CPD transmitter failure

J

Thread Starter

jk

I have worked in GE Frame 9E machines with out DLN and seen CPD transmitter failure will not trip the GT. I would like to know the same in DLN machines like Frame 9FA.
 
Dear JK,

I don't know if there is a single answer to your question. I think a lot depends on the type of control system and the vintage of the machine and type of combustion system. Obviously CPD is more important in a DLN combustion system than with an older diffusion burn system.

I know with older machines there was typically a single CPD transmitter (I think due to cost mostly), and its input would be shared by all processors if the system was a TMR type of system.
In later machines transmitter 3 CPD transmitters were used (DLN combustion, lower transmitter pricing, and newer control systems), one typically for each controller core.

But how logic is written can be different site to site, machine to machine.

I think the preferred control is using CPD to help calculate compressor pressure ratio, and finally fuel control. But as you know there is typically a backup firing curve based on megawatts, which is useful but not typically the desired way to operate. But the decision of if the machine will be tripped or not, will lie in the application code running, and I just don't think or know for certain that this is the same for all machines and years.
 
> I think the preferred control is using CPD to help calculate compressor
> pressure ratio, and finally fuel control. But as you know there is
> typically a backup firing curve based on megawatts, which is useful but not
> typically the desired way to operate.

Hi Mark

can you explain CPD Vs MW back up control in case CPD fails? There are some gray areas for me to understand. I hope your explanation may make them clear.

Thanks
Sardar9
 
Dear Sardar9,

I don't think I can do the job of explaining the way that CSA would, but I will give it a quick try.

In the GE heavy duty gas turbine the attempt is to control fuel into the turbine to control emissions, protect hardware components, and maximize heat and or megawatt output. At some point as you raise megawatt output or heat output of the machine the firing temperature at the first stage nozzles will reach the design limit of the material.

This temperature at the first stage nozzle is not measurable due to constraints of sensor technology. So in this case it is derived from calculations based on exhaust temperature and compressor pressure ratio (CPR), which CPD is used to calculate. For a given CPR and average exhaust temperature reading, the temperature at the first stage nozzle can be calculated, and so firing rate can be controlled to protect turbine hardware. This use of CPD and CPR is the most accurate way to calculate mass flow and in the end the firing temperature(TTRF1).

In the case of a failure of the the CPD sensor system, the backup way to "estimate" compressor flow is to look at megawatts. For a given load or megawatts the compressor mass flow can be estimated and used in the calculation of TTRF1 to control firing temperature. Of course this is not as accurate and leaves more room for inaccuracies due to compressor cleanliness, ambient conditions, etc. that effect the actual mass flow through the compressor. But usually this backup curve is more conservative than the CPD based curve and has some buffer to it for protection.

I know this doesn't do justice to the explanation of this concept, but it is not a simple concept to explain or understand. I hope that I have helped you understand it at least slightly better.
 
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