GT Exhaust Thermocouple Failure Alarms

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Thread Starter

niazbibiyana

Hello,

In our 9FA.03 machine from last few days TTXD_28 thermocouple is held on negative value (as like fault condition) and while it starts fluctuating, several times "FIRST FAILURE HIGH EXHAUST THERMOCOUPLE, L30TXAH1_ALM" OR "FIRST FAILURE LOW EXHAUST THERMOCOUPLE, L30TXAL1_ALM" alarms appears at MARK VIe and also others relevant alarms are appearing.

My question is: Can anybody tell me, how HIGH and LOW exhaust thermocouple failure is detected? what is logic? I noticed while fluctuation is started never it goes to higher than highest thermocouple value. So how it is detected that "1st failure of high exhaust thermocouple"?
 
niazbibiyana,

As with all things which emanate from GE Belfort, they change--frequently and often. Failed exhaust thermocouple detection is no exception. (GE Belfort has been given responsibility for Frame 9 machines, Es and Fs and FAs, as well as Frame 6s.)

So, without being able to see the exact application code running in the turbine control system, it's very difficult to say with any certainty exactly how the two conditions are detected (but anyone with access to Toolbox should be able to easily work this out--GE application code is fairly simple to follow as it's very much relay ladder logic diagram-based (even the FBDs (Function Block Diagrams)).

One of the failure modes of exhaust thermocouples--but not the most common failure mode--is to be intermittent, or to fluctuate. The most common failure mode is fail in the open circuit condition--and when that happens GE turbine control systems treat an open circuited exhaust thermocouple as a low, sometimes negative, value.

ALSO, an intermittent thermocouple value is an indication of loose terminations along the thermocouple circuit, somewhere between the exhaust thermocouple leads, to any intermediate junction boxes/terminations along the circuit to the turbine control panel input terminals.

It's rare for turbine control panel thermocouple inputs to fail intermittently, but that does happen and in some cases it might be indicated with a Diagnostic Alarm.

Perhaps someone else with a similar turbine/turbine control system and with some experience with analyzing the application code running in the turbine control system could answer your more specific questions. But, in the end, someone is going to have to investigate and resolve the issue.

While most operators aren't aware of the feature, nuisance "dithering" alarms (one that come in and go intermittently) can be LOCKed to prevent unnecessarily using Alarm Log printer paper, and from filling up the Trip History Alarm Queue with alarm messages. When the condition is resolved, then the alarm can be UNLOCKed.

Again, in the end, the problem will have to be solved. If site procedures prevent opening junction boxes and checking for loose terminations then one could disconnect one of the offending exhaust thermocouple wires from the turbine control panel input terminal board; the value would go low and stay low until such time as the wire was re-terminated and the problem resolved (either by replacing the thermocouple; or finding and fixing the loose terminal; or replacing the thermocouple input terminal board).

Please write back to let us know how you resolve the problem!
 
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