What is the advantage enabling Inlet Guide Vane temperature control in combined cycle operation? how is heat input to the boiler increased at lower gas turbine loads during start-up in combined cycle operation?

 

> What is the advantage enabling Inlet Guide Vane temperature control in combined cycle operation?

I am talking about Frame7E(mark VIe). Consider IGV Temp control OFF. During startup, IGV opens to keep exhaust temp ref to about 700(*F). Till GT reaches 40 MW,IGV is about 57 degree open (from FSNL to 40 MW). As load increases temp reference goes up and IGV start opening more to admit more inlet air to keep temp ref is about 700 and at base load IGV is full open (84) and temp ref is never 700 but about 1000. So this additional air is diluting the heat for HRSG. In opposite action when IGV temp control is ON this excess air is not required to cool the temp ref to 700 so air intake is minimised and net heat gain is far more then the case if it were IGV TEMP CONTROL is OFF. So this heat is utilized in HRSG to make more steam. One more thing at base load IGV TEMP CONTROL should be OFF. I hope CSA or other experts will give more detail about this. Waiting for their inputs.

Thanks

 

It provides a higher exhaust temperature at a lower load for more steam production. A higher ttrf1 is also achieved at a lower load thus allowing for greater turndown of the low emissions mode.

It is done by reducing the IGV angle to a lower limit of 42 degrees. A lower air fuel ratio will create a higher ttrf1/ttxm temperature. To prevent compressor surge with the low IGV angles the inlet bleed valve will open. IBH is effectively a compressor recycle valve. IGV angle to IBH flow is a linear relationship. Typically 5% IBH flow at an IGV angle of 42degrees, and decreasing to zero at 57degrees, with some deadband and other compensating factors. IBH may also provide S17 and anti ice protection in some installations.

 

sardar9,

This is just to clarify your post. For units with conventional combustors (such as yours) and the ability to select IGV Exhaust Temperature Control (sometimes simply called IGV Temperature Control) ON and OFF, when it's selected OFF and the unit is loaded from initial breaker closure, the the IGVs will remain at minimum (usually 57 DGA for a Frame 7EA) until the exhaust temperature reaches 700 deg F. At this point as load is increased (fuel flow is increased) the IGVs will open to maintain 700 deg F until finally the IGVs are fully open. So, the IGVs will "modulate" to hold 700 deg F from minimum operating angle to maximum operating angle.

When the IGVs are fully open (usually 84 DGA for your units), as load (and fuel flow ) are increased the exhaust temperature will increase until it reaches the CPD-biased Exhaust Temperature Control Reference (TTRX) at which time the unit will be on CPD-biased Exhaust Temperature Control, affectionately referred to as "Base Load."

When IGV Exhaust Temperature Control is selected ON when the unit is being loaded from initial breaker closure, the IGVs will remain at minimum operating angle as the unit is loaded (as fuel flow-rate) is increased until the exhaust temperature gets to within approximately 10 deg F of the CPD-biased Exhaust Temperature Control Reference (TTRX). At that point as load (and fuel flow) is increased the IGVs will open to keep the exhaust temperature very close to TTRX until the IGVs reach maximum operating angle (again, usually 84 DGA for units like yours), and at that point the unit is at or very near Base Load and it will not be possible to increase the load (or exhaust temperature) by very much, if at all.

By holding the IGVs at minimum during loading when IGV Exhaust Temperature Control is selected ON, the exhaust temperature is "maximized" which increases steam production if the gas turbine exhaust is being used in a HRSG (Heat Recovery Steam Generator), or "boiler" at loads less then Base Load. Because, with IGV Exhaust Temperature Control selected OFF, the exhaust temperature is much less than maximum during loading from low load to Base Load.

Reducing the IGV angle during loading does increase (maximize) exhaust temperature during low loads, but it also decreases the heat rate of the gas turbine--meaning that the gas turbine is less efficient. However, the increased steam production causes the overall plant heat rate to rise because of the increases steam production, so it's a good trade-off.

If the unit does NOT have an HRSG (boiler) on the exhaust, or if the exhaust is not being directed through a damper to an HRSG (boiler) then it's NOT recommended to select IGV Exhaust Temperature Control ON, as it reduces the efficiency of the gas turbine.

