Hi guys,

We have two Frame 9E gas turbines namely GT1 and GT2. As you can see in the attached data (plant data jpeg file), the heat rate for GT2 is higher than GT1. Compressor in GT2 is replaced early 2012 with honey comb brush seals at the inner barrel. The CPD achieved after the compressor replacement is 11bars compared to 10.3bars previously.

My question is, what can cause the gas consumption to be higher? Both GTs have same control settings. Is tuning necessary after the Major Overhaul works? We suspect that the turbine is working harder to drive the compressor due to the honey comb brush seals. I appreciate any information or guidance given. I am willing to provide all the data required.

Design Data: https://docs.google.com/open?id=0B9IltUsseK6LbEk5dk9vUW14NVU

Plant Data: https://docs.google.com
/open?id=0B9IltUsseK6LR0NKQlFtSDhxTFk

 

Hi RY,

Heavy duty gas turbine performance and efficiency are a function of MANY parameters and internal clearances. Turbine inlet air filter type and condition; axial compressor cleanliness; turbine hot gas path components and fit/alignment; turbine nozzle and -bucket condition; exhaust back-pressure; and IGV LVDT calibration, are just a few of the many parameters that all affect performance and heat rate. Do both units have exactly the same turbine inlet air filters, with exactly the same differential pressure (bot dp's measured with one set of instrumentation) and are the IGV LVDTs on both units calibrated properly and are the axial compressors on both units in nearly identical states of cleanliness and are the turbine nozzles and -buckets in both units in exactly the same condition (age; wear; clearances)?

Even if you have two automobiles built one right after the other on the same assembly line by the same workmen with all of the same equipment and accessories and driven by the same individuals (at separate times, of course) the performance and mileage of the two cars will vary.

Many times I find the instrumentation used to measure gas fuel flow is less than ideal and yet it's being used to measure heat rate and compared against revenue-quality metering. Differences in instrumentation calibration, metering tube and orifice conditions, etc., can contribute to some error in the readings.

You would be correct that the compressor should require more horsepower if it were more efficient, but gas turbines are mass flow machines and the increased air (mass) flow should increase total power output AND allow for slightly more fuel to be burned <b>when operating at Base Load.</b>

There are simply too many intangibles to be able to say with any degree of certainty why there is a discrepancy in heat rate between two units, even two units installed and commissioned at the same time and being operated with the same parameters and in a similar manner. Generally, the periods between maintenance outages are staggered somewhat, as are the periods between off-line compressor washes. The components being used in the maintenance outages are never exactly the same, and are never assembled exactly the same (with the same tolerances). If the gas turbines exhaust into HRSGs ("boilers") then the back-pressures of the boilers are never usually identical.

I wish the news was better, but it's virtually impossible for us to pinpoint anything for you without a lot more information, and even then, this is a free forum and what you are asking for requires a lot of effort--some of which should require some very detailed calculations and data analysis, and which should be contracted and appropriately compensated.

The last comment I wish to make regards your reference to the 'Expected Operating Parameters' section of the Control Specification. Those values were calculated at the time the turbine was being built based on information provided to the OEM about the expected fuels available on the site at the time the turbine is to be commissioned. Many sites experience changes in fuel composition over time; sometimes, gas fuel composition can vary greatly over 10 or 20 years, or even shorter periods depending on the site and where the gas fuel originates. Those values are guidelines and it's also interesting that the date of the drawing is missing from the bottom of the .pdf file.

Whenever using these sections of the Control Specification, they should be used as references and guidelines and for a proper comparison to current operating conditions one should have a very recent fuel analysis to use to note any differences between the Control Specification values and current fuel make-up and composition.

 

Nice reply by CSA.

G.Rajesh

 

Great reply by CSA. Most of his answers are stock (right out of the book) but I would assume that you have already done your homework and are looking for something not quite so obvious. Let me reference you to an article I wrote back in 1997 and I believe is still not fully realized by many in the industry. Although most refurbishment shops added the procedure of "polishing" their refurbished components after the article was published, I still believe most power plants and even some refurbishment shops don't even have a surface finish measuring device. I gave a presentation on this subject at PowerGen (I can't remember the year) and as I walked around the show floor there were all kinds of booths peddling their refurbishment prowess and I was stunned to see components with the surface finish of sandpaper! Note in the article I had measured newly refurbished nozzles with a 400 micro inch surface finish. If this was the case on your first stage nozzle, using GE's information (Table 1), you will have approximately a 2.8% increase in heat rate on just that one component alone! If you remember installing any gas path components ANYWHERE on GT 2 that didn't have a velvety smooth finish, bingo, that may be your problem.
http://www.power-eng.com/articles/p...nozzle-surfaces-hurt-turbine-performance.html