Gas Turbine GE 9E IGV disabled


Thread Starter


The station is being started and Gas turbines/Generators are synchronized, then the IGV's are disabled to increase station output(so to Grid the generation MW profile is higher, making us more attractive to be called on).

The DCS gives a start point of say 60MW's per GT, but some operators are putting in a profile to grid indicating each GT start point is 95MW. So once the Gens are sync'd, the GT's are loaded to 95MW's.

My question please is simply although the station output looks commercially attractive, and the GT exhaust temps are lower (so the HRSG's are not experiencing high thermal shocks/gradients) with the IGV's disabled (that's the only functions for IGV enabled or disabled). I believe its most inefficient, and possibly detrimental to the HRSG (over time). has anyone experienced/researched this situation. Thanks

Process Value

Ok first of all let us start off with the efficiency calculation for gas turbine and HRSG
For GT
Efficiency = 860*MW output (MwHr) / (fuel flow(kg) * calorific value of fuel(in kcal/kg))
this is the base formula where fuel flow is normally available in M3 which you have to convert to mass with the known density.

for HRSG
efficiency = steam flow rate * enthalpy of steam / ( (HRSG inlet temp - HRSG outlet temp) * 0.25 * air flow rate)

the combined efficiency of the co generation is given by

efficiency = ( (860*MW output (MwHr)) + steam flow rate * enthalpy of steam) / (fuel flow(kg) * calorific value of fuel(in kcal/kg))

now let me calrify a few points

1. IGV temp control On and Temp control off is indeed used to regulate the air flow into the turbine. this is done so that you can increase the exhaust temp during "part load" conditions , thus having IGV control helps in maximizing the steam output during part load operation. the igv does two things , reduces the air flow and increases the HRSG inlet temp , thus from the HRSG efficiency equation you can see that the IGV operation increases the overall efficiency of the HRSG.

2. with IGV control OFF , during part load conditions , even though air flow is higher , the exhaust temp reduces so there will be a reduction in the steam flow. this means that the over all co generation efficiency will come down. Thus for part load operation IGV operation is a must if you need to improve the plant efficiency.

3. you seems to have been misinformed about thermal shock. thermal shock is caused by sudden raise in temperatures not by sustained high temperatures. HRSG's ( and boilers) tubes are all in fact designed for 100000 hours of operation, with the maximum temperature gradient ( calculated from LMTD) of the particular section. you can operate the HRSG with IGV control for sustained periods and it will not cause any thermal shock to the HRSG.

4. all gas turbines are designed primarily for base load operation and they are optimized for that. the performance guarantee of your gas turbine , which specifies heat rate , HRSG steam flow and efficiency are all for full base load operation. part load efficiencies are well naturally lower. thus operating the plant close to the base load of the machine (you have mentioned frame 9E , which has a iso rating of 122-127 MW and probably depending on your site condition it will be around 100 i guess ) is always the best way to achieve efficiency.

5. the IGV control has been designed so as to automatically open for higher load operation as you need more cooling air to maintain a stable exhaust within the temperature range. so IGV will be open almost fully for the 95 MW operation . thus enabling or disabling it will not cause any significant effect in the operation.

6. IGV operation in a GT ranges from 50 t0 85 deg. however , the amount of air control is quite small. for a igv at 50 deg to IGV at 85 deg , the air flow variation will be only 10 - 15 % depending on the design and site condition.

from the above i will summarize this

a. for part load operation go for IGV control , as it will increase your total overall plant efficiency

b. for base load operation IGV control on or off does not have any significance , IGV by default will be fully open so you don't have to worry about it.

Moral of the long story is, put the machine in IGV control , it will automatically take care of the rest :) . If you are having any abnormal conditions at your site, ie with IGV control on you are experiencing high spread or any other combustion difficulty , please provide that information also. If that is not the case you can always go for IGV control ON.
Many thanks to "Process Value" for the information, it's appreciated.

So it is as you state an inefficient operation.However the thought process by the Operator (as you can imagine) was not concerned with efficiency (in this Stn start up scenario)but purely offering more MW's to NGC,making it more attractive for NGC to call the station on (only concerning the MW start point value, or first generation elbow point)in doing this the GT's (2 off) exhaust temps would be higher eventually causing the HRSG's (2 off) outlet steam enthalpy and the STA Stop Valve enthalpy to exceed the max start up value before the STA run up sequence permissives have been met, so the IGV's are disabled, once the STA/GEN is syc'd then the IGV's are enabled.
Many thanks again for your advice.