Dear All,

I work for ESBI @ Uch Power Station. We have 3 GT's (MS9001E) and 1 ST with three Deltak HRSG's. Plant's ISO rating is 586 MW.

In Sep, 09, we (GE rep) increased the Isotherm temp. on the Gas Turbines. From 1025 to 1070 degF.

Initially 7-8 MW was increased on the GT's and the complex load increased aopprox. 30 MW.

Now a days the GT output is very low, compared to last year in same ambient conditions. We expect that the GT should be running on higher MW, because last year the Isotherm was not changed.

We have done compressor offline washing in Oct, 09 before the Complex HGPI in Nov, 09.

Could the isotherm temperature increase is not working in these conditions and will work in hot humid summer days?

Please let me know your views, if you require more data, than i an forward that too.

 

Uch. Or is it oUch? (the 'o' is silent) I've seen it expressed both ways. Now there's a site name that brings a shudder to many, many, many people. (I think I can hear otised echoing the same thought.)

Have you asked GE this same question? If so, what was their response?

We do not know the reason the Isotherm was increased, or probably more correctly, the reason<b>s</b> (plural) the Isotherm was increased. And, that seems like a pretty significant change, which might not be intended for long-term operation; we just don't know enough about the situation.

I would also suspect that it's highly likely that other changes were made to the control curves than just the Isotherm. And if not, making this single change is even more intriguing, making the need to understand why the change(s) was(were) made more important.

It's not clear if the output of all three GTs is down, if they are all down equally, or if only one is experiencing lower than expected output.

I think doing an off-line water wash in October of 2009, which is almost four months ago, makes it a long time since the last off-line compressor wash. We don't know if the site/units has on-line compressor wash capability or if it's been used since then.

We don't know how the unit has been operated or is being operated currently. We don't know if there have been higher than normal exhaust temperature spreads or how many T/Cs have been jumpered (a common practice in my experience in your part of the world, even by ESBI personnel, which can skew the average exhaust temperature calculation).

We presume the units are being operated at Base Load, but we don't know that. I don't recall if the units at Uch have Peak Load capability or not, or if it does if it's being or been used or for how long it might have been used.

We don't know what the grid frequency is doing when the units are running, and from all we're hearing about grid frequency in that part of the world it's pretty *$%#ed unstable, which will definitely affect power output at or near Base Load.

You also indicated there was a recent "complex" Hot Gas Path Inspection (HGPI), and reassembly techniques and components used in any outage can all greatly affect the efficiency of the machine.

Uch has some trending capability I recall, and I believe I even heard the DCS was recently upgraded to include more functionality. What is the CPD now compared to them for the same ambient and fuel flow-rate? (Good news can sometimes spread as fast as wildfire!)

What is the condition of the inlet air filters? What is the inlet air filter dp now compared to the previous similar conditions?

What is the back-pressure on the gas turbine exhaust (very important to know!)? I don't remember if the boilers at Uch were fired or not.

Lastly, the Isotherm change is extremely difficult to understand. It has to be understood in the context in which it was done. Increasing the firing temperature (which is what would happen) increases the temperatures in the hot gas path, which increases the wear on the parts in the hot section of the turbine. I would imagine that someone at the site was told by GE that there would be 'consequences' related to increasing the Isotherm (there is no such thing as a free lunch, after all). In other words, operating at a higher firing temperature will decrease the parts life by "x" amount over "y" period of time. So, it's still important to understand the entire situation, and the explanations and admonitions which would likely have been made to understand the condition you have described.

It's not known if the hot gas path parts were replaced with ones which could withstand the higher firing temperatures or not. If not, I would expect continued operation at the higher firing temperature would have the effect over time of reducing efficiency and output.

But I think you are trying to relate two things which may not (or may) be related, but we don't have enough information to know or to be of help.

Performance is about machine conditions, including inlet, exhaust, compressor (cleanliness, clearances, condition), exhaust conditions (back-pressure), exhaust T/C conditions, spreads, fuel conditions (has the fuel changed recently), hot gas path part conditions, a LOT of things, not just Control Constants and values.

You should be talking to GE about this if you believe the Control Constant change is related to the decrease in power. Machine conditions, even the condition after the Isotherm increase, are what control the power output. And, the grid frequency, which is questionable in that part of the world. And, the fuel which may or may not have changed.

Just too many variables, and no context for the Isotherm change.

I'm not trying to be harsh, but this type of question is extremely difficult to answer without lots and lots of details. I don't want to discourage you from posting here, questions or responses (I would have to believe that there is, or should be, lots of operational experience at Uch that many people could benefit from). But, as with many things in life while it might seem a simple question a proper answer requires a lot more understanding.

 

CSA,

Fortunately I never visited the Uch site. I did spend some time at Guddu, an earlier project.

I am not sure why isothermal temp was increased, unless the firing temperature was also raised. The isothermal line is the exhaust temperature limit of the control curve and really only comes into play during start up and low loads. If the constant firing temperature line (the portion of the control curve that slopes down) was not changed, I don't see how the output would have increased, unless outside ambient was so high that they hit base load limit on the isothermal line. (It does get pretty hot over there - I experienced 48C at Guddu!) If so, and only the isothermal limit was changed, this would offer an explanation as to why the output is not higher on the colder days - they now are on the constant firing temperature line which did not change.

I would doubt that they have peak capability on these units. Combined cycle GE gas turbines do not have peak capability.

 

Previously the min select was taking TTKo_I (552 deg C), while TTRXp was 560 deg C…Now GE increased the TTKo_I to 570 deg C. These were changes to raise the exhaust temp. which in turn will decrease the CPD and in turn will increase the Firing temp. and the MW of the machine. We generally experienced very low outputs in the hot humid conditions from April – Sep.

Outputs of all 3 GT’s is down.

Yes we are planning an offline wash in next month and yes we have weekly routines for online washing on all 3 machines.

I have been here since May, 08 and we operate at base load all the time (99%). We do not operate on Peak Load.

Yes you are right, the Grid Frequency is unstable. We are considering that as a contributor to low output.

How can we calculate/estimate/claim the low output is being caused by the HGPI?

Average CPD’s for Jan, 2009 is 11.05 and 11.34 for Jan, 2010. After the control curve change, it was approx. 10.41.

Air filters are replaced with new ones this outage. Air Dp is similar to last year.

Boilers are not fired. Back pressure is also similar.

HRSG OEM studied the higher inlet temp. and gave us a report on that and guaranteed their parts. GE did a study but did not guaranteed their parts. We were also asked for compressor reblading. This issue has been raised to GE, but I know that they will come back and will be asking us for data, than after some time More Data and MORE DATA and ….

I hope you understand the situation and I also would like to know more about you and your past relationship with Uch or ESBI?