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Can someone explain the meaning of "Turn-down" in case of gas turbines and how to calculate the same. Is some data available comparing turn down of advanced class GTs of various makes (GE 9FA, Siemens, MHI etc).
"Turndown" is a term which has been derived from 'turndown ratio' which is a fairly common term used when describing characteristics of many flow-rate measuring devices.
In this usage, turndown generally refers to the operating range of the combustion turbine in low emissions mode (by low, I'm really referring to lowest). Most combustion turbines with multiple combustion modes cannot operate in low emissions over the entire operating range of the unit. Some turbines can operate over a wider range in low emissions mode than others. Some turbines can be modified to operate in low emissions mode over a wider range than would otherwise be possible.
So, turndown refers to the load range that's possible while remaining in low emissions mode. A higher turndown would mean a wider load range, while a lower turndown would mean a smaller load range. I've never seen it expressed as a ratio as it is when flow measuring devices are being discussed, but it certainly could be. I've just seen it more used in conversation, such as, "The unit has a larger turndown when Inlet Bleed Heat is in operation than when it's not." (Inlet Bleed Heat--an <b>extremely</b> poor name for the function it provides--is one of the ways some GE-design heavy duty gas turbines can increase the low emissions operating range.)
You should contact the various manufacturers' sales people for exact numbers for any machines you are considering. In today's world, technology advances makes some data obsolete almost before it's released! And there is much competition in this arena as for many plants the ability to operate over a wide load range while keeping emissions low is very important.
Hope this helps!
In this usage, turndown generally refers to the operating range of the combustion turbine in low emissions mode (by low, I'm really referring to lowest). Most combustion turbines with multiple combustion modes cannot operate in low emissions over the entire operating range of the unit. Some turbines can operate over a wider range in low emissions mode than others. Some turbines can be modified to operate in low emissions mode over a wider range than would otherwise be possible.
So, turndown refers to the load range that's possible while remaining in low emissions mode. A higher turndown would mean a wider load range, while a lower turndown would mean a smaller load range. I've never seen it expressed as a ratio as it is when flow measuring devices are being discussed, but it certainly could be. I've just seen it more used in conversation, such as, "The unit has a larger turndown when Inlet Bleed Heat is in operation than when it's not." (Inlet Bleed Heat--an <b>extremely</b> poor name for the function it provides--is one of the ways some GE-design heavy duty gas turbines can increase the low emissions operating range.)
You should contact the various manufacturers' sales people for exact numbers for any machines you are considering. In today's world, technology advances makes some data obsolete almost before it's released! And there is much competition in this arena as for many plants the ability to operate over a wide load range while keeping emissions low is very important.
Hope this helps!
D
Debashish Das
Dear CSA
thanks a lot for your reply. Can you give me an idea of the turn down as quoted for the 9FA machine. Now a days many OEMs are offering retrofits for improving turn down. I presume by reducing the mass flow through the compressor, at the same time by maintaining exhaust temperature. Can this have any adverse effect in machine performance in the long run ? Regards
thanks a lot for your reply. Can you give me an idea of the turn down as quoted for the 9FA machine. Now a days many OEMs are offering retrofits for improving turn down. I presume by reducing the mass flow through the compressor, at the same time by maintaining exhaust temperature. Can this have any adverse effect in machine performance in the long run ? Regards
P
Process Value
Turn down ratio in gas turbines - (for those who just want the answer just skip the first paragraph to avoid my blabbering about turndown ratio's abusive and murky history and usage , those interested can just read it for fun and forget it as soon as possible 
)
well , turn down ratio and turndown is a much abused term , it means so many things in so many places. In boilers turndown means " the min continuous operating load possible to maintain a required steam quality , ie the min load the boiler can be run at so that the specific steam temperature and pressure can be met" , and the turn down ratio is the expressed as the ratio between the min to max load possible in the boiler. Eg. If the boiler can be operated continuously at 30% load then its turn down is 30% and the turn down ratio is given by 30%/100% = 1/3.33 , but i have also seen in literature that the turn down ratio expressed as the ratio of max to min operating load , ie in this case it will be 100%/30% = 3.33. ( assuming that the boiler has a 100% operation capacity). In flow meter terminology it is the ratio of operating range of flow meter , expressed in terms of ratio of operating range to its design flow.
In Gas turbines it is predominantly used ( as in the case of boilers ) to refer to the min possible part load operation. There is actually no restriction to the gas turbine as such , you can run the gas turbine at FSNL as much as you want , but what is the point eh ? the restriction comes in the form of external factors
1. Emission control
2. Downstream HRSG steam quality
In case you have signed a treaty to reduce emissions for the greater good of the world , you don't have to worry about it. But sadly that is not true, you need environment clearance to run the GT and you will only get it if you adhere to the emission norms. Typically the emission rates (<15 PPM of nox for DLN burners and <60% ppm for conventional) promised is for loads from 50% to full load , if you operate at less than 50% load your emission will go up. Thus the turn down ratio for your turbine is 0.5 or 2. sometines it is just expressed as turndown , in that case it is 50% itself. GE and a lot of other guys promise all sorts of packages to reduce the turndown. Opflex is the ge package for DLN burners.
