what is exactly combustion dynamics in Gas turbine? can someone please explain me about it. as it is very important for me to understand the combustion philosophy. i am fresher in Combine cycle power plant.

 

Combustion dynamics refers to the combustion process inside the combustion "can" and "liner". When fuel is burned, there is a pressure increase, and depending on the design of the combustor, the fuel nozzles, the liner, etc., the combustion process can be smooth or it can be subject to pressure oscillations or pulsations. These oscillations or pulsations, if not minimized, can lead to premature failure of combustion components as well as unstable flame.

When fuel is burning in a combustion turbine, there are very high air flows and this causes turbulence which is both desirable and undesirable. It's desired to achieve good mixing with the fuel for efficient combustion, but it's not desirable because it can lead to high pressure oscillations/pulsations.

Some pressure oscillations or pulsations can be like pressure pulsations in a pipe or vibrations, they can be "exaggerated" at some points and become very, very destructive. They can be said to have resonance or resonant frequencies which need to be attentuated or avoided.

It's practically a science all unto itself and for some combustion systems, especially it seems those with lean fuel/air ratios, they can be very difficult to achieve a balance of stable combustion, stable "flame", low dynamics (pressure oscillations/pulsations), and low emissions (which is the purpose of lean fuel/air ratios in combustion turbines).

 

Thanks for quick reply.
as i am working on GE 9FA turbine can you please shed some light what GE has done to reduce turbulence as you said it can lead to high pressure oscillations/pulsations which might damage the combustion parts (like liner , TP and combustion sleeve)?

 

As the saying goes, "I could tell you; but then I'd have to kill you."

In other words, it's proprietary to GE's DLN (Dry Low NOx) combustion system. There are a couple of "knobs" depending on the type of DLN system; some older systems used to have valves between combustors. Most of it has to do with liner and fuel nozzle design. And some of it has to do with fuel splits, which are optimized during DLN tuning.

Some units have "permanent" combustion dynamics monitoring systems which are used to detect possibly damaging situations and the operators are then notified to take some kind of action. This can be through alarms in the turbine control panel, or via a phone call from GE's Remote Monitoring & Diagnostics center (for those units with On-site Monitors (OSMs) and a contractual services agreement or M&D agreement).

 

We have CDM system on our HMI screens. I want to learn, what is the critical values for CDM values. 4 or 5 or 16 psi is critical value. Also critical values are same for peak-1, peak-2, peak-3?

Another case: one of our machıne has 7psi in peak-1, 20psi in peak-2, 6psi in peak-3. is it a dangerous situation? But adjacent nozzle's CDM values are very normal like 2 - 1 - 1,5 and something like that.

regards..

 

Hi,

You did not tell which turbine model you have.

On Frame 9FA with DLN2+system dynamics bigger than 2.5psi are considered high and would require strict monitoring. This is valid for all three frequency ranges. Dynamics higher than 5psi are considered excessively high and would require corrective action - DLN tuning.

It can happen that single combustion chamber has much higher dynamics than adjacent but provided by you figures: 20psi v.s. 2psi are not normal and really dangerous.
It is highly probable that the root cause of excessive dynamics in one chamber are already damaged components of the combustion chamber; damaged by excessive, long lasting dynamincs.

Another option is that pressure sensing loop in that chamber is not working properly. In the JB with electronic components that is installed typically on the side of the turbine compartment (where are landed cables from dynamics monitoring pressure sensors) there is a testing plug. By replacing the connections from pressure transmitters with this testing plug it is injected testing signal into direction of CDM monitor - if I am not mistaken testing signal simulating 1psi.
If site has a special testing generator ( I think it is not standard part of the system - site should purchase it separately) it can be installed near the combustion chamber, replacing the plug from pressure sensor and it can be tested additionally cable from pressure transmitters to CDM monitor. It is not possible to test easily pressure transmitters.

Combustion dynamics are affected by gas property represented by wobbe index and this in turn depends on gas composition and gas temperature.
- Do you monitor gas composition? Is it the same as during the last DLN tuning? Even small changes in the gas composition can provoke big change in dynamics.

- Is gas heated on your site? Do you keep this temperature the same as during the last DLN tuning?

- Are gas valves properly calibrated? Was calibration of the gas valves changed from the last DLN tuning?

- If you have on your site purge valve PG-2,did you check that valve opens properly? On standard configuration, there is no continus feedback of this valve position, only limit switches and on system with heated gas, high temperature damages the I/P converter.

Regards

 

Hi,

Thanks for your detailed answer.

I'll try to write a detailed problem description and what happened about this problem. Maybe these information will be helpful for somebody in the future.

As a beginning,our machine is GE 9FA with DLN2+.

-This sudden increase in these values are occurred in nearly 243Mw and as I indicated before, peak1-2-3 increased to 17-44-19 psi.

-This abnormality occurred after 120 hours of the start.

-During S/D there was no maintenance activities in gas skid, neither on valves nor on software about gas and purge valves.

-There was not any abnormal situation with the adjacent cans. They were nearly 1-2 psi

-Also there was not any abnormal vibration

-Everything was normal with the continuous emmissioning system.

-There was no wobbie index alarm, also HHV has not any significant change.

-In our site gas heated up-to 175C, on the date of event there is not any change in the gas temperature.

Now its time to reply your questions:

> - Is gas heated on your site? Do you >keep this temperature the same as during the last DLN tuning?
YES WE ARE HEATING GAS. YES WE KEEP IT AS THE SAME IN OUR LAST DLN TUNING.

> - Are gas valves properly calibrated?
YES GAS VALVES ARE PROPERLY CALIBRATED.

> Was calibration of the gas valves changed from the last DLN tuning?
NO CALIBRATION DID NOT CHANGE AFTER THE LAST DLN TUNING.

> - If you have on your site purge valve PG-2,did you check that valve opens properly?
YES THAT VALVE OPENS PROPERLY

As a result:
-This situation was discussed with GE urgently and they stated that: This seems not a real reading. It seems like a thermocouple failure. We can run the unit safely like that.

-After 8 hour from the sudden increase, values decreased as sudden as they increased. They declined to 6-15-1 peak1-2-3 respectively.

-After 24 Hours from the sudden increase, values were 4-10-1 peak1-2-3 respectively.

If anybody need any more information or have any question please feel free to contact with me.
Regards..

 

Hi,

Additional information provided by you suggests, as stated by GE in your message, for defective measuring instrument/bad connection, not real reading.

You write: 'It seems like a thermocouple failure.' Do you see excessive exhaust spread that could be linked (by swirl) with the chamber having high dynamics?

I think that dynamics of the magnitude 17...44psi (as you stated) should be possible to hear around the turbine: please compare dynamics readings (around 7psi) at the instant of the transfer to Premix with the noise near the turbine at this moment. The conditions are not directly comparable (one can with dynamics vs. 18 cans during the Premix transfer) but can be used as some reference.

Regards

 

HI
LOOK AT THIS
http://www.skf.com/files/058467.pdf
THANK YOU

<b>Moderator's Note:</b> checked out the link. It is to a paper titled "Dynamic Pressure Monitoring in Gas Turbines."