Frame 6 Machine with Mark-V. IGV is operating at 57 DGA with TTXM=562 degree C. The IGV moves to 62 DGA and is operating between 59-62DGA. What is the logic. GTG is not operating in Temperature Control mode
There is 'exhaust temperature control' and there is 'exhaust temperature control.' Which exhaust temperature control are you referring to?
Most GE-design heavy duty gas turbines have CPD-biased exhaust temperature control (this is what defines Base Load--and is the maximum allowable exhaust temperature based on operating conditions <b>when the IGVs are at their maximum operating angle.</b>
And, most GE-design heavy duty gas turbines also have IGV exhaust temperature control--which is the method of maximizing exhaust temperature at part load (below Base Load) by keeping the IGVs closed as long as possible which restricts the air flow through the machine and increases the exhaust temperature.
When the unit is being loaded from synchronization and IGV exhaust temperature control is enabled and active the IGVs will be held closed until TTXM (the average exhaust temperature) gets very close to TTRX (the CPD-biased exhaust temperature control reference--again: the maximum allowable exhaust temperature for the current operating condition). So, the IGVs will be held at 57 DGA until TTXM gets very close to TTRX (sometimes, depending on the vintage of the control algorithm, it will be when TTXM equals TTRX) as load is increased the exhaust temperature would tend to increase above TTRX--and the IGVs are opened slightly to maintain the TTXM/TTRX relationship. This is IGV exhaust temperature control.
This continues during loading until the IGVs are fully open, and when TTXM equals TTRX then the unit is said to be operating on CPD-biased exhaust temperature control. Again, CPD-biased exhaust temperature control only occurs when the IGVs are fully open--that's part of the definition of Base Load: when the IGVs are fully open <i>and</i> fuel flow is such that TTXM equals TTRX.
Hope this helps!
Two last things. First, if the turbine you are working on has DLN (Dry Low NOx) combustors, IGV exhaust temperature control is not operator-selectable; it's always on.
Second, when a GE-design heavy duty gas turbine without DLN combustors is operating with IGV exhaust temperature control disabled and not active when the exhaust temperature hits 700 or 900 deg F (sorry; I can't remember which temperature exactly) the IGVs will start opening to maintain that temperature (either 700 or 900 deg F) as the unit is loaded until such time as the IGVs are fully open. The operator interface display will show "EXHAUST TEMPERATURE CONTROL" during this period. The purpose of this 'exhaust temperature control' is to limit combustor pressure pulsations during loading (which occurs around 700 or 900 deg F--which I could remember which, but I can't; sorry). So, really, there's 'exhaust temperature control' and there's 'exhaust temperature control' and there's 'exhaust temperature control.' (Isn't this fun?)
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