Another area where the turn down is specified is when you run a cogeneration of a combined cycle power plant. Your HRSG is designed for a particular steam output , ie at a particular pressure and temp. The min load at which this particular pressure and temp is attainable is called the turn down. For eg you have a HRSG which produces a VHP steam of 50Kg and 430 deg , then you need a min exhaust of 440 deg to attain the required superheat , the gas turbine you have will only reach a exhaust temp of 440 at lets say 40% of the load , anything below it the steam temp is going to fall down. thus for this particular machine the turndown is 40%. this is quite important in case of a combined cycle power plant , and have a steam turbine in the downstream expecting superheated steam and not a saturated one . It is also important in process industries which employ co-generation to supply the steam at rated temp , lest they should be chastised by the process group for damaging precious equipment downstream , which uses the steam for turbo drives, stripping , steam tracing , product heating etc etc ...
" I presume by reducing the mass flow through the compressor, at the same time by maintaining exhaust temperature. Can this have any adverse effect in machine performance in the long run ? "
well , from the above statement i assume that you are having a HRSG downstream and it is used for combined cycle. Yes one of the ways to reduce turn down is to reduce the turndown is to reduce the air flow. the main advantage apart from a lower part load is that the part load efficiency increases. as far its effect on the gas turbine performance , i think you have to trust the manufacturers. The modulated IGV is a kind of turn down increasing device as well, but i would go with anything the original manufacturer provides , for a ge machine go for a GE provided retrofit.
)well , turn down ratio and turndown is a much abused term , it means so many things in so many places. In boilers turndown means " the min continuous operating load possible to maintain a required steam quality , ie the min load the boiler can be run at so that the specific steam temperature and pressure can be met" , and the turn down ratio is the expressed as the ratio between the min to max load possible in the boiler. Eg. If the boiler can be operated continuously at 30% load then its turn down is 30% and the turn down ratio is given by 30%/100% = 1/3.33 , but i have also seen in literature that the turn down ratio expressed as the ratio of max to min operating load , ie in this case it will be 100%/30% = 3.33. ( assuming that the boiler has a 100% operation capacity). In flow meter terminology it is the ratio of operating range of flow meter , expressed in terms of ratio of operating range to its design flow.
In Gas turbines it is predominantly used ( as in the case of boilers ) to refer to the min possible part load operation. There is actually no restriction to the gas turbine as such , you can run the gas turbine at FSNL as much as you want , but what is the point eh ? the restriction comes in the form of external factors
1. Emission control
2. Downstream HRSG steam quality
In case you have signed a treaty to reduce emissions for the greater good of the world , you don't have to worry about it. But sadly that is not true, you need environment clearance to run the GT and you will only get it if you adhere to the emission norms. Typically the emission rates (<15 PPM of nox for DLN burners and <60% ppm for conventional) promised is for loads from 50% to full load , if you operate at less than 50% load your emission will go up. Thus the turn down ratio for your turbine is 0.5 or 2. sometines it is just expressed as turndown , in that case it is 50% itself. GE and a lot of other guys promise all sorts of packages to reduce the turndown. Opflex is the ge package for DLN burners.
Another area where the turn down is specified is when you run a cogeneration of a combined cycle power plant. Your HRSG is designed for a particular steam output , ie at a particular pressure and temp. The min load at which this particular pressure and temp is attainable is called the turn down. For eg you have a HRSG which produces a VHP steam of 50Kg and 430 deg , then you need a min exhaust of 440 deg to attain the required superheat , the gas turbine you have will only reach a exhaust temp of 440 at lets say 40% of the load , anything below it the steam temp is going to fall down. thus for this particular machine the turndown is 40%. this is quite important in case of a combined cycle power plant , and have a steam turbine in the downstream expecting superheated steam and not a saturated one
. It is also important in process industries which employ co-generation to supply the steam at rated temp , lest they should be chastised by the process group for damaging precious equipment downstream , which uses the steam for turbo drives, stripping , steam tracing , product heating etc etc ... " I presume by reducing the mass flow through the compressor, at the same time by maintaining exhaust temperature. Can this have any adverse effect in machine performance in the long run ? "
well , from the above statement i assume that you are having a HRSG downstream and it is used for combined cycle. Yes one of the ways to reduce turn down is to reduce the turndown is to reduce the air flow. the main advantage apart from a lower part load is that the part load efficiency increases. as far its effect on the gas turbine performance , i think you have to trust the manufacturers. The modulated IGV is a kind of turn down increasing device as well, but i would go with anything the original manufacturer provides , for a ge machine go for a GE provided retrofit
.
