We operate 02 GE LM 6000 PC sprint machines with inlet cooling at a combined cycle power plant. I unable to understand the logic behind exhaust temperature increase at part loads at full load temp are 460C at 42 MW and 450 C at 40 MW with all augmentations in operations

If we switch off Sprint and machine looses 6 MW the exhaust temp high alarm appears for HRSG exhaust temp goes up to 465 C.

Sensing is OK.

 

I do not profess to be any kind of expert on aero-derivative gas turbines. But, anything that decreases inlet air temperature (like SPRINT) increases the mass flow through the machine, and that has the effect of reducing the exhaust temperature since all of the air does NOT get consumed by combustion, even if more fuel is burned. It's counter-intuitive, but it's the way it works. I'm sure there's maths to prove why this is true, but I'm not a theoretical physicist.

Most of the aero's I've worked on don't use CPD-biased exhaust temperature control like heavy duty gas turbines and actually try to control an "intermediate" hot gas temperature to maximize power production and optimize engine parts life. I'm sure exhaust gas temperature is important, and should be limited to protect the exhaust components, but I'm not sure what the criteria are, other than protecting the exhaust components (including the HRSG).

 

I'd have to dig through some of my documentation (and it's a lot of stuff not really organized), but I seem to remember that LM6000's where primarily controlled by compressor discharge temp (T3 they call it if I remember correctly).

Also, LM6000 is a twin shaft unit and the HP section changes RPM as load changes. SPRINT injects water into the compressor section (not the inlet) and effectively lowers compressor discharge temps.

It's been a while since I've been around LM6000's and I work on many things so I'm not an expert on them...but the SPRINT injection scheduling may be more water at higher loading which could reduce exhaust temps. Exhaust temps are going to be lower with SPRINT activated vs deactivated.

 

krush,

Thanks! I confuse SPIRITS and SPRINT all the time.

 

You are welcome. I remember seeing the control philosophy (or limitations) presented in temp/pressure charts, but I can't pinpoint the document. Sometimes having too much information is just as useful as having no information (aka, not useful at all).

I keenly remember that LM6000 output is limited differently than LM2500 and others.

Aero-derivatives are very high-tech and use really high firing temperatures. They are fun to work on too. For GE brand stuff, most of the original packaging thinking was done by Stewart and Stevenson before GE bought/absorbed them. Controls were and still are mostly are Woodward components.

In the 60MW and less range, I would pick aero stuff hands down. But that's another topic and another debate!