Exhaust Thermocouples Spread MS5001


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The gas turbine installed on site MS5001 having Mark-VI control system has 13 thermocouples installed on its exhaust. Upon the failure of one (ttxd_5), my understanding is that it should be declared unhealthy and removed from the spread calculation logic. However, the thermocouple's value (-17) is being read by the system and consequently spread value being shown is 426.3'C while the allowable value is 51.4'C

Please access the running logic values and cimplicity screenshot from the following link:



I need to know if this could trip the turbine on high spread reading? If so, is this an error on the programming part because the Turbine control system does recognize the thermocouple as unhealthy (logic snapshots are in the mentioned link)? Currently the exhaust spread security was forced on our turbine and so are some other exhaust thermocouple related securities based on the past experience of nuisance trippings. GEs workmanship issues were found to be the root cause earlier, and also the fact that Mark-VI even reads the transitional values of a failing thermocouple to generate trip signals.
To answer your question about the trip. After examining all the pictures, your machine needs that a thermocouple exceeds the high temp spread limit (TTHT) to trip, because already we have:

1) the actual spread (TTXSP1 is higher than the allowable TTXSPL)
2) the system detects that a Tc is lower than the TTLT limit

And i think everything is normal, because the ttxd_5 would be excluded only when calculating TTXM. however it won't be deleted in the exhaust thermocouples arrays:

1) TTXD1 (Tcs sorted by position) to be compared with the TTHT and TTLT limits
2) TTXD2(Tcs sorted by value) used to calculate the actual spread, and as consequence all the details are shown in the pictures.

PS: i think it is not good to force the the exhaust spread
Waiting for your feedback

Your understanding of the working of the Combustion Monitor is not correct. "Failed" T/Cs are removed from the average exhaust temperature calculation--NOT the Combustion Monitor. Removing failed T/Cs from the Combustion Monitor function would defeat the purpose (which you have already done by forcing the logic NOT to trip on combustion trouble).

There has been LOTS written about the Combustion Monitor and how it works on control.com--all accessible by using the 'Search' feature of control.com. It takes a high exhaust temperature spread <b>AND</b> adjacency checks to trip on combustion trouble--NOT just when a single exhaust T/C causes one spread to exceed the allowable spread. It would be unreliable to just trip on a single exhaust T/C spread--and the Speedtronic turbine control system is all about reliability.

Your comments about "workmanship" without specifying details and providing data are inflammatory and likely incorrect--based on your "understanding" of how the Combustion Monitor works.
A single exhaust TC causing the spread and crossing any value out of TTHT, TTLT will not cause the speedtronic to activate the trip, if you see the logic.

TTXSP1 should cross the TTXSPL which gives L60SPME to activate and any two adjacent TC exceeding the TTHT and TTLT will; and the PCAA IO packs in MARK VIe should indicate communication link OK (l3vtcc) will only cause the high exhaust temp spread trip (L30SPT5).