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Gas Turbine FRAME9FA IBH Opening at Full Load
Why is gas turbine FRAME 9FA IBH opening at full load?

Dear all.

we have in our site 3 gas turbine 9FA and IBH one of them is opening at full load (temperature control) by 18% and IGV fully opened 84 degree. i checked IBH valve reference. i found its value like feed back 18%. i traced the IBH reference. i found its value coming from signal CSRDLNX (inlet bleed heating-extended DLN turndown), but i don't catch the reason. can any one help me why it is happened? and How to solve this problem?

thanks for all

msalem,

Something is amiss, because as you suspected the IBH control valve should NOT be opened during Base Load operation when the IGVs are at the maximum operating angle (which you indicate is 84 DGA for your machine).

IBH is used to allow low-load operation while maintaining low emissions--on most GE-design heavy duty gas turbines with DLN combustion systems. On F-class machines, it' also to protect the axial compressor during starting and shutdown.

A lot of sites don't actually select Base Load when the intend for the unit to be running on CPR- (or CPD-) biased exhaust temperature control. Rather, since they make extensive use of Pre-Selected Load Control for Part Load operation they just put in a "high" setpoint for Pre-Selected Load Control and let the Speedtronic automatically go on exhaust temperature control.

The problem with this is if the Pre-Selected Load Control setpoint is right on the edge of the transition between Droop Speed Control and exhaust temperature control the Speedtronic can "toggle" back forth between Droop Speed Control and exhaust temperature control. And any function that relies on being on exhaust temperature control will NOT recognize this because the customary logic used to tell other functions that the turbine is on exhaust temperature control requires it to be on exhaust temperature control for at least approximately 30 seconds.

So, this is one possibility, not very likely, but a possibility none-the-less. (This is just one of the problems of operating a turbine constantly using Pre-Selected Load Control.)

If the grid frequency is unstable, then this can contribute to any controls problem(s) and make them worse.

Something else which I've seen occur recently is that some logic signal related to DLN control gets latched when it shouldn't be and causes a problem like the one you are describing. It's a very unusual occurrence, but it does happen on occasion--especially if the grid frequency is unstable, and/or there has been instability at some point prior to reaching "full load."

Another cause for something like this to occur is when operators or maintenance personnel are forcing logic signals without understanding the knock-on effects of doing so.

Unfortunately, without being able to see the sequencing running in the Mark V at your site we can't be of much help, because the logic (sequencing) is supposed to be written to prevent this from occurring--but sometimes it does happen. And GE learns of this they usually issue a TIL (Technical Information Letter) to fix it.

You are on the right track; you're just going to have to dig a little deeper into the sequencing. For most GE-design heavy duty gas turbines IBH is necessary to maintain low emissions only at low loads.

The "good news" is that this isn't probably hurting the turbine--but it is reducing the power output of the turbine-generator (because it's recycling a portion of the axial compressor discharge air back to the inlet of the compressor, wasting the energy used to compress the air AND increasing the inlet air temperature, which isn't helpful to performance, either).

On a very recent troubleshooting expedition a particular DLN logic signal was latched prior to a turbine trip and did not unlatch as it should have when the turbine was tripped. It prevented loading the unit past a particular point because the Speedtronic thought it was already at that particular point (even though it wasn't). It took a considerable effort to locate the logic signal and manually force it back to "0" and then the turbine could be loaded to Base Load.

Again, this probably isn't hurting the turbine but it is decreasing output. It's kind of like running an Anti-Icing System--which also decreases output when protecting the IGVs and axial compressor during low temperature and high humidity operating conditions. But, you're going to have to dig a little deeper into the logic (sequencing) to see if this is the problem, and what to do about it.

Please write back to let us know what you find!

Hello everyone,

I do agree with Mr CSA in his theory. Since we have almost the same machine 9FB, with DLN 2,6+, MKVIE, I'll try to answer the question at the first topic.

Well I tried to tune on the IBH:

Actually It works on two principal modes:

1/ Extended DLN Turndown: as its name indicates it seems that it is designated for DLN works in minimum load, and since we are a bit comfortable against compressor operation (CPR is too far from CPR Limit). here we care rather of the DLN function, taking care about the ratio of IBH mass flow/compressor inlet flow. keep in mind at min load that ratio is high, then we need to handle that.

actually the IBH Ref for this mode is smoothly decreasing when the IGV position increasing by Load increase. More exactly the mode is enabled until IGV reach the 63 DGA.

And before reaching that position and since effective IBH ref is the maximum value between (extended DLN Turndown Ref and the second mode which is Compressor operating limit Ref), we always working under this mode.

(When IGV reach 63 Degrees this mode is disabled at, and no more need to work under DLN turndown mode),

working toward the rated load needs to close IBH (somehow, whenever IBH is open this will reduce GT performance ,because some of air mass flow is returning to the compressor inlet through IBH opening).

Now the IBH is at minimum. IGV is opening which means more compressor parameters get involved especially around the CPR (compressor pressure ratio), in other word the CPR is getting closer from his limit or CPRLIM, which we must be taken in consideration!

then control system starts to command IBH taking care all parameters around that CPR. By enabling the second mode: Compressor operating limit through a new Ref.

this new mode is enabled and reference is calculated even before reaching the 63% DGA of IGV, but it become effective only when it get greater than the older ref which continues decreasing until 0% after reaching 63% of IGV.

2/ Compressor operating limit: here we take care about compressor operation, how it does safely work?

this mode of operation get enabled if a kind of CPR CMU Error is less than 0 (GET NEGATIVE VALUES). the mode is latched until the CPR CMU Error be greater than 0.13 (Hysteresis).

Or

the Filtered CPR Error also get negative values (< 0). also its latched until that value get again greater than 0.13.

here two new ref are calculated and the greater one is the effective and be taken for commanding the IBH.

a/ Compressor pressure ratio CSRPR is the first one: which should be (calculated ref) between 3 - 100% not less than 3 %. it's calculated basically from CPR Error (difference between CPR Limit and filtered CPR). CPR Lim is function of your compressor degradation (this is for GE people).

b/ Compressor pressure ratio CMU error is the second one: CPR CMU Error protection (kindly, someone could tell me what CMU stand for?).
the CPR CMU Error is calculated again from the CPR CMU Limit Error which is calculated from a CPRLIM and a filtered CPR.

those both ref supposed to be equal and somehow they aren't. I think cause in their calculation we use different gains, filtration and offsets...etc.

but since they are too close each other, why we do calculate two? we could just calculate one for the IBH REF, either CPR or CPRCMU.
should exist a difference somewhere? someone can explain please?

A question:

How its been affected the base load in summer? (in our case we have three units put in service in 2013).

two of them getting base load (FSR Exhaust temp control FSRT is active. IVG is fully open 89.5 DGA), and the IBH is still opened about 3% then we are not at maximum output (we are losing some Mwatts).
the third one its ok we reach base load and the IBH is commanded close.

Hopefully this helps someone, and helps me too.

Regards.