because I think that's what you're looking for: What is the cause of "large" exhaust temperature spreads in the exhaust?
And, really, I'm not so much talking about exhaust T/Cs in the descriptions above as I'm talking about the sensors which measure the outlet of the hot gas producing section (usually the HP section). I guess the discharge of the HP turbine could be called an "exhaust" but since it still travels into at least one, sometimes two, more turbine sections to my way of thinking (and training) it's not really an exhaust. So, here's another point of discussion which needs to be clarified: Is the original poster talking about the outlet temperatures of the HP (hot gas producing section) of the aeroderivative combustion turbine, OR about the outlet of the whole turbine (after the LP section)? Because that is also important to the discussion.
But, aeroderivative combustion turbines don't operate exactly the same as heavy duty gas turbines, so some of these charts and write-ups (besides being copyrighted material) aren't perfectly applicable. They look nice, but start-up times and exhaust temperatures during starting/acceleration aren't exactly the same for aeroderivative combustion turbines as they are for heavy duty gas turbines. Why? Because the axial compressors can be VERY different and have different operating characteristics.
Secondly, the write-up provided in post #11 to this thread is copyrighted material. And I don't see any claim of having reprinted (or posted) the information with the permission of the firm holding the copyright. (The copyright is CLEARLY posted at the bottom of the second page of the write-up....)
Thirdly, the question to the original poster is: How is the spread problem resolved on the aeroderivative units on the site when the spread becomes too large? Are the T/Cs replaced and the problem goes away? If so, that would indicate the problem is failed or failing T/Cs, possible poor quality T/Cs and/or interconnecting cabling/wiring. So, the method for solving the spread problem over time can tell us a lot about the nature of the problem.
Are individual fuel nozzles (sometimes called 'fuel injectors' on aeroderivative combustion turbines) replaced to reduce or eliminate the temperature spreads? Again, that would indicate it's not an instrumentation problem, but a fuel quality or nozzle/injector problem.
If the annular combustor has cracks or is missing sections and has to be replaced that's another indication of how the problem starts and what it might be. Could even be leaky seals between the annular combustor sections.
So, if you understand how the temperature problems are solved on the site that would help understand what the cause(s) might be. Because I think that's what you're looking for: What is the cause of "large" exhaust temperature spreads in the exhaust?
And, really, I'm not so much talking about exhaust T/Cs in the descriptions above as I'm talking about the sensors which measure the outlet of the hot gas producing section (usually the HP section). I guess the discharge of the HP turbine could be called an "exhaust" but since it still travels into at least one, sometimes two, more turbine sections to my way of thinking (and training) it's not really an exhaust. So, here's another point of discussion which needs to be clarified: Is the original poster talking about the outlet temperatures of the HP (hot gas producing section) of the aeroderivative combustion turbine, OR about the outlet of the whole turbine (after the LP section)? Because that is also important to the discussion.
But, aeroderivative combustion turbines don't operate exactly the same as heavy duty gas turbines, so some of these charts and write-ups (besides being copyrighted material) aren't perfectly applicable. They look nice, but start-up times and exhaust temperatures during starting/acceleration aren't exactly the same for aeroderivative combustion turbines as they are for heavy duty gas turbines. Why? Because the axial compressors can be VERY different and have different operating characteristics.
The exhaust thermocouples are used to measure the temperature of the exhaust gases from an engine. The large spread in temperatures is because these thermocouples operate at different temperatures, which can lead to errors in calculation.
Say what?.?.?!!!???!!!
These thermocouples do measure "high" temperatures, but if they're in the exhaust (the exhaust of the HP gas generator (gas producer)) then the temperatures they measure should be very uniform around the periphery of the turbine--because if they are not fairly equal (uniform) then that can indicate some kind of problem with the combustors or fuel nozzles (injectors), etc.
Just because the thermocouples measure temperatures which can change from ambient to several hundreds of degrees Celsius doesn't mean they should be anything less than accurate--which is what you seem to be saying, that it's okay for there to be high temperature spreads because of the high, varying temperatures being measured.
And that's just not so. Read the copyrighted write-up in Post #11 above; it explains why the temperatures should be uniform and how there is no mixing of hot combustion gases as they flow through the turbine section of a combustion (gas) turbine.
These thermocouples do measure "high" temperatures, but if they're in the exhaust (the exhaust of the HP gas generator (gas producer)) then the temperatures they measure should be very uniform around the periphery of the turbine--because if they are not fairly equal (uniform) then that can indicate some kind of problem with the combustors or fuel nozzles (injectors), etc.
Just because the thermocouples measure temperatures which can change from ambient to several hundreds of degrees Celsius doesn't mean they should be anything less than accurate--which is what you seem to be saying, that it's okay for there to be high temperature spreads because of the high, varying temperatures being measured.
And that's just not so. Read the copyrighted write-up in Post #11 above; it explains why the temperatures should be uniform and how there is no mixing of hot combustion gases as they flow through the turbine section of a combustion (gas) turbine.
