Hello,

We are having ge frame 9e machine- dln.when machine is on base load ttxm is 567 *c is manintained which is being controlled by cpb biased exhaust temp. Control. when machine is on part load ttxm ( exhaust temp )is 600*c ( igv is <60 degree) which is being maintained by igv temp control. when igv reaches <60 deg inlet bleed valve starts opening.

My query is base load is the max temp limit.but during part load exhaust temp is 600*c.
Kindly explain igv temp control philosophy. function of inlet bleed heat system. what is cpb biased exhaust temp.

 

See http://www.control.com/1026226559/index_html.

If you have a Frame 9E packaged by GE you will have a Control Specification-System Settings drawing and GE Instruction Manuals. Section 3 of the Control Spec. should have a very informative description of CPD-biased Exhaust Temperature Control (or, more likely in your case, CPR-biased Exhaust Temperature Control since you seem to have a DLN-I combustor-equipped unit). In either case, the "curve" has the same shape.

You should also be able to read the Control System tab of the GE Instruction Manual for even more information on CPD-/CPR-biased Exhaust Temperature Control and IGV Exhaust Temperature Control and Inlet Bleed Heat system operation.

As has been said before, the operation of Exhaust Temperature Control is counterintuitive, that is, it seems that when the exhaust temperature limit increases that power output (load) should increase--but it doesn't, power output (load) decreases when the exhaust temperature limit increases.

The reason is because the sloped portion of the Exh. Temp. Control Curve (which is the Exhaust Temp. Limit, also) represents a constant "firing" temperature (in your case, that's the temperature of the combustion gases at the trailing edge of the first-stage turbine nozzles). The sloped portion of the curve has a negative slope--to the right and down. Since actual "firing" temperature can't be/isn't monitored, CPD (or CPR) and exhaust temperature are monitored and through decades of operation and data-gathering GE has established a relationship between "firing" temperature and CPD/CPR versus exhaust temperature--and that's what the Exh. Temp. Control Curve is trying to control: "firing" temperature.

So, "firing" temperature remains constant along the line that's a plot of CPD/CPR versus exhaust temperature. As ambient conditions change and compressor efficiency decreases and hot gas path components degrade, CPD/CPR will decrease and the corresponding exhaust temperature will increase FOR THE SAME "FIRING" TEMPERATURE.

When IGV Exhaust Temperature Control is active and when the Inlet Bleed Heat System is active, the exhaust temperature can still never exceed the CPD-/CPR-biased Exhaust Temperature Control Curve/Limit--that represents the maximum allowable exhaust temperature under any condition, Base Load, or Part Load.

When the IGVs are modulated to decrease air flow through the unit to maximize exhaust temperature (when IGV Exhaust Temp. Control is active) or when the IGVs are being modulated closed during Inlet Bleed Heat system operation (and IGV Exhaust Temp. Control is also active), the IGVs are never closed such that the exhaust temperature would exceed the CPD-/CPR-biased Exhaust Temp. Control Curve/Limit.

This description is basically for unloading, when the IGVs are modulated in the closed direction to allow Premix Combustion Mode operation below approximately 80% of rated power output. When the unit is being loaded, the IGVs are held closed to minimize air flow through the unit while the IBH (Inlet Bleed Heat) system is in operation in order to cause the unit to transfer into Premix Combustion Mode sooner than would otherwise be possible--but as the unit is loaded and exhaust temperature increases the IGVs must be opened in order to prevent the exhaust temperature from exceeding the limit until the IBH system is de-activated, at which time the unit will be loaded on IGV Exh. Temp Control until it reaches Base Load and the IGVs are fully open.

markvguy

 

Does it mean when part load to maintain constant firing temperature exhaust temperature can increase around 600 deg.c. and on base load to maintain constant temperature exhaust temperature should decrease? or if exhaust temperature increase on base load to 600 deg.c. then it will reach isothermal constant.

