Why using bleed valve's at lower power generation?

Hi there,

Bleed valves used within Gasturbines like GE are taking precautions against Ice - stall (surge), … at starting up moments or stopping.
But why do they use the bleed valves out these periods - we also see that this happens at a certain time in production mode - lower power at same speed ref.
Please advice.

Kind regards HoltW
 
Hi Holtw

There is some very good threads on this forum explaining the operations and benefits for IBH on GT .

I strongly advise you to have a look on them.

There is surely the right answer for your response ,

Regards,
James
 
HoltW,

GE designs and manufactures many different gas turbines, which fall into two basic groups: heavy duty gas turbine and aero-derivative gas turbines (aircraft engine gas generators with power turbines attached for land-based applications). Both some kind of "bleed valve" but they can take several different forms.

In general, "bleed valves" (I'm specifically referring to axial compressor bleed valves) are used primarily for protection against stalls or surges during low-speed operation during starting or shutdown--not low power operation. (I'm defining low power operation as AC power generation at low power output (low load AC power generation); whereas I'm defining low-speed operation as turbine/compressor operation during starting and shutdown at less than rated speed. AC power generation occurs at rated speed (when the frequency is at rated also).) I have seen some aero-derivative gas turbines that use bleed valves for anti-icing protection, but mostly in aircraft applications, not so much in land-based applications (AC power generation; mechanical drive applications (process compressors or pumps, etc.).

Heavy duty gas turbines typically have the axial compressor bleed valves open during starting and shutdown (low speed or part speed operation); some heavy duty gas turbines keep the axial compressor bleed valves open for a short time after reaching rated speed and during low power generation periods (say, 5 MW or less). Again this is usually done when the air flow through the axial compressor of the unit is at or near less than design conditions to protect the axial compressor from stalling or surging (which can be very destructive to the axial compressor). Most of these types of axial compressor bleed valves are connected to mid-axial compressor taps (for example, the 7th stage, or 9th-, 10th or 11th stages of the axial compressor). They are open to reduce the amount of air that is flowing through the axial compressor by directing some of the air flow to the turbine exhaust area--again during starting or stopping. This is because axial compressors are designed to operate at rated speed, but during starting and stopping they have to accelerate from zero speed or decelerate to zero speed--necessitating the need for reducing air flow to prevent compressor stalling or surging. (Axial compressors are a very different type of machine from centrifugal compressors or piston-type compressors and have very unique characteristics. They are excellent at moving LOTS of air at rated speed, but they are not so good at moving air at low (part) speed operation and need protection--which the axial compressor bleed valves provide.)

In my own personal experience, axial compressor bleed valves on heavy duty gas turbines are not used for anti-icing functions. Some GE-design heavy duty gas turbines have variable valves which can extract ("bleed") a portion of the axial compressor discharge air (from the discharge of the axial compressor, but not from intermediate stages of the axial compressor) and recirculate it to help prevent icing at the axial compressor inlet, or to protect the compressor when the variable axial compressor inlet guide vanes are closed to very low angles (air flows).)

Hope this helps! Again, there are different types of gas turbines (heavy duty and aero-derivative), and there are different types of "bleed" (extraction) valves. And, the purposes of each, and the times when they are used (opened) are also different, as are the reasons for opening the valves (because they are usually closed during normal operation at rated speed at rated operating conditions)

.
 
Hi Guys,

Thanks for the intell, I knew what it does at start stop, but my guestion goes more specifically at the point where the power ramps at his 'lowest' power ratings. It happens that when we go from 350MW to about 100MW or les, then at this lowest point the bleed valve opens for a certain timelaps. So correct me if I'm wrong, I suppose for these lower ratings they do not want to risk a stall/surge operating?

It's my best guess ...
 
HoltW,

All gas turbines are not alike, not even all GE-design gas turbines--be they heavy duty or aero-derivative. And, when it comes to the differences between F-class (including FA and FB) and B/E-class units (the original workhorses of the GE fleet of heavy duty gas turbines) they (the differences) can be great. Yes; they all suck, squeeze, burn and blow (draw air into the axial compressor; compress it; add fuel to the compressed air and burn the fuel; and exhaust to atmosphere or an HRSG ("boiler") or possibly to some other process)--but often-times the similarities end there.

So, when you're asking about a specific process or point in the process and a specific machine, please--be specific about the machine and the process or point in the process.

And for the record, 100 MW out of 350 MW isn't really "low power" (not in many people's estimation; and as a percentage it's more than 28.5% of the rated power you listed, not exactly "low power").

If the unit you are asking about has DLN combustors AND the unit is being unloaded very quickly, not at some normal rate of say 1 MW/sec or thereabouts, then it's possible the designers of the unit have decided to use the axial compressor bleed valves to help protect the axial compressor against stalling or surging if the IGVs are also closed very quickly while the unit is at rated speed. Again, axial compressors are really unique machines all by themselves, and the manufacturers and designers of large heavy duty gas turbines are doing some very unique and unusual things to maximize air flow through the machine (so the power output of the machine can be optimized and maximized under normal operating conditions) and that may include using the axial compressor bleed valves in some fashion.

You haven't told of the axial compressor bleed valves on the unit you are asking about are "bang-bang" bleed valves (meaning they are either fully closed or fully open--and don't move or can't be moved to some intermediate position), or if they are modulated bleed valves with some type of actuator/positioning device that can move the bleed valve to some number of or any number of intermediate positions as necessary).

In decades past (or is it passed ...???), modulating axial compressor bleed valves was not typically done on GE-design heavy duty gas turbines. For several reasons, but mostly for two reasons: it wasn't necessary, and it was costly. As the machines have become more sophisticated (and, too, the control systems!) and the desire to maximize power and efficiency and yet try to maintain operability it has become more necessary to devise and implement new and different control schemes. And, as control systems have become more sophisticated and more capable, including actuators, new control (and protection) schemes have become the norm.

So, when you're asking what seems like a simple and general question--please don't assume that all gas turbines, of any manufacturer, are alike in all or most respects. It's just not true. And if you're fortunate enough to be working on a machine that was manufactured and/or designed by GE in Belfort, France, you can be assured they have pulled out all the stops on implementing new and untried and untested control (and protection) schemes, all in a conscious effort to maximize complexity at the expense of simplicity and operability. They do seem to do a little better job of documenting some (but not all!) of their schemes, so do have a look at the Operations and Service Manuals provided with the unit for information (one should always RTFM (Read The Fine Manual) when trying to understand a unit or its control and protection schemes).

Best of luck!
 
Hi CSA,

Thx for the intell. I've already took a better look at the Original manuals, but some thing have been changed the last few years. The turbine is a FA type of GE9 and also like you've said DLN. I'll take some new approche to the system operating to clear up some things. When I know some new specific elements I'll keep in tough.
Nevertheless, I'ts good to hear some people to know the skills about these magnificant machines. And yes indeed it's not Always black and white like you've said before. There are at other plants GT from different manufactures and like Siemens or RR. Let's say we have a GT with 2 different pressure modes, HP en LP compressors at one GT. So it's not correct to say it's the same thing.
My apologies for the inconvenience about the simple question that may lead to a simple question, that rather would be better to a specifically question. The same if I would ask how big a inductance is 1H, not knowing, in air-core material-amps, ...
However, I'ts a good practice and hopefully could lead us to a better understanding.

Kind regards,

HOLTW
 
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