GE 9FA Gas Fuel Purge System

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Thread Starter

niazbibiyana

My curiosity is, I know in base load compressor discharge pressure is around 225 PSI. But when base load, turbine is on Premixed mode. Thats why there is continuous purge in the D5 manifold. I don't understand, if by D5 manifold, continuous purge pressure is 225 PSI, how the flame in combustion chamber establish? Because, 225 PSI is so high pressure, flame can be blown off. So, How this is happening?

Again referring GE Document, "GEK 110832a", in description, it is said that, all 63PG (1A/1B/1C/3A/3B/3C) pressure switches are used for give proper block and bleed system. again referring, "0414 - Device Summary", it is said that, all switches will start acting, while cavity pressure will be higher than, 50 PSI. I am so much confused, how is it possible to maintain 50 PSI? because CPD pressure is around 225 PSI during base load.

I will be very grateful to someone , who will kindly make it clear to me.

Thanx a lot
 
niazbibiyana,

The answer to this question--and to many questions--lies in reviewing and analyzing the P&IDs for the various systems. When one is trying to understand these kinds of questions it's always best to refer to the P&IDs. In fact, every operator and technician should have their own copies of the P&IDs, and those copies should have lots of colorful markings (from highlighter pens) and lots of notes--both of which means the owner of the drawings has taken the time to review and understand the P&IDs, which means the owner likely has a very good understanding of the systems and how they operate and what the various components do and what their purpose is.

I believe you'll find that when running on gas fuel purge air comes from axial compressor discharge--so it is basically CPD. The pressure inside the combustor (where purge air "discharges") is slightly less than CPD (I think the pressure is a very low, single-digit percentage less than CPD). And, it has to be less than CPD--otherwise there could be no flow (if the pressure in the combustor was higher than CPD, the flow would be from the combustor to axial compressor discharge--and that doesn't happen!). So, with a very low differential pressure (between CPD and combustor pressure) the flow is not that great to begin with, and it's purpose is just to ensure that hot combustion gases don't flow backwards into the fuel nozzles and manifolds where can can cause very bad damage.

The pressure switches you mentioned are sensing the pressure between the double block valves that are OPEN during purging. One block valve is used to shut off the flow of axial compressor discharge pressure to the manifold/nozzles, and the other block valve is used to shut off the flow of fuel gas to the axial compressor (which should NEVER happen). And, there's s solenoid-operated bleed (vent) valve that is to be CLOSED when the double block valves are OPEN (this arrangement of two block valves and a bleed (vent) valve is commonly referred to as a "double block-and-bleed valve arrangement). And since there is flow through the double block valves from the axial compressor discharge to the combustor when they are open, there will be pressure--CPD, to be exact--between the valves. And that's perfectly normal, and acceptable--and to be expected. And, the pressure switch isn't trying to sense pressure when there SHOULD BE pressure--only when there SHOULD NOT BE pressure. Read on.

When there is gas fuel flowing in the manifolds and nozzles that are otherwise being purged, the double block valves are CLOSED and the bleed valve (solenoid-operated vent valve) is OPEN. So, there should be NO pressure between the double block valves, especially since the vent valve is open to vent any pressure between the double block valves when there is gas flowing in the manifolds. If any pressure builds up inside the cavity between the double block valves that means one (or both) of the two block valves is leaking--and the leak is so bad it is pressurizing the cavity between the double block valves even with the solenoid-operated vent valve open. And, that's NOT normal, acceptable or desirable--so the pressure switches will actuate and alarm and trip or runback the unit. Since it's not possible to tell which double block valve is leaking AND since it could be the valve which is blocking gas fuel, there's potential for a very bad situation--hence the alarm, and trip or runback.

Does this satisfy your curiosity? The pressure switches are there to sense pressure when the double block valves are closed and the bleed (vent) valves are open--when there SHOULD NOT BE pressure. They are not there to sense when there SHOULD BE pressure. And if there is pressure between the double block valves when the bleed (vent) valve is open that means the leak is relatively substantial, and since it could be leaking through the block valve that is supposed to be blocking (isolating) gas fuel, pressure in the cavity is deemed to be dangerous.

Double block-and-bleed valve arrangements are used when its critical to have positive isolation (shut-off) between two systems--in this case, the axial compressor discharge and the gas fuel manifold(s). If gas fuel (which is at a much higher pressure than CPD) should flow into the axial compressor discharge the results could be explosive, catastrophic and even deadly. So, the pressure switch is used to monitor for pressure when there should be NO pressure--especially when the bleed (vent) valve is open.

Hope this helps!
 
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niazbibiyana

Dear CSA sir,

I am very very satisfied to your answer. I actually wanted to know this. As you suggest, for following the P&ID, I always follow this.

