Fluctuating Allowable Spread in Hitachi H-25 Gas Turbine


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Recently I am facing an issue of fluctuating allowable spread in Hitachi H-25 gas turbine. The value of allowable spread is fluctuating from 80 degrees (which is usually the normal value at load of 21MW) to 150 degrees at 21MW. Values of actual spreads are normal as per previous history and showing no abnormal behavior.

Note: There is no substantial change in compressor discharge pressure which is one of the factor in calculation of allowable spread.

When did this problem start?

What Diagnostic Alarms are present and active (even the ones thought to be irrelevant to this problem)?

>Note: There is no substantial change in compressor discharge
>pressure which is one of the factor in calculation of
>allowable spread.

One of the other major factors in the allowable spread calculation is CTD (Compressor Temperature-Discharge), if I recall correctly. Most people don't think the compressor discharge temperature T/Cs do anything--but, if I recall correctly, they are key to the TTXSPL calculation. What are the individual compressor discharge T/C readings doing (usually signal names CDTA-1 and -2 (Compressor Temperature-Discharge, Annulus 1 and -2), or something similar; if a Mark VI, the signal name may be in lower-case alpha characters)?

Hope this helps. Please write back to let us know what you find
Thank you CSA for your response.

We are facing this issue after recent HGPI performed two months ago.

There is no such alarms or indications on the system. Only concern are the peaks in allowable spread chart at regular interval. The fluctuation is even more enhanced during load change.

No variation is observed in the CTD. Also there is hardly difference of 1-2 degrees between CTD-1 & CTD-2 temperatures. And yes you are absolutely right about TTXSPL, it caters CTD in its calculation.

Is there any possibility of instrumentation malfunction, as in Thermocouples reading or in transmission of signal that is further used in Allowable Spread calculation?

I don't have access to any software with the block that calculates TTXSPL, but you say both CPD and CTD are stable.

If I recall correctly, TTXM is another of the major inputs to the block that calculates TTXSPL. What is TTXM doing?

Whenever a load change is underway, the allowable exhaust temperature spread is "biased" out of the way. So, whenever L70R or L70L is a logic "1" (which are driven by a LOT of different conditions, including an operator-initiated RAISE SPEED/LOAD or LOWER SPEED/LOAD, or commands from Pre-Selected Load Control) then the allowable spread is sent to a maximum value and then it slowly returns to it's calculated value over some period of time.

If the exhaust T/Cs are "spiking" that would have an effect on TTXM (the average exhaust temperature, minus the highest and lowest values and ignoring any failed exhaust T/Cs) which would likely have an effect on TTXM. Are any of the exhaust T/Cs spiking?

When you say "...peaks in allowable spread chart at regular intervals..." can you be more specific? If you are trending TTXSPL with Trend Recorder, add L70L and L70R to the list of signals being trended and tell us if you see any relation between when L70R & L70L and the peaking of TTXSPL.

Hope this helps. Please write back to let us know what you find!

I should have added that GE (the writer of the block used in some turbine control systems provided with Hitachi units) recognizes that, at times, during loading and unloading the fuel flow-rates and IGV movements can cause unusual changes in exhaust temperature profiles, and so programmed the function to temporarily increase ("bias") the allowable exhaust temperature spread up to prevent nuisance trips.

In actual practice, present-day machines rarely experience exhaust temperature spread transients during loading and unloading during normal operation. Of course, there can be times (such as when a unit is switching from Droop Speed Control to Isochronous Speed Control when the load on the machine may suddenly increase or decrease) when during load changes when fuel flow-rates or air flow-rates may cause uneven combustion in one or more combustors.

In my personal opinion, the need for biasing of allowable exhaust temperature spread is not required for most present-day GE-design heavy duty gas turbines with digital control systems. It's a legacy of control philosophies from decades past, and it doesn't seem to cause any issues.

I would be very surprised if you had evidence that this was not occuring (with load changes--even small load changes when using Pre-Selected Load Control) prior to the HGPI. It could just be that it was noticed after the HGPI, but if it is due to the normal load-change bias applied whenever L70R or L70L toggle to logic "1" (which is what happens during any load change, however large or small) then it's normal, expected behavior.

If the jump in TTXSPL is not followed by a slow return to normal of the value, then that could be cause for concern (unless L70R and/or L70L are toggling fairly quickly--which could be indicative of some other problem, or of poor tuning of Pre-Selected Load Control).

I was able to look at an older version of the block that calculates TTXSPL and the logic that controls the addition of the bias, and the bias is NOT added during load changes when the unit is at Base Load (on CPD-biased exhaust temperature control).

Again, please describe the nature of the fluctuations--does TTXSPL "jump" to 150 deg C (about the usual value of the maximum bias) and then slowly return to approximately 80 deg C? Or, does it jump to 150 deg C, and then quickly return back to 80 deg C? And, again, what is the period (frequency; timing) of the spikes?

In H-25 machine the combustion monitor algorithm calculates the allowable spread TTXSPL.

The allowable spread will be increased at the following cases like fuel change over operation,at nitrogen purge operation,at high error of fuel fraction ratio and at high rate of FFD changes.

Please check that at 21 MW any of the above mentioned cases except for N2 purge operation is happening and that can be the reason of the change in allowable exhaust spread from 80 to 150.
I have to confess--my experience is with typical GE-provided sequencing (application code), and not with Hitachi-provided sequencing (application code). It seems Hitachi is using the typical GE heavy duty gas turbine control allowable exhaust temperature spread block, but there is a logic signal (L83SPMB, I believe) that controls the addition of the bias.

Your application code may be different from my experience--Hitachi may have a slightly--or very--different control philosophy when it comes to the addition of the allowable exhaust temperature spread bias.

Sorry if my replies have caused any confusion. Without being able to review the application code running in the turbine control system at your site and actionable data from events you are trying to describe, it's virtually impossible to say for certain exactly what is happening--and why it is happening at your site.