what is the need of omiting highest and lowest temperature values in exhaust temperature (ttxm) calculation? Even without deleting those 2 values, if we are calculating means almost same values only coming. please help me..

 

The idea is to remove "outliers" from the calculation. You are correct in that if all of the readings are basically the same, deleting the highest and lowest will not appreciably change the calculation.

A lot of sites run with failed T/Cs, and the typical failure mode is open circuit, which results in a low, negative reading in the Speedtronic turbine control system. This would have a large impact on the average reading.

Thinking about the combustion system of a gas turbine it's not really likely that a single T/C reading will be significantly higher than any other. The likely failure mode of a component of the combustion system is to allow excess compressor discharge air into the gas path, resulting in a cold spot, not a hot spot. And, some T/Cs do fail in the high direction initially, then go open circuit.

So, the idea is to try to prevent skewing the average ("median") value with excessively low or high T/C readings which are likely failed or somehow aberrant. I've seen insulation fibers blown out from behind failed flex seals block air flow over T/C radiation tube openings.

Remember, everything about the Speedtronic is about reliability and availability. Understanding predominant failure modes of devices and instruments and circuits and how the combustion turbine operates is all part of the philosophy of Speedtronic turbine control systems. In addition to the hardware being designed to accommodate turbines and the philosophy of GE to control and protect them, the software used in the Speedtronic is written to optimize reliability and availability. It's another reason that PLCs aren't really a good choice for optimal control and protection of combustion turbines, without a lot of careful programming and configuration (which usually isn't done, or isn't done very well).