By the way, the reason the IGVs are modulated when IGV Exhaust Temperature Control is selected OFF on units with conventional combustors is that it is found to reduce combustor dynamics at the load range where the IGVs are modulated, thereby improving hot gas path parts life (combustion liners; combustion transition pieces; cross-fire tubes and clips; etc.).

 

But be careful in what we are saying, Inlet Bleed Heat for DLN control is different from IGV Temperature Control. As CSA indicated IGV Temp control will normally be at an IGV angle of 57-58 Deg. whereas Inlet Bleed Heat for DLN will be at 42 Deg. as 309Guy said

 

Well explain CSA, I am still confuse about the inverted slop of exhaust temp Vs CPD/FSR bias. Any good insight would be highly appreciated not by me but by all on this forum.

Thanks

 

sardar9,

The topic of the CPD-biased exhaust temperature control slope has been covered many many times on control.com, some with "drawings". Use the 'Search' feature at the far right side of the Menu bar at the top of every control.com page.

The thing to remember when looking at the negative slope of the line (curve) is that the line, itself, represents a b>constant firing temperature</b> as CPD varies. So, what the graph tells you is that as CPD increases, exhaust temperature decreases in a combustion turbine.

Yes, it is counter-intuitive, when you consider that as you increase fuel during loading the exhaust temperature increases. But, when holding <b>firing temperature</b> constant as CPD changes.

After you read some of the many threads about that, if you still have questions--ask them here.

 

If we reduce the IGV angle less than 57.... What happen?
Heat rate improve?...What about efficiency of Ga turbine? What is effect on combined cycle performance?

sardar9

 

CSA,

With 'IGV temperature control' ON, it is limiting IGVs opening until TTXM is very close to TTRX, would you expect the IGV reference set point to match this?

We have a condition where IGV reference setpoint 90TV is 100% but the position feedback 96 is flatlined around 60%.

TTXM has not reached TTRX yet which is my train of thought. Can you confirm if this is correct logic on my part?

Many thanks

 

methzzz,

When IGV Temp Control is enabled an active, it will keep the IGVs closed to try to maximize exhaust temperature (TTXM) without exceeding TTRX (Exhaust Temp Control Reference--which is ALSO the maximum allowable exhaust temperature for any given operation condition).

CSRGV, the IGV control reference, is the output of a MINimum SELect block, which looks at all the various IGV references and passes the least of them all to the servo (usually signal name CSRGVOUT, or something like that).

The IGV reference is going to be expressed in DGA (DeGrees Angle), and as such it will be a number between CSKGVMIN and CSKGVMAX. We don't know what Frame size machine you have (nor what control system the unit has), so we can't say it will be a value between 34 DGA and 84 DGA, but those are pretty typical min and max IGV values for a lot of non-DLN machines. (We also don't know if your machine has conventional or DLN combustors.) But, 100%? I don't recognize that number as an IGV position reference at all.

I really don't understand the "flatlined" statement at all.

Finally, a LOT of machines (especially machines which DO NOT exhaust into an HRSG (or boiler)) reach a point during loading and unloading when the STATUS message on the display reads 'IGV Temp Control' or something similar and the IGVS are modulated as the unit is loaded to maintain a particular exhaust temperature (900 deg F, if I recall correctly). And, then when the IGVs are finally fully open (CSGV=CSKGVMAX), the message goes away. (I believe something similar happens during unloading.)

TTXM at 900 deg F is much less than TTRX, and the Status message is really incorrect--technically it's correct, but it's technically incorrect at the same time. (Another one of the inconsistencies of Speedtronic turbine control systems, that's been broken for so long, no one will fix it.)

Again, IGV reference is a position reference--not a percent. (Unless there's something very unusual about the turbine control configuration at your site....)

Other than this, I can't offer any more help without a lot more information.

 

It's a 6b, non dln unit with a 3rd party control system. I am pretty sure what is happening is exactly what you are describing. I just need to find the right data to prove it. The challenge is that some of the conventional tags have changed/are missing/in different units.

Thanks for your explanation, its is very helpful and much appreciated.