Effect of Supl firing grid plate on gas turbine air flow


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We have Frame6 MarkV m/c 30MW. We have implemented inlet air chilling to improve the throughput of the gas turbine. Originally there were pulse air filters and I have seen m/c generating 36MW at 15degC amb temp, 50Hz freq (India) with TTRX 545degC. As we have gone for air chilling, air filtration sytem also changed. At present we have 2 stage filtration, primary filters T-50 type and Secondary MV95. With this system the following problems are being faced: Lower base load o/p 27-28MW than earlier (Donaldson) filters. TTRX remaining 575-78 degC.

Secondly we have switched over to HRSG supl. firing from OIL to Dual firing in which we have installed GRID plate prior to duct burner (whether this can cause lower output), but CPD is also less compared to earlier. We have measured compressor inet pressure which is -175 inwc with IGV full open and base load o/p 25 mw at 36 degC amb. What is the inlet press of compresor with Donaldson filter at base load? (We do not have previous data.)
The exact order of the changes is important in order for people to understand what was done when. Additionally, it would be best to know exactly when each data point was taken, as well as the conditions when the data was taken because we would need to know if all or any of the data was taken with the inlet chiller running or not.

I think you have said that you have a GE-design Frame 6 turbine-generator, rated at approximately 30 MW which was originally equipped with self-cleaning inlet air filters (made by Donaldson). Then a chiller was added to the inlet and the power output increased approximately 20% (and I think by "15degC amb temp" you actually meant compressor inlet temp???) to 36 MW.

Then you say you changed inlet filters and you now see a much lower Base Load output, but is that with the inlet chiller running or not? Because you say that's at an ambient of 36 deg C; what was the compressor inlet temp if the chiller was running?

Then you say you have swithced supplemental firing from oil to dual (what's the second fuel? nat gas, propane, naptha, butane?).

Again, the exact order of the changes, the conditions when the readings were taken (as well as proper descriptions of the readings), and the nature of the changes are all important to try to troubleshoot any problem. All of these changes probably didn't occur at the same time, or even in the same month or quarter

Any time you put something into the inlet or the exhaust of a gas turbine that was not originally included in the design you are affecting air flow through the unit. I have seen an extra set of economizer tubes added to an HRSG cause a 3% power output decrease of the gas turbine. (And of course, it was the gas turbine's fault; NOT!) I've also seen insulation drawn from behind plates in the exhaust transition duct get stuck in the superheater tubes and cause a 10% decrease in performance (again, initially (incorrectly) blamed on the gas turbine!).

Remember how gas turbines operate: Suck, Squeeze, Burn, Blow. If you make it harder to suck (draw air into the compressor), squeeze (dirty compressor, increased clearances), burn (fuel restriction, low supply pressure, nozzle problems), or blow (increased exhaust backpressure) you will decrease the power output of the gas turbine.

We can't possibly understand the nature of the changes you've made. You need to get someone to site to help with collecting all the manufacturer's data (expected differential pressures, flow-rates, etc.) and observe the running conditions and make some observations and recommendations. Something you haven't included is the exhaust duct back-pressure at the gas turbine exhaust.

Usually, there are conditions which were stated and taken into account when the unit was built and installed, such as ambient pressure, ambient temperature, inlet conditions (static pressure/pressure drop), exhaust pressure, etc., which can be referred to in your analysis. You can work with the OEM (Original Equipment Manufacturer) to obtain the information to use in your analysis.

Best of luck!

