what is the function of IBH. How does it operates according to the load and IGV angle? what factors and conditions influence the IBH to operate during part and low loads?

please anyone clear my doubt

 

Dear PAWAN,

I can give you a quick explanation of the operation of IBH as it relates to GE Heavy Duty gas Turbines, how it operates on other manufacturers machines I can't say.

Simply, IBH is a way to decrease compressor efficiency and reduce mass flow through the compressor. Inlet bleed heat(IBH) takes compressor discharge air and routes it back to the inlet of the compressor.

By taking compressor discharge air away from the combustion wrapper you reduce mass available to the combustor. Also introducing this very hot air into the turbine inlet raises the temperature of the air and decreases the efficiency of the compressor since warm dry air has less mass than cold damp air.

All of this is typically done to reduce mass flow through the compressor at low load conditions to help control emissions. It also helps to decrease the formation of ice in cold climate areas, and lastly helps to provide compressor protection at low loads, again by reducing the efficiency of the compressor.

Inlet bleed heat demand is typically a logrithmic function of IGV angle, although you would be advised to review application code for the particular unit you are familiar with.

I hope this helps your understanding of IBH systems.

 

IBH is used in conjunction with modulated IGV to extend the pre-mixed steady state range. PMSS is the preferable operating mode due to the lowest emissions.

To achieve PMSS the combustion firing temperature TTRF1 needs to be above a certain temperature. Without IBH this occurs at about 80% load. With IBH/IGV modulation PMSS can be achieved at about near 50% load. Modulating or closing the IGV's at part load will reduce the airflow which will in turn lead to a higher TTRF1/TTXM temperature. However lower airflow reduces the surge margin of the compressor. The IBH valve is used to prevent a compressor surge. Also the lower IGV can lead to ice formation on the front compressor stages. IBH flow is not that great and limited to 5% of total compressor flow. IBH position is also linear relation ship versus IGV angle. At 42 IGV angle IBH is at maximum flow wth flow reducing to zero at 57, i think,degrees IGV. On F class units the IBH is also used to provide 17th stage compressor protection as well.

 

Good discussion. One topic related to this. We are operating 2 Frame6B DLN-1 machines with IBH (Inlet Bleed Heat) for power generation. Is it economically viable for capacity enhancement using inlet air cooling in a GT having IBH? Few agencies has approached in this regard. To me the principle is not logically fitting. Inlet air is cooled to enhance capacity and IBH is heating it to maintain DLN. Can some one throw some light in this?

 

Dear AK,

I think you understand the concept correctly as far as IBH and inlet cooling for power augmentation.

IBH is used to decrease compressor mass flow. This allows for lower unit turndown, the ability to produce lower power output, while still maintaining low emissions. Also as 309EGuy notes it affords some compressor protection at lower loads as well.

Inlet cooling is typically installed to increase unit output by cooling the inlet air, allowing for more mass flow, more fuel, and more power output. As your question goes there would be no reason to cool the inlet air if the unit was operating at anything less than maximum output (baseload). Typically the inlet cooling system is interlocked so that is not active unless the unit is operating near baseload and or the IGV's at full open position.

In many parts of the world the gas turbines in service are always operating at baseload. The areas typically need all the power available, and also that is the most efficient way to run the gas turbine.

In other parts of the world power companies need units that can "swing" or change load quickly, and often. Typically these units are working in conjunction with large wind or photovoltaic systems that have output that is not consistent (wind stops or clouds cover photovoltaic panels). Here it is important and valuable to have a unit that can operate in low emissions mode over a large area of output. For instance a GE Frame 7EA can "swing" from approximately 84MW to 40MW and still maintain low emissions. As I am sure you understand this is needed to maintain proper grid frequency, since large coal plants and large nuclear plants don't "swing" well.
So I hope that helps you understanding for the need of IBH and inlet cooling. Both systems have value and a purpose, but may not always be used in all parts of the world.