Thanks a lot. But sir, you said that, "The pressure inside the combustor (where purge air "discharges") is slightly less than CPD (I think the pressure is a very low, single-digit percentage less than CPD). " Do you know, the approximate amount of pressure for GE 9FA ,DLN2.0+ turbine on base load?

Again, As sir, you are a C & I expert, do you know? why in fuel purge system, on D5 purge line the last valve "VA13-2" has some control function to control the opening percentage of that VA13-2 valve? but in PM4 purge line, the last valve "VA13-6" does not have this type of control. Do you know the exact reason?

Thanx again.. Have a nice day
 
niazbibiyana,

You're welcome. And thank you for the feedback.

I'm not very familiar with DLN 2.0 or any of its variants; it was a version which wasn't produced for very long before the change to 2.6, and (fortunately) I never had to work on any 2.0-equipped units. I had a course on DLN 2.0 in the late 1990's, and I understand the basics--but that's about all. I've never seen a P&ID or any information about DLN 2.0+.

When purge air is being re-introduced after gas fuel has been flowing in the manifold/nozzles, the concept is to open the purge valves (the double block valves) slowly. There is fuel trapped in the manifold and nozzles and if the purge valves open quickly they will blow the fuel in very quickly--which will cause a load spike and an exhaust temperature spike. By slowing the opening rate of the purge valves the fuel is not purged out so fast and this minimizes any load- or exhaust temperature spike.

But, that's about as much as I know about purging--and "D5" is not something I recognize or have seen on a P&ID. If you could scan and post a copy of the gas fuel P&ID to a (free) Web-hosting site, such as tinypic.com, and then post the URL/link to the posting that would be very much appreciated. I might be able to posit some thoughts after reviewing the P&ID--but that's just a guess on my part.
 
niazbibiyana,

The pressure drop across the combustion liner is somewhere in the range of approximately 2-4%, so if CPD was 225 psig the pressure inside the combustion liner would be between 216 - and 220.5 psig.
 
niazbibiyana,

Thanks very much! The resolution of the drawing is not very good; when I zoom in the drawing the print is too grainy to be able to read it (it's very small print for such a large drawing).

Also, there is only one page of the set; it seems there are three pages, total, in the Gas Fuel P&ID. I also wonder if there is a Fuel Purge P&ID which might be useful also.
 
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niazbibiyana

Dear CSA Sir,

Should I send you full P&ID of DLN 2.0+ gas fuel purge system? I am uploading P&ID of "Gas Fuel Purge System" and "Gas fuel System" on a free file sharing site for you.

http://www.mediafire.com/view/7o0kaewbn8qfn3x/Fuel_Purge.pdf

http://www.mediafire.com/view/76cyze32fcks90n/Gas_Fuel.pdf

Thanx a lot again. Sir, I here uploaded for you. But , kindly inform me in D5 purge line the last valve "VA13-2" has some control function to control the opening percentage of that VA13-2 valve? but in PM4 purge line, the last valve "VA13-6" does not have this type of control. Do you know, why? if you know, please give me some idea.
 
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niazbibiyana

Dear CSA sir,

I am so much grateful to you for your continuous response. Believe me, I am new on GE 9FA Gas turbine. only 6 months experienced. Previously, I did a job on Wartsila engine based plant. By the way, Can you please tell me, How you calculate this?

"somewhere in the range of approximately 2-4%"

Thanx a lot
 
niazbibiyana,

I've been able to download the drawings, and when I get some "free" time I will have a good look over them.

GE measures pressures inside the combustors in their Combustion Engineering Laboratory, and that's how the general difference is known. It's not calculated, it's just a number derived from empirical data gathered over years and years of testing and monitoring in the lab, and also at some selected sites in the field.

It's not something that's measured or monitored on running turbines; it's just a known- and understood given (because gases can only flow from an area of higher pressure to an area of lower pressure--another one of those proved physical "laws").
 
N
Thank you very much CSA sir. Please, when you have a free time, please have a look, and kindly make me understand my desired things.

Have a nice day.
 
niazbibiyana,

Would you please post Sh. 1 of the Gas Fuel system P&ID? Many times there are Notes which can be very helpful.

Thank you in advance.
 
N
Dear CSA sir,

I am now out of office for 8-9 days leave. If you don't mind , after reach Office I will send you again.

Many many thanks for your fast reply.... Have a nice day
 
niazbibiyana,

> But, kindly inform me in D5 purge line the last valve "VA13-2"
> has some control function to control the opening percentage
> of that VA13-2 valve? but in PM4 purge line, the last valve
> "VA13-6" does not have this type of control. Do you know,
> why? if you know, please give me some idea.