I did not also get the point of your writing 'the operation of Exhaust Temperature Control is counterintuitive, that is, it seems that when the exhaust temperature limit increases that power output (load) should increase--but it doesn't, power output (load) decreases when the exhaust temperature limit increases'. Could you please explain a bit more.

regards,

anonymous

 

hai,

thank you for your informative reply.

when the inlet bleed valve opens as you said compressor inlet temp increases as a results in density decreses.how this will help in increse in compressor operating regime or surge limit.con you explain by graph.

thanks
kumar

 

As for the compressor operating limit/margin, one kind of has to take GE at their word on this. Axial compressor design is very high science, and is considered very proprietary. While GE has on occasion published some graphs, they have never been very detailed.

You can search the Internet for information on axial compressors; unfortunately most of it contains a lot of mathematics to explain what can happen when a compressor is operated near or at the limits of its design. Suffice it to say that if one has ever heard an axial compressor stall or surge, one will never quite forget the sound--nor the fear it brings to those who hear it!

You might also try searching the Internet for 'axial compressor stall' or 'axial compressor surge.'

Since this author isn't one for higher mathematics (y=mx+b is about the limit), and has experienced axial compressor stalls twice, if the OEM says this is necessary to prevent compressor problems--he gladly accepts it without much question!

One of the reasons this question gets asked is because the efficiency of the machine is reduced when a portion of the compressor discharge is recycled to the compressor inlet--and when people realize this, they think they're being "cheated" of power output.

Well, on some units Inlet Bleed Heat can be disabled with a target on a display--but when it is, a unit equipped with DLN-I combustors cannot be operated much below about 80% of rated power output and remain in Premix combustion Mode. So, if one wants more "turndown" (the term used to describe an expanded operating range of Premix combustion mode), one has to be content to live with a little inefficiency.

The alternative is to muck with the sequencing and configuration and try to run in Premix combustion mode without Inlet Bleed Heat. Just don't expect any sympathy when the unit crashes or keeps losing flame at part load....

You can also ask this question of the packager of your GE-design heavy duty gas turbine. GE has provided them with some documentation which they may or may not have included with the unit they provided you.

As for an explanation of Exhaust Temperature Control and IGV Exhaust Temperature control, well, that is taking some time.... It's not easy in this forum since one can't post graphs....

They are both also very difficult subjects to describe concisely (and briefly) in writing.

markvguy

 

<p>Gas turbine design engineers have found over decades of operational experience that a relationship between "firing" temperature (in your case, the temperature of the combustion gases at the trailing edge of the first stage turbine nozzle), exhaust temperature, and CPD (CPR) can be determined, such that for any combination of CPD (CPR) and exhaust temperature, the "firing" temperature is known/can be determined.

<p>By controlling "firing" temperature, parts life and power output can be optimized. However, "firing" temperature is not measured or monitored by any sensor--there is no sensor that can be used to monitor "firing" temp which is economically viable (which will last long enough and is inexpensive") and which, when damamged, will not cause and damage to the turbine nozzles and buckets.

<p>The relationship between exhaust temperature and CPD (CPR) is represented on an x-y plot usually referred to as the CPD (CPR) Biased Exhaust temperature Control Curve. (It's not really a curve, but that's what it's called...) There is an upper limit to the allowable exhaust temperature, often referred to as the "isothermal" limit. The "isothermal" limit is set to protect exhaust components (diffuser, duct workd, etc.) from damage at high temperatures/low exhaust flows.

<p>The sloped portion of the curve represents a constant "firing" temperature that is the optimum, maximum power output of the unit--Base Load. If more fuel were to burned, the "firing" temperature would increase and the turbine nozzles and buckets would not last as long (of course, more power output would result but the period between maintenance outages and hot gas path component replacement would decrease--which increases cost of operation...).

