please explain about inlet bleed heating in GE9FA machines in detail

 

Well, IBH is a very poor name for a system that protects the axial compressor during operation with the IGVs (Inlet Guide Vanes) less than approximately 57 DGA (DeGrees Angle). And, F-class turbines (including FA turbines) start and stop with the IGVs at angles below approximately 57 DGA, so the IBH Control Valves must be open during starting and stopping (specifically at speeds less than 100%). Further, the IBH Control Valves usually used on F-class turbines are fail-open valves (as opposed to the IBH Control Valves used on B/E class turbines which are fail-close valves).

The axial compressor operating limits with IGV angles less than approximately 57 DGA are such that it's necessary to reduce the back-pressure on the axial compressor as well as to decrease the mass flow through the compressor. This is done with a system that recirculates up to approximately 5% of rated air flow from the axial compressor discharge casing, through the IBH Control Valve, to a manifold in the inlet air duct to the axial compressor bellmouth. The axial compressor discharge air is hot (as much as 700 deg F) and that air heats the air flowing into the axial compressor inlet, thereby reducing the mass flow through the compressor. The air being drawn off the axial compressor discharge is sometimes referred to as "bleed air", and when it's recirculated to the axial compressor inlet it heats (by just a few degrees F) the inlet air, hence the (poor) term Inlet Bleed Heat.

IBH is only in operation during acceleration and part load operation; it is not required for compressor protection above approximately 57 DGA. There is usually some deadband, sometimes about 5 DGA, so sometimes it stops during loading at about 62 DGA and starts during unloading at about 62 DGA, the important thing is that not all turbines are alike in this respect, but very similar. IBH Control Valve position reference is usually a function of IGV position.

Some, but *not* all, gas turbines, also use IBH for Anti-Icing to protect the IGVs against icing conditions when operating at or near rated load, but this reduces the power output of the unit and most owners get very upset when they realize they lose output when Anti-Icing is in operation (there ain't no such thing as a free meal!). In general, Anti-Icing is only required when there is a higher than normal humidity in the inlet air which can cause icing of the IGVs at some conditions. Most normal humidity conditions don't require Anti-Icing, nor do most cold air conditions.

control.com has a very good search feature; unfortunately when I searched there wasn't a very detailed explanation of IBH for F-class units, but there was a fair amount about E-class units. The concept is the same for E-class units, except that IBH isn't required for most E-class units, but it is mandatory for F-class units (though, true to form, there is usually an exception to most every GE "rule" and there is one exception to this "rule" also).