We operate 2001 vintage 7EA units with Mark V control systems.
During some recent cold weather runs (8 deg F), most of our units had the following alarm:

<pre><b>LFSRCPR_ALM: COMP LIMIT FSR BACKUP CONTROL ACTIVE</b></pre>

While the LFSRCPR condition existed, the L70R (raise load) commands were prevented, therefore the unit did not load up to meet pre-select. This condition cleared on all but one of the units within a few minutes and they loaded normally from there. The other unit remained in that state for ~45 minutes, before it finally got past it and loaded up.

Have any of you all in the North experienced this alarm?

Do you have insights and advice to offer?

 

><b>LFSRCPR_ALM: COMP LIMIT FSR BACKUP CONTROL ACTIVE</b>

From my friend at 1-800-IT-DON'T-FIRE, LFSRCPR_ALM SEQ_TRB3 Rung 18. The compressor pressure ratio limit error is greater than -.25 PR (pressure ratio above limit) which biased down FSR to maintain an error less than -.25 PR. which kept the unit from loading as designed.
Pressure ratio is calculated by CPRV2, SEQ_TBR2, rung 172. Inputs are CPD, AFPAP AFPCS. Check those inputs for accuracy.

Also check 96CD-1, 96CD-2 and 96CD-3.

If you have bleed heat, it is designed to maintain an upper limit on the compressor ratio. Check its operation.

Good Luck.

 

The alarm is saying the turbine control system has detected that the compressor is approaching or has reached the limit of normal operation, and is limiting the fuel to prevent damage to the compressor. This can occur when the inlet air temperature is very cold; the air is more dense and the mass flow through the axial compressor is very high--possibly too high for the axial compressor to prevent damage to the axial compressor. So, the turbine control system is limiting the power output (by limiting the fuel flow-rate) which also keeps the IGVs at a stable (limited) opening.

Inlet Bleed Heat (a VERY poor choice for the function it provides) can be used to allow the unit to to go a slightly higher load. However, the air for the IBH system comes from the axial compressor discharge, and extracting this air and recirculating it to the inlet reduces the power which can be produced (by reducing the mass flow out of the compressor and into the combustion- and turbine sections of the unit). (It should be noted that Inlet Bleed Heat and Inlet Air Heating (to prevent icing of the axial compressor inlet) are two different functions, which can be provided by the same system of extraction, control valve, and manifold. Again--the choice of names for the functions is very, very poor.)

Heavy duty gas turbines can produce lots more power--if they could get more air to flow through the axial compressor. However, materials and axial compressor design limit the amount of power which can be produced--and when ambient conditions cause the mass flow through the compressor to increase to levels which can cause damage to the axial compressor the control system acts to limit that mass flow. And, that's by limiting the fuel flow-rate--not allowing it to increase. Increasing the fuel flow-rate also results in the IGVs opening which increase the mass flow-rate through the axial compressor.

DLN combustion systems, especially, are very prone to flame instability. They operate (when in Premix mode) in extremely lean fuel/air ratios, so the fuel flow-rate can't be allowed to increase while holding the IGVs at a fixed angle. That would cause the emissions to increase, and add to flame instability in the combustor and probably to high dynamic pressure fluctuations in the combustor as well--neither of which are good for the combustor or for emissions.

So, keeping the IGVs from opening is accomplished by limiting the fuel flow-rate.

What can be done? There's really not much which can be done under these conditions--except to limit load to protect the axial compressor from exceeding its operating limits.

The alarm text message is cryptic and doesn't accurately describe what's happening very well. But, then, there's only so much one can do with an 80-character alarm text message limit.

Hope this helps!

 

LFSRCPR_ALM: COMP LIMIT FSR BACKUP CONTROL ACTIVE

<p>Thanks Selkirkjim and CSA</p>

<p>Found the 96CS transducer calibration was off resulting in that unit's AFPCS reading being twice as high as the others. Although the 4-20 mA only scales to 0-11.08 inH2O. So "twice as high" was still only a slight difference numerically, but maybe had a significant affect on the CPRV2 calculation.</p>
<p>We have not had another opportunity to run at such a low ambient temperature since. And it might be a long time before we can test under similar conditions.</p>

 

Thank you for the feedback!

 

> LFSRCPR_ALM: COMP LIMIT FSR BACKUP CONTROL ACTIVE

Here's an update:

Found the 96CS transducer 4-20 mA scales to only 0-11.08 in producing an AFPCS reading that was slightly off but it was actually twice as high as the other turbines.

This apparently had a significant effect on the "compressor pressure ratio" calculation in the CPRV2 block, which in turn resulted in the " COMP LIMIT FSR BACKUP CONTROL ACTIVE" condition.

PS - there was no ice in the air lines of the differential pressure transmitter.

We got to run this unit again, with an ambient temp that was even several degrees lower. It loaded normally with no alarm.