IBH (inlet bleed heat) has lost its marbles

Hey folks, quick question for those of you with experience running GE frame 7AE or similar units with DLN combustors.

Recently our unit's IBH system lost its marbles. We think that its a problem with the CV positioner. Long story short, the IBH valve's feedback was becoming flaky (dropping to negative values intermittently) along with some diagnostic alarms:
  • G1.PMVE-21-R 5435.403
  • G1.PMVE-21-S 5435.403
  • G1.PMVE-21-T 5435.403
ringing in on the Mark VI.

An hour later GTG BH SYS FAIL TRIP IBH came in along with GT IBH CV POS TRBL and IBH FLT-CMPR LIMIT REF FAIL OPEN

The inlet bleed heat CV initially went to 100%, then the air tripped to the CV solenoid and the valve failed closed. (not sure why it did all of this)
For some reason the bleed heat reference is still calling for 100%, although the feedback and valve position are reading zero.

Now- my question is, is there any concern with running the unit with bleed heat unavailable? I understand that the IBH allows the unit to run in premix below it's normal threshold, and I have seen it operate when we move to part load. I've been making it a point to keep the unit at full load until the IBH system can be repaired but is there anything I might be missing? Ambient temps are high now so icing won't be an issue.

Thanks for any input.

Cheers,

Jeff
 
There are SOME 7EA gas turbines that must use IBH during loading and unloading (namely, units that DO NOT use a Gas Transfer Valve/function); these units are said to be "transfer-less" DLN-I units). They just can't get enough fuel through the second-stage nozzles at high loads to sustain operation and maintain flame/load, and IBH allows the unit to transfer into Premix at lower loads when the fuel flow is lower than it would otherwise be. So, without knowing if your gas fuel system has a Gas Transfer Valve/function, we can't say you could or couldn't start/stop/load/unload the unit without a working IBH system. (Units without IBH or units which can enable/disable IBH) will transfer into Premix at about 80% of rated load (on an ISO day), and transfer out of Premix when unloading at about 75% of rated load. Using IBH, it's possible for a unit to transfer into Premix at about 40% of rated load and stay in Premix when unloading until about 35% of rated load. THAT'S what IBH does--and when there's no Gas Transfer Valve/function, it's necessary to transfer into Premix at lower loads (lower fuel flow-rates) to maintain stable operation during the transfer from Lean-Lean to Premix.

IBH has NOTHING to do with anti-icing. It's STRICTLY for being able to close the IGVS below 57 DGA in order to get into Premix sooner (at lower loads) than a unit without IBH, and for staying in Premix longer (at lower loads) when unloading a unit operating in Premix mode. (SOME--but NOT many GTs--also use the IBH system for anti-icing but only when there's an unnatural source of humidity at low ambient temperatures, but most sites stop that (Anti-Icing operation) in a hurry when they figure out it limits load on cold days (and that means revenue is limited on a cold day... remember: It's all about generating revenue for the bean counters--and many think it's about generating MWs!!! well, it is, but it's more about the revenue that the MWs).

SOME 7EA gas turbines do not even have a button on an HMI display to enable/disable IBH, so if your HMI doesn't have one then it's probably because it shouldn't be operated without IBH (which means that unless some logic is forced the unit will always try to move the IBH control valve during loading and unloading--and if the unit doesn't have a Gas Transfer Valve/function that's NOT going to be good--not at all).

I'd also suggest the 7EA has some perversion of a Mark V turbine control system that's been upgraded with Mark VIe-compatible components to extend it's life (the key is the reference to PMVE in the alarm information you posted). Mark VI and Mark VIe share some terminal boards, but they are different control systems (trust me on this).
 
Hello,
Thanks for the detailed response! Precisely the kind of information I was hoping to find.

Our unit does indeed have a transfer gas line that operates when moving between primary firing and premix mode on the way up, as for IBH control I am able to both raise/lower it or select between auto/manual on the HMI, although I don't see anywhere to actually disable or enable IBH.

Our unit does also rely on IBH for anti-icing (its a necessary evil operating in this blighted Canadian wasteland...), in fact almost everyone who works with this unit had no idea that IBH had anything to with unit downturn capabilities and thought that its use was confined to anti-icing operation.

Having said all of that, based on your response and the fact that unit did not trip when the IBH system failed and we are equipped with a transfer line, I assume we are safe to continue operating the unit until the IBH system can be repaired. I've made a point of holding MSG at a higher point (greater than 80% rated capacity) to avoid accidentally dropping out of premix in the mean time.

Another question, the markVIe has a premix upholding function that prevents the DCS MW controller set point from pushing us to lean/lean. I believe it become active when TTRF1 drops below 1085*C and will then bump the MW set point up to bolster the reference temp. Will the operation of the premix uphold be impaired by the loss of IBH? My assumption is that is should still work, only we'd reach a lower reference temp sooner (at 75-80% rated capacity) than we might have otherwise (usually closer to 45% rated capacity) if IBH was available. Does that sound correct?

Thanks again!
Jeff
 
You are correct.

I would suggest looking at a way to disable IBH (there is probably a rung and signal to do so--it's just not used on any HMI display and may be blocked with an LFALSE or something like that). Forcing the IBH off UNTIL the control valve operation can be restored (there have been reports of minor problems with the 4-20 mA position sensor causing problems like this--a loose set-screw, likely). But, that's just my conservative controls nature.

Best of luck. Please write back to let us know how you resolve the problem.
 
Ended up replacing the IBH valve feedback positioner, that sorted out all of our problems.

Stroking the valve to calibrate the new positioner swung the machine around a little, but nothing too surprising. The mark VIe required a 'master reset' to clear the IBH trip alarm that came in when it failed initially.

Thanks again WTF? for taking the time to write back.

Cheers
 
Thanks for the feedback!

When they were looking for a way to dissipate the extraction air when the IGVs were closed below 57 DGA, the idea of using an anti-icing manifold was tried. The idea worked reasonably well and with some tweaking it became IBH (a really crappy name for the purpose it serves—to protect the axial compressor when the IGVs are closed below 57 DGA while trying to get into Premix sooner during loading or stay in Premix longer during low load operation). Heating the axial compressor inlet air even a little bit makes it less dense and helps protect the compressor. And, if the unit needs anti-icing protection it can be used for that, also. (Though the tweaks made to make it protect the compressor at lower loads kind of make it a little less effective at anti-icing, but it is still adequate for anti-icing protection.)

Be safe!
 
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