IGV angle and pulsation protection control

After studying the CSRGVPS algorithm of a 5002B heavy duty gas turbine, I found that the inlet guide vanes are automatically positioned closed during a start-up and a shutdown sequence to avoid gas turbine compressor pulsation or combustion pulsation. I found also to protect the axial compressor from the rotating stall region at low speed.

Can any one please explain to me what does this mean exactly (physically)!

What can happen to the compressor exactly?

Thanks in advance.
 
Isulamu,

Axial compressors are unique machines, unlike centrifugal or reciprocating compressors. As such, they have unique operating characteristics.

One of them is that during starting and shutdown (during acceleration and deceleration) as air is being pushed along from stage to stage there are conditions under which the air cannot be pushed to the next stage--it stalls, or does not move. Imagine what happens--the air behind the stalled air is still trying to be pushed to the next stage, but there's no place for the air to go. And, the air that's stalled, well, it just keeps getting pushed around and around the compressor and getting more heated in the process. Very large physical forces can be created, and these forces can cause pulsation as well as mechanical damage to rotating and stationary compressor blades.

The IGVs are used during starting and shutdown--along with the compressor bleed valves--to ensure that air can be pushed from stage to stage without stalling or without causing vibration and/or mechanical damage to the compressor blading. For two-shaft machines that don't operate at one speed during loaded operation, there is some need to also protect the axial compressor at less than rated speed, also.

During operation, there are also conditions (especially for two-shaft machines) involving axial compressor inlet temperature ranges (usually very high temperatures as well as very low temperatures) where the IGVs are used to limit air flow through the axial compressor to protect against compressor damage from similar phenomena as during low-speed operation when starting and shutting down.

Again, axial compressors are really very unique machines and they can move huge amounts of air when at or near rated speed, but it's during starting and shutdown, and some ambient temperature ranges when the operating limits of the compressor can be compromised and so the IGVs (and compressor bleed valves) can be used to protect against damage, which can be very serious.

You can use your preferred World Wide Web search engine to research axial compressor operating characteristics, but, most of it is full of high-order maths with very little down-to-earth explanations. I can tell you from personal experience that you never want to experience compressor problems during starting or shutdown, the vibrations can feel like an earthquake and the unit speed can slow down several hundred RPM in one or two seconds.

It is VERY scary, and fortunately the machines I experienced it on were not severely damaged (but could have been). The borescope inspections and finger-pointing required several days of lost commissioning and generation (power and steam) and were very expensive monetarily, as well as mentally draining and physically exhausting and took weeks to write the reports and go to meetings about who, what, why, when and where--and responsibility.

Finally, during loaded operation some machines have a range of air flow through the combustion section when pulsations can occur and these pulsations can cause flame instability as well as load swings. They are VERY rare, but there is generic programming/sequencing/control schemes in which IGVs are modulated to limit the chances of these combustion pulsations from occurring--again, during loaded operation.

Hope this helps!
 
Isulamu,

You're welcome, but, I'm not clear what other information you might need. The OEM decides what the IGV angles should be for the protection of the axial compressor during start-up and shutdown. Is there some problem with the operation of the IGVs or the machine?

Is there some "need" to open the IGVs sooner, or some other specific question about the IGV schedule/configuration you would like clarified?
 
Dear CSA

Could you please give us bit more explanation about principle of stall detection .. I've followed "compressor stall detected" Alarm but couldn't find thing because you know it is high level of control..

I mean principle, , I know in Siemens gas turbine they predict it by sensing a severe and sudden drop in pressure deferential across bell mouth as air flow rate is proportional with bell mouth deferential pressure.

Does GE use the same philosophy?

How GE predict stall phenomenon when it occurs?
 
Amr1589,

This thread was started regarding GE-design Frame 5 heavy duty gas turbines. Your past posts have been regarding GE-design Frame 9FA heavy duty gas turbines. There are some pretty large differences in the materials and construction of the two types of machines, and as has been said in the past--there is very little engineering margin in the F-class series of GE-design heavy duty gas turbines.

So, active compressor stall detection is used much more frequently (almost always) on F-class units and very infrequently on older GE-design heavy duty gas turbines (such as Frame 5s and Frame 6Bs and Frame 7B/7E/EA and Frame 9E machines).

F-class machines were designed and built at the very limits of the materials and design capabilities at the time, and have very little compressor operating margin (meaning they quite often run at the very limits of stall/surge), and the situation is made worse when the grid frequency is unstable or chronically low. This is something people fail to recognize--the effect of low speed axial compressor operation due to low grid frequency or unstable grid frequency.

It's something which is being addressed by installing multiple stages of variable axial compressor blading on older machines, and which was heavily researched and tested in the new HA- (Harriet) class machines which also have multiple variable stages of axial compressor blading (very similar to many aircraft engines).

Active compressor stall detection is accomplished in I/O card firmware and the algorithm is considered proprietary and not "visible" to technicians or operators. It is accomplished using the axial compressor discharge pressure transmitter(s) and that's about as much as I can tell you about it. I believe some Mark VI and Mark VIe System Guides have some sections about active compressor stall detection which might be helpful, but I've never really read or studied those sections.

Most of the older GE-design heavy duty gas turbine (i.e., non-F-class units) have a fair amount of margin and there is LOTS of empirical data about the way the units react when approaching the limits of compressor operation and so the control system can be programmed to prevent most unsafe operation. F-class machines operate so close to the safe limits of compressor operation during normal operation that a faster and more detailed, intricate system of detecting compressor operation and stall/surge are used.

But, again, most of the active detection is done at the I/O card level using proprietary algorithms which are not visible to operators or technicians. And, if the unit has been programmed with MBC (Model-Base Control), then even more aspects of the unit operation are hidden from view to the operator/technician (I think you have mentioned MBC in the past).

I wish I could offer more information, but that's all I have been able to glean from various sources--which are few and far between (meaning, there ain't much written about active compressor stall detection).

Lastly, I don't think GE turbine control systems are really programmed to detect stall as much they are programmed to try to prevent it. This angers many operators and owners because when the protection becomes active it acts to reduce the output of the unit--which means less revenue is being earned, even though the unit is being protected and improving it's reliability and availability by trying to prevent stall and thereby reduce its effects (which are usually destructive and sometimes catastrophic, and can lead to long periods of non-operation and lack of revenue generation). I've heard that other OEMs actually let the units encroach into stall before taking action (which often includes tripping, which seems to reduce reliability and availability).

Hope this helps!
 
CSA

first of all i'm sorry fo my delayed reply.

Second so as an other protection to the axial compressor from stall, when the turbine below the operation speed are the bleed valves! (10th stage bypass to exhaust) THEY MUST BE OPEN. And during normal operation the valves are automatically closed. is it right?

And i want to know is it possible that the turbine operates high loads withing the valves open? and when closed what they affect?

Thank you very much indeed.
 
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