hello,

we r having GE frame 9E - DLN machine. when the machine is on base load TTXM is 567*C, which is being controlled by CPD biased exhust temp control. but when the load comes down around 85% load, inlet bleed heat valve is opening and the TTXM is 600*C. (IGV is 60 degree) My query is base load is maximum load which can be acheived that v r limiting by exhaust temp. in my case 567*C. but in partial load it is 600*C. then normal loading why it is limited to TTXM 567*C. how is the IGV temp control loop works? kindly explain.

 

Briefly, Inlet Bleed Heat is used to allow a GE-design heavy duty gas turbine equipped with DLN-I combustors to remain in Premix Combustion Mode at lower loads than would otherwise be possible. In other words, without Inlet Bleed Heat, a unit can only be operated in Premix Combustion Mode in a very small range, between approximately 80-100% of rated power output; but with Inlet Bleed Heat (IBH) the unit can be operated in Premix Combustion Mode from approximately 50-100% of rated power output, sometimes as low as 40% of rated power output depending on ambient conditions.

To operate the unit in Premix Combustion Mode below approximately 80% of rated power output the IGVs must be closed to angles less than the usual minimum operating angle of 57 DGA--but doing so can cause abnormal axial compressor operation under certain conditions.

IBH reduces the flow through the axial compressor by recirculating a portion of the axial compressor discharge air (which is hot!) to a manifold in the Inlet Air Duct and heating the axial compressor inlet air a few degrees making it less dense, thereby increasing the compressor operating limit, sometimes referred to as compressor operating margin. This is done in conjunction with lowering the IGV angles, which also reduces the air flow through the compressor.

The IGV (Inlet Guide Vane) Temperature Control curve mirrors the CPD-biased Exhaust Temperature control curve which has a negative slope up to a horizontal limit which is sometimes referred to as the "isothermal limit", which is usually approximately 1100 deg F. As the unit is unloaded and CPD (Compressor Pressure-Discharge) decreases, the Exhaust Temperature Reference increases up to the maximum isothermal limit value.

The negative slope of the Exhaust Temperature Limit Control Curve represents a constant first-stage turbine nozzle temperature which is also called "firing temperature"--which varies with CPD. As CPD decreases, the air flow through the unit decreases and one of the results is that for a constant firing temperature the exhaust temperature increases.

To protect the exhaust diffuser components the exhaust temperature can never be allowed to exceed the isothermal limit, so as the unit is operated at Part Load the IGVs are modulated to maintain the Exhaust Temperature Limit--which has a maximum value equal to the isothermal limit at part load operation.

There is usually approximately a 5 DGA deadband (overlap) for IBH operation, meaning that IBH can be active at IGV angles of approximately 62 DGA.

It should be noted that without Inlet Bleed Heat units would not transfer into Premix Combustion Mode until approximately 80% of rated power output. So, units with IBH will transfer into Premix Combustion Mode sooner than units without IBH (at approximately 40-50% of rated power output).

markvguy

 

Would welcome advice on the wisdom of using bleed heat to avoid ice build up in the air inlet of GE Frame 6 GT.

The idea is to use the bleed heat system to increase the inlet air temperature when ambient temperature drops below approx. 1deg C to protect the GTG from ice buildup. Preliminary studies here are showing that the use of bleed heating significantly reduces the capacity of the turbines to values comparable with summer duration.

For instance for the Frame 6 GTGs ISO rating is 38.8MW, whilst the Summer rating at 44degC is approx 31.5 MW and the Winter rating at -36degC appears to be approx. 31.8 MW.

Are these figures correct? It would mean that bleed heat is not the preferred method for avoiding ice build up. Are there alternative methods that would have less impact on turbine site rating? Thanks for any ideas.

 

The air flowing through the Inlet Bleed Heat system comes from the discharge of the axial compressor. This is air that is "extracted" from the axial compressor discharge. Energy was used to compress this air and instead flowing into the combustors and through the turbine and exhaust it is re-directed to the inlet of the turbine.

Since gas turbines are mass flow machines, decreasing the mass flow through the turbine will decrease the power produced by the gas turbine.

You are correct: Using Inlet Bleed Heat to prevent or reduce the formation of ice <b>on the inlet guide vanes</b> will reduce the power output of the turbine. And relatively significantly, too.

You have not provided enough information about the turbine at your site. Does it have DLN combustors? Does it have IBH?

We recently had a thread about ice formation on the inlet air filters. You just talked about ice formation at the "air inlet". We don't know where, specifically, you are referring.

This topic has been covered in other posts on control.com. In general, the formation of ice on inlet air filters or on the IGVs is a function of a source of abnormal humidity being drawn into the turbine. This typically happens a turbine is located on a site such that when the prevailing wind causes the vapors from evaporative cooling towers or evaporative ponds to be blown into the inlet of the gas turbine. In general, normal humidity doesn't result in icing of the inlet filters or IGVs--in general. Usually, when a turbine is being ordered for a particular location, the ambient conditions are taken into account and if necessary an option called Inlet Anti-Icing is recommended and/or provided. IBH and Inlet Anti-icing use many of the same components, and I have been at sites where the same components serve as IBH during start-up and shutdown, and in certain ambient conditions is also used as Inlet Anti-icing protection.

However, when the operators of the turbine see how much power is lost when Inlet Anti-icing is enabled and working, they usually scream very loudly, "Shut it off!" They want the MW and will live with the possibility of icing in order to produce more power.

So, it's as you say: There is no such thing as a free lunch.

Lastly, if you have DLN combustors you should know that enabling IBH as Inlet Anti-icing at Base Load may cause combustion instability and high combustion dynamic pressures. You really should work with the OEM or some other knowledgeable company to ensure the hot gas path parts will not be damaged when using IBH as Inlet Anti-icing at Base Load.