from the Forum department...
combustion trouble
 Posted by ESKAY on 3 August, 2010 - 12:51 pm
Dear All,

Combustion monitor algorithm of MK5 and Mk6 GT, automatically and internally calculates the allowable spread and Spread1- SP2 and SP3. One of our turbine GE Fr.5 Mk6 control
running at 12 MW reads TTXSPL =55degC TTXSP1=35degC diff and TTXSP2=34 deg.C diff.

Could someone Pls tell me,what must be the minimum acceptable value in deg.C between TTXSPL and TTXSP1. Is the 20deg.C differential between TTXSPL and TTXSP1 in my example,OK to keep the turbine running continuously. Why the spread1-2-and 3 mentioned in Degree differential?

Pls someone clarify my point and explain..

 Posted by CSA on 4 August, 2010 - 3:17 pm
ESKAY,

Combustion monitoring is a complicated subject. It involves an algorithm BBL performing some sorting of exhaust T/C values and locations, and then performing comparisons of values and locations to determine if there is a problem. And some of the signals which are derived in the algorithm are then used in some relay ladder rungs in the sequencing/application code to determine if there is a combustion problem.

To complicate matters further, different Frame size units use different Combustion Monitor algorithms, so it's very difficult to explain exactly how Combustion Monitoring works.

The allowable spread is a number which is calculated in order to set a limit (which is dynamic as opposed to fixed, and can change (slightly) with machine/ambient conditions). And, then the highest and lowest exhaust T/C values, and their locations are compared to determine if they are adjacent (next to or close to each other). [If the highest and lowest T/C readings are adjacent to each other that's a very good indication of combustion trouble. And, if the next highest or next lowest value is within some percentage of the difference between the highest and lowest, then that's even a better indication. And so on.]

The 20 deg C differential between the allowable and the #1 spread (usually the difference between the highest and lowest values) is not bad; not great, but not bad. But it's relative. The real problem occurs when the highest and lowest values are adjacent, so that's what you really want to be looking at in conjunction with the differential. Because, that's what the Speedtronic is doing; it's looking at the magnitude of the differences AND the position of the highest and lowest values with respect to each other.

There is no hard and fast differential which will cause the unit to trip. It's the magnitude of the differential as well as the adjacency of the differentials that are used to determine combustion problems and alarm or trip the unit. However, the

To understand the actual parameters in use for the turbines at your site, you would need to review the sequencing/application code and the Control Constants used for the turbines at your site. While many units use typical or default values, not every turbine does so and most people don't want to know how things work "generically" they want to know how their machine works so please refer to the configuration of your machines' control systems.

I don't have access to any Speedtronic information at this time, but a hint for everyone is that the "sorting" done by the Combustion Monitor algorithm results in two arrays. I believe the arrays are JXD1_0-nn and JXD2_0-nn, where nn = (Total Number of Exhaust T/Cs minus 1). So, for example, if the unit has 18 exhaust T/Cs then nn would be 17 (18 -1).

If you put these into a User Defined Display or Watch Window or display, you can monitor them on a real-time basis. They will become very evident when you get them all arranged on a display, and then you can add the allowable spread and the three calculated differentials, and then everything should become much clearer. A little bit at least.

The topic of Speedtronic combustion monitoring and exhaust temperature spreads has been covered many times before on control.com, and those previous posts can be found using the 'Search' feature in the upper right corner of the top of every control.com page.

 Posted by ESKAY on 7 August, 2010 - 2:41 am
Dear CSA,
Thanx for the explanation to workout thro combustion algorithm arrays to identify the real cause of the spread and its magnitude.

Combustion spread is indeed a difficult subject to understand and evaluate.I shall feed the two arrays values in watch window OR display page to find out the real tme values of Exhaust temperatures during the next opportunity {Turbine under shut down at present} and revert back to you.

Meanwhile, I would like to know How much minimum Deg.C diff. is allowed between TTXSPL and TTXSP1 in order to keep the turbine running continuously, as the combustion trouble alarm does not activate with the differential of 20 deg.C between TTXSPL and TTXSP1.

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