M/C installed in 1995 with self cleaning filters with no chilling facility (GE is not in favour of chilling) even after running for 5-6years, i.e. in 2001 to 2002 during winter season when ambient temp used to fall at 15 degC at that time, m/c used to deliver even 36 MW (no chilling). 3-4 months back we have incorporated inlet air chilling with self cleaning (Donaldson) filters. It is not possible as it requires primary filters>>chilling coil>>secondory filters. So Donaldson filters were discarded. At the same time we have gone for the new duct burner system for HRSG (dual firing-Naphtha & Natural gas). For this duct burner supplier has installed GRID plate before burners. Also we have removed Economiser-2 of HRSG and installed LP evaporator between ECO-1 and STACK. After completing these three jobs at a time, we have started the m/c and gone up to base load but we have seen that m/c is delivering 26 MW at 32 degC ambient temp. Then we have started Chilling system and brought down ambient temp. to 20 degC but we have gained only 2 MW, i.e. 28 MW with chilling. After running for 2 months, the primary air filter got choked and at one time m/c was at base load with 20 MW without chilling with new air filter DP was 6 inwc (fresh filter DP 0.5 inwc). I was asking about inlet pressure of compressor. We had not measured it for Donaldson filters but presently we have measured it in between INLET DUCT & INET PLENUM (Do you know this pressure for Donaldson filters?). Also GT exhaust press is 185 inwc at 26 mW base load (I see there is not much change in this press.) CPD is also less compared to earlier. (As three changes made at the same time we are finding the exact problems). I will come to you with more data.

Thanks again.
It's really difficult for me to understand what you're describing. I repeat, have a consultant or a turbine manufacturer/packager representative visit your site and observe the modifications you have made, take some measurements, perform some analysis, and make recommendation(s).

Can't your duct burner supplier tell you how much pressure drop the new grid plate will add to the gas turbine exhaust (how much it will increase the back-pressure)? Can you measure, or have you measured, the pressure drop across the new exhaust duct components? It really seems to me (from what I can glean from your description) you are fighting more than one problem, and if I had to guess (and that's all I can do right now) which one was the bigger contributor to the reduced power output I would say it's the exhaust back-pressure. Exhaust duct pressure is difficult to measure; I've usually seen it accomplished using something commonly referred to as a "pancake probe" for accurate measurement, and the placement of the probe is also important. 185 in wc seems excessive if it's a real number, especially if it's right at the gas turbine exhaust where it transitions to the HRSG.

You keep asking about Donaldson inlet filter pressure drops; these are usually measured with a differential pressure gauge mounted in the self-cleaning control enclosure measuring ambient pressure (atmospheric; barometric) upstream of the filters and the pressure downstream of the filters. In my experience, if the pressure drop exceeds approximately 6 in wc, an alarm, and in some cases an automatic shutdown, is initiated. Quite often, with self-cleaning inlet filters the differential pressure gauge has adjustable contacts built in to the assembly, and those contacts can be set to start the cleaning at some adjustable differential pressure setpoint, and also to stop the cleaning when the differential pressure drops below some differential pressure value (also adjustable). Have you tried measuring the differential across the "chiller assembly" (including all the filters and the chiller coil)? It should be a relatively simple task to use the differential pressure gauge from the Donaldson control panel to measure the differential across the "chiller assembly". Or, just make a simple water tube manometer and leave one side open to ambient (atmospheric; barometric) pressure, and connect the other side to a point downstream of the secondary filter of the chiller assembly. You should be able to compare this against the expected pressure drop which your chiller supplier can provide or should have provided.

There is usually an inlet silencer section downstream of any inlet filter; was this section removed or left in place when the chiller assembly was installed? (It's usually just some "baffles" made of perforated sheet metal with insulation between the sheets, and there are usually several of them arranged vertically in the air flow. These don't add too much to the differential, but do contribute something.)

Again, I really think you need to have someone visit your site, observe the configuration, possibly assist with taking data, perform some kind of analysis, and make some recommendation(s) to improve your power output from the gas turbine. (I'm sure the gas turbine exhaust temperature is higher than it was before; so the HRSG is probably making a little more steam, or at least it's hotter than it was before!)

It would be really interesting to follow this and learn from your findings and experiences; please let us know how this progresses and is ultimately resolved.
Thanks a lot. We are collecting related data (a lot of data at maximum possible points). Surely I will be back to you. Thanks again.