I believe you are referring to the I/P (current to pressure) positioner on VA13-2 that isn't present on VA13-6? The only guess I can hazard is that possibly the system designers though that at some time it would be necessary to be able to control the purge valve position using a 4-20 mA (current) signal. Sh. 1 of the Purge System drawing has two notes, 5 and 8, that are very important--and I presume, as you suspected earlier, this is how it's hoped that the diffusion flame will not be "blown out"--and how it's hoped that the fuel being blown into the combustor during the start of purging will not cause a large load swing.

Device 65GP-E1P is likely connected to a 4-20 mA output of the Speedtronic turbine control panel which is used to control the intermediate position of VA13-2 by sending a current signal to the I/P positioner (which the I/P converts to a pressure reference that is proportional to stroke, which is then compared to the actual stroke and adjusts the air pressure to keep the actual stroke (position) equal to the reference from the Speedtronic). By controlling the opening (stroke; position) of VA13-2 the flow-rate through the valve can be controlled and the amount of purge air flowing into the combustor through the five diffusion flame nozzles in each combustor can be controlled (limited).

So, 65EP-G1P should be listed on the Device Summary, and should also likely be connected to a milliamp output of the Speedtronic turbine control panel. You can review the sequencing/application code in the Speedtronic to determine how the output signal is derived by working backwards from the milliamp output.

Hope this helps!
 
niazbibiyana,

It would also be helpful to know what each of the four limit switches on VA13-2 do (in other words, what position they are to indicate)? Perhaps the sequencing/application code in the Speedtronic turbine control panel adjusts the milliamp output to 65GP-E1P to make the valve move to one of the four positions indicated by the four limit switches. (GE Belfort would definitely do something like this....!)
 
N
Dear CSA sir,

As per your question, from device summary about VA13-2 position limit switches<pre>
33PG-3 : FUEL GAS SYSTEM PURGE VALVE LIMIT SWITCH
NORM=(2) NO-CES; CLOSED WITH VALVE CLOSED;

33PG-4 : FUEL GAS SYSTEM PURGE VALVE LIMIT SWITCH
NORM=(2) NO-CES; CLOSED WITH VALVE 17% OPEN;

33PG-15 : FUEL GAS SYSTEM PURGE VALVE LIMIT SWITCH
NORM=(2) NO-CES; CLOSED WITH VALVE 55% OPEN;

33PG-16 : FUEL GAS SYSTEM PURGE VALVE LIMIT SWITCH
NORM=(2) NO-CES; CLOSED WITH VALVE CLOSED;SPARE</pre>
Our GT expert told me few days ago, when D5 nozzle start purging, at first VA13-2 is opened 55%. then it starts reducing up to 17% and then continue purging from this position. But he could not explain me the exact reason to control the VA13-2 position. But I think at first it starts purging from 17% opening of VA13-2. Then slowly starts opening up to 55% ( Of course when GT is in Premixed Mode). Am I right?

In our Cimplicity screen, the VA13-2 valve position is not showing in percentage. So, I could not see the exact position from Cimplicity.

It is our bad luck to enter in toolboxST to check any logic. Because our management dont gave us the permission to touch toolbox ST.

Thanx a lot for your response. Have a nice day.

NIAZ
 
niazbibiyana,

Thanks for the information on the limit switches. And, without being able to see the application code running in the Mark VIe of the turbine at your site it's impossible for us to say exactly how the unit at your site is controlled and protected.

I will say that if the Mark VIe only has the limit switch feedback from the VA13-2 that the control scheme must be very clunky and complicated (something the engineers at GE Belfort, France, are very inclined to do--overly complicate something.)

As for what is or isn't on the HMI display--that's an oversight on the provider's part, not to at least show which limit switch is "active" during operation to indicate valve position (presuming there's no other valve position feedback to the Mark VIe).

>It is our bad luck to enter in toolboxST to check any logic.
>Because our management don't gave us the permission to touch
>toolbox ST.

This is one of my personal pet peeves--and reflects a total lack of understanding of how ToolboxST works. Not allowing ambitious and self-motivated individuals to have access to ToolboxST to be able to learn and monitor operation and eventually become better operators and technicians is just simply short-sighted and misguided. ToolboxST has password-protected levels that prevent inadvertent modification of parameters or forcing of signals--and if you don't people to modify things (unintentionally or intentionally without permission) then don't give people the password(s). But, don't block their access to be able to learn and understand--and even become a better troubleshooter.

Unfortunately, this is all too common. Sorry to hear about this; perhaps you could suggest that the knowledgeable people with access to ToolboxST be made available periodically to answer written questions and help others to be better operators and technicians (and supervisors, even)?

I'm very saddened to hear about this situation; very, very sad, indeed. It comes from a completely misguided, short-sighted and ill-informed perspective--and in many cases causes morale and motivation to be a problem for operators and technicians (and supervisors).
 
niazbibiyana,

Thanks for the feedback!

(And, please stop calling me 'Sir.' I work for a living just like most everyone else on this forum.)
 
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