I'm an shift engineer working in GE 7FA power plant.

Why start permissive of Evaporative coolers is linked to IGV angle of 70 degrees and above in GE frame 7FA?

 

There are probably several reasons. The reason(s) may be listed/described in either or both of the System Description in the Operations and Service Manuals and/or the Control Specification (probably Sect. 06).

My best guess is that Inlet Bleed Heat starts operating as the IGVs close below 70 DGA during unloading, and ceases operation not too long before reaching 70 DGA when loading.

Also, evaporative cooling increases mass flow at low IGV angles, and this could result in the compressor approaching or exceeding its operating limit and protection.

Evaporative cooling is most effective when the air flow across the media is high. Low air flows can result in carryover due to low evaporation rates at low air flows. It's entirely false that evaporative coolers are "self-regulating"--but they do require less manual adjustment when operated at or near Base Load most of the time. But, plant managers being beholden to the owner who is always trying to maximize revenue and reduce fuel cost are always dreaming up new schemes to increase power output even though some really negatively impact heat rate at part load and really are only truly effective at Base Load--the definition of which includes the IGVs being at maximum operating angle.

There used to an ambient temperature permissive below which evaporative cooler operation was not permitted, but many plants routinely bypass or defeat this permissive even though evaporation rates decrease as ambient temperature decreases....

And, there used to be a permissive that only allowed operation when at Bsse Load, which was also routinely bypassed/defeated.

If you have a GE F-class machine you probably have access to the GE PAC (Power Answer Center). If the intent of this question is to determine if it's possible to operate evaporative cooling at loads where the IGV angle is less than 70 DGA and you are not satisfied with the documentation or can't find anything about the restriction in the documentation and you're not satisfied with the information provided in this thread you should ask this question of GE, using the PAC. They would have the best answer for your site and it's operating conditions. There are so many versions of axial compressor configurations on F-class machines these days that there may be something particular to your machine and ambient conditions which only GE could answer.

Hope this helps.

If you do ask the PAC, please write back to let us know what you learn!

 

Thank you very much for an excellent reply.

any person working in GE Frame 7FA can explain me in detail.

 

I do not have access to PAC case and it involves may top level management people. Please help me out with the best answer.

 

>Please help me out with the best answer.

What is the context of the question? Is it being considered to operate the evaporative coolers at IGV angles less than 70 DGA?

 

The context of the question is that any threat can be expected to unit if evaporative cooler will be operated below IGV angle of 70 degrees?
If ok to run evaporative cooler below 70 degrees,then up to what IGV angle can be evaporative cooler be kept in service.

Thanks for your support CSA.

 

I would say yes--if the OEM/packager has a restriction on operating the evaporative coolers below 70 DGA then it's likely because of a risk of damage to the axial compressor--either the IGVs or the first stages of the compressor. Again, because of the increased mass flow at reduced IGV angles the axial compressor may exceed the operating limit.

If you're being told by your supervisors to operate the unit with the evaporative coolers in service below 70 DGA, then they have made a decision to take the risk and operate the unit against the advice of the OEM/packager. That's what they get paid the big bucks for--to analyze situations and perform a risk assessment and make a decision.

Is it always the correct decision? No. Is it their machine? Yes; they have the responsibility for it, and when making such decisions to go against the OEM/packager guidelines they are, hopefully, making reasoned and well thought-out decisions.

These kinds of decisions are not to be taken lightly, and rarely are. Unfortunately, many times everyone isn't aware of all the discussions and information that go into making the decisions. But, if the unit doesn't suffer any ill effects AND the efficiency (heat rate) improves--meaning the revenue per MWH (megawatt-hour) increases then the people making the decisions are heroes. And, if the decision does have ill effects--well, then depending on their management's involvement in the decision they may be held accountable, and they may not. Hopefully, upper management and ownership is aware of the potential risks (per the PAC case), have done their due diligence (research) and have reached a decision based on sound knowledge, experience and reasoning.

That's (supposed to be) how people get into positions of authority to make these decisions--by experience. Good judgement comes from experience; experience comes from bad judgement (and sometimes, good judgement, too).

That's all that can be said. If you don't have access to the PAC Case, and can't get access to the Control Specifications and the System Descriptions to do your own research and reach your own conclusions and you're being told to operate down to nn DGA (below 70 DGA), well, then as an operator/technician one has to do what one is told to do. It could be a learning experience for everyone--whether there is damage or not.

The OEM/packagers of these turbines put such guidelines and conditions in place mostly because when the unit is under warranty if something breaks they are responsible for the repairs. So, it's in their best interests to be cautious, and they usually are cautious and conservative in such matters. But, if someone has already contacted the PAC and has received a response and has decided to operate the evaporative coolers below 70 DGA--the decision has been made. And, all one can do is follow directions--and learn from the experience.

I wish I could offer more, but, without being able to see the Control Specifications and System Descriptions for the unit at your site--and without being able to be party to the PAC Case discussions--it's really not possible to say any more. If people want to bypass operating parameters and the unit is not under warranty--it's their machine and they can do what they want.

For years, the OEM recommended against on-line compressor water washing. The finally relented and now supply on-line compressor washing equipment--though they will NOT make a recommendation for detergent or washing solution, only providing guidelines. And a LOT of people went against the OEM recommendations and paid third-party/after-market firms to install on-line compressor washing systems and equipment. And with but a couple of exceptions (those exceptions being people who mis-used the on-line compressor washing systems for power augmentation) most units didn't experience any ill effects at all. So, there is precedence for such decisions--and, again, it's management and ownership that make these calls, based on their experience and judgement.

 

Thank you very much for the reply. But I would like to know like exactly what happens when compressor operating limit is crossed? what protection acts? is it related to surging?

Does 7FA machine takes care of that in protection?

Regards
Venkatesh