<p>An Exh. Temp. Control "curve" looks something like this:
<pre>
| isothermal
|________________
| + + + + +\
| + +\
| + +\
| + +\
| + +\
TTRX | +\ Constant
| + "Firing"
| * * Temperature
| * \
| * \
| * \
| * \
| * \
|_________*____________________
CPD (or CPR)
</pre>
<p>The above is an attempt to draw a plot of TTRX (Turbine Temperature Reference-Exhaust) on the y-axis and CPD (Compressor Pressure-Discharge) or CPR (Compressor Pressure Reference--a fancy way of measuring compressor discharge pressure) on the x-axis. TTRX is a function of CPD (CPR). TTRX is also the Exhaust Temperature "limit" for the unit--meaning that for any value of CPD (CPR) the exhaust temperature should never exceed the corresponding Exhaust Temperature (TTRX).

<p>On the graph, CPD (CPR) increases from left to right; TTRX increases from bottom to top. The flat portion of the graph represents the "isothermal" temperature limit of the Exh. Temp. Control Curve--the maximum allowable exhaust temperature
under any operating condition.

<p>Without IGV Exhaust Temperature Control, as one loads a gas turbine, more fuel is being burned and the usual result is that the actual exhaust temperature, TTXM (Turbine Temperature-Exhaust, Median--really, it's the average value not the median value...), the "firing" temperature, and the CPD (CPR) all increase.

<p>When the actual exhaust temperature (TTXM) equals the Exhaust Temperature Reference (limit) for the current value of CPD (CPR), the unit is then on CPD (CPR-)biased Exhaust Temperature Control, or, Base Load. The actual exhaust temperature of a unit without IGV Exhaust Temperature Control is represented by the "*" line; TTXM increases with load until it equals TTRX, and then the unit is on CPD (CPR)-biased Exhaust Temperature Control.

<p>The "+" line represents what happens to exhaust temperature (TTXM) while loading and unit with IGV Exhaust Temperature Control active. The IGVs are held closed as long as possible to maximize exhaust temperature; TTXM will never be allowed
to increase above TTRX. (The same thing happens when the unit is being unloaded with IGV Exhasut Temp Control active. As fuel is reduced and TTXM would tend to decrease, the IGVs are modulated closed to maximize TTXM--but never exceed TTRX. NOTE: The "+" line cannot be drawn "on top" of the isothermal or Exh. Temp. Reference lines in this forum; but in reality it would be.

<p>Now here's the counterintuitive part: When the unit is operating on Base Load control, as the CPD (CPR) increases, the Exhaust Temperature Reference (limit) DECREASES--and load INCREASES--for a constant "firing" temperature. During part load operation, as load increases exhaust temperature increases and "firing" temperature increases--but on Base Load control as load increases exhaust temperature decreases while "firing" temperature remains constant! That's represented by the negative slope of the Constant Firing Temperature portion of the curve--as CPD (CPR) increases to the left, the corresponding Exh. Temp. decreases and the "firing" temperature remains constant.

<p>Remember: The sloped potion of the graph represents a constant "firing" temperature and the relationship between TTRX and CPD (CPR) when the firing temperature is constant. The Speedtronic is controlling fuel to control exhaust temperature based on the current value of CPD (CPR). Exhaust Temperature Reference (TTRX) is a function of CPD (CPR).

<p>Now, the question should be: Why is IGV Exhaust Temperature Control necessary for a DLN-I unit with Inlet Bleed Heat? The answer is simple: The design of a DLN combustor forces almost all of the air entering the combustion liner to do so through the primary combustion zone for premixing. If air is not reduced while fuel is reduced the air-fuel mixture in the primary combustion zone will be too lean, and high dynamics and/or flame instability will occur (even though GE says there's no "flame" in the primary combustion zone when operating in
Premix combustion mode...there is no diffusion flame, but where there's a temperature increase (and there is a temperature increase in the primary combustion zone when operating in Premix combustion mode) this author says something is burning...).

<p>Most GE-design heavy duty gas turbines with DLN-I combustors are not designed to be operated at rated speed with IGV angles less than approximately 57 DGA (DeGrees Angle). In order to remain in Premix combustion mode below approximately 80% of rated power output, the IGVs must be closed less than 57 DGA. To close the IGVs below 57 DGA, Inlet Bleed Heat is required.

<p>markvguy

 

Hai,

Thank you very much. your reply is very informative. thanks lot to this forum.