PG6581B Operating in IGV Temperature Control Mode


Thread Starter


I am operating 2 PG6581B gas turbines in combined cycle mode. At part load the speed FSR is lowest value, however HMI indicating IGV temperature control. Why?

What curve are we operating on at part load if not the CPD-Temperature curve?

What curve are we on when peak load is selected?

The isothermal constant is set at 615C, should the high exhaust temperature and over temperature trips be above this value?

Do the units have DLN combustors?

Part Load does not use a curve. Load is controlled using Droop speed control.

If the units do not DLN combustors but do have IGV Exhaust Temperature Control (sometimes called Combined Cycle mode) and its enabled and active, then the IGVs are modulated (positioned) to maximize exhaust temperature in order to maximize steam production and improve the total plant heat rate when the unit is at Part Load. The maximum exhaust temperature is limited by the CPD-biased exhaust temperature control curve--the same one that's used for Base Load. Load is controlled by Droop speed control; exhaust temperature is maximized and limited per the Base Laos CPD-biased exhaust temperature control curve.

When the IGVs are full open and the exhaust temperature equals the CPD-biased exhaust temperature control curve value the the unit is at Base Load

Peak Loaf is just another CPD-biased exhaust temperature control curve that's just slightly higher than the Base Load curve.

The exhaust over temperature alarm and trip setpoints are set to mirror the CPD-biased exhaust temperature control curves--not the isothermal value (unless the isothermal value is the controlling value. CPD-biased exhaust temperature control curves have a negative slope--counter-intuitively. So, in general, unless the unit is old and/or dirty and/or the ambient temperature is very high the exhaust temperature reference is less than the isothermal (when the unit is operating at Base-or Peak Load) and the exhaust over-temperature alarm and trip setpoints will be 25- and 40 deg F , respectively, higher than the exhaust temperature reference.

Hope this helps!
CSA Thanks for incite. The answers have helped very much to clear up some misconceptions.

I am new to this forum, and it was by chance that I was conducting some searches and found your responses to some questions posted by others in the field. I see you are rated as 5 stars and I would think quite appropriately. I find your answers to be expertly clear and direct.

Sorry I did not mention that the units are currently operating on liquid fuel (No2 distillate) with planned DLN conversion within a year.

Back to the CPD-TTXM curve. Could you explain the corner value TTKN_C(6.54bar for my unit)and how they fit into the TTRX calculation?
I think the TTRX for the unit in combined cycle mode is typically about 593 deg C. Am I correct to think that this should be the reference exhaust temperature throughout the full range of loading up to base load? Ambient at my location is typically about 30 deg C and base load CPD of app. 11bar with a clean compressor.

Have a read of this thread:

It should help your understanding.

As the unit is loading the exhaust temperature reference, TTRX, will be equal to TTKn_I, the isothermal temperature--until CPD increases and reaches TTKn_C, then TTRX will start to decrease as CPD continues to increase. If IGV exhaust temperature control is active and enabled the IGVs will be held closed as the unit is loaded to make TTXM equal to TTRX--to maximize steam production.

When the IGVs are full open <b>AND</b> TTXM = TTRX, the until will then be on CPD-biased exhaust temperature control.

It's not normal for a machine to have a curve for simple cycle and one for combined cycle, but it does happen.

Peak Load is just another "curve" to the right of Base Load.

Hope this helps. Responders earn stars when people reading the posts click on the 'Thumbs Up' icon.

Sorry for all the typographical errors in the previous post. I'm not the best person to proof-